August, 2000

Nicotiana tabacum as an Experimental Host for the Study of Plant–Xylella fastidiosa Interactions. S. A. Lopes, D. M. Ribeiro, P. G. Roberto, and S. C. França, UNAERP, Dep. Biotecnologia Vegetal, Av. Costábile Romano, 2201, Ribeirão Preto, SP; and J. M. Santos, FCAV, UNESP, Rod. Carlos Tonanni, Km 5, Jaboticabal, SP, Brazil. Plant Dis. D-2000-0620-01R, 2000 (on-line). Accepted for publication 9 May 2000.

Xylella fastidiosa causes the citrus variegated chlorosis disease (CVC), which is present in all citrus growing areas in the state of São Paulo, Brazil. Affected trees show reduced growth, orange leaf lesions, and small fruits. The pathogen is limited to the xylem of infected trees and is transmitted from plant to plant through a few species of xylem feeding leafhoppers. Despite the adoption of measures for disease control such as pruning of infected trees, use of healthy stock material for planting, and application of insecticides to reduce vector populations in the field, the pathogen disseminated quickly throughout S. Paulo and other states since CVC was first detected in 1987. More effective and environmentally safe strategies for disease control are necessary. Such strategies might result from the study of the Xylella genome sequence that was just completed by a consortium of laboratories located in the state. This study may provide a better understanding of how the pathogen colonizes and causes disease in orange trees. However, lack of an experimental host has been an obstacle to the study of pathogenicity genes in X. fastidiosa. Citrus plants are not appropriate for this due to the extended periods (6 months to 1 year) required for symptom expression in inoculated plants. In order to overcome this problem, we tested more than 20 plant species as potential hosts for X. fastidiosa. Tobacco was the only one that expressed unequivocal symptoms, consisting of orange leaf lesions approximately 2 months after inoculation. CVC symptoms were observed in citrus 1 to 5 months later. The presence of the pathogen within the xylem of symptomatic tobacco was confirmed by scanning electron microscopy, phase contrast microscopy, polymerase chain reaction (PCR), isolation on agar plates, and more recently by enzyme-linked immunosorbent assay (ELISA), employing specific antibodies produced against the citrus pathogen. The tobacco tested in this work is a native variety of Nicotiana tabacum not used for commercial planting. It may represent an important tool in the search for an effective measure to control CVC and to reduce losses in citrus caused by this disease.

Severe Yellowing Outbreaks in Tomato in Spain Associated with Infections of Tomato chlorosis virus. J. Navas-Castillo, R. Camero, M. Bueno, and E. Moriones, Estación Experimental "La Mayora", Consejo Superior de Investigaciones Científicas (CSIC), 29750 Algarrobo-Costa, Málaga, Spain. Plant Dis. D-2000-0602-01R, 2000 (on-line). Accepted for publication 25 April 2000.

A novel yellowing disease has occurred in tomato crops in southern Spain since 1997. Symptoms resemble those reported for Tomato infectious chlorosis virus or Tomato chlorosis virus, species of the genus Crinivirus of the family Closteroviridae. Interveinal yellowing developed on lower leaves and progressed to the upper part of the plant. Affected plants were less vigorous and yielded less due to reduced fruit growth and delayed ripening. During 1999, severe epidemic outbreaks occurred associated with high populations of the whitefly Bemisia tabaci. The yellowing disease was readily transmitted from symptomatic to healthy tomato plants by means of B. tabaci. Analysis of symptomatic plants indicated that they were infected with Tomato chlorosis virus. This is the first report of the presence of this virus in Europe.

Geminiviruses Infecting Tomato Crops in Nicaragua. A. Rojas, Escuela de Sanidad Vegetal, Universidad Nacional Agraria, Km. 12 Carretera Norte, Managua, Nicaragua, and Department of Plant Biology, Swedish University of Agricultural Sciences (SLU), 750 07 Uppsala, Sweden; and A. Kvarnheden and J. P. T. Valkonen, Department of Plant Biology, Swedish University of Agricultural Sciences (SLU). Plant Dis. D-2000-0522-03R, 2000 (on-line). Accepted for publication 18 April 2000.

Geminiviruses transmitted by whiteflies are believed to be responsible for the devastating epidemic in tomato crops in Nicaragua as well as in other Central American countries. The technique of polymerase chain reaction was used to identify the presence of geminiviruses in diseased tomato plants collected from all the major tomato-growing areas of Nicaragua. Further analyses showed that they corresponded to four different geminiviruses related to the other begomoviruses native to the Americas. One of the viruses, which was detected in three regions of Nicaragua, is probably Sinaloa tomato leaf curl virus. Two of the other detected viruses showed close relationships with several geminiviruses, including Tomato mottle virus, Tomato leaf crumple virus, and Sida golden mosaic virus, all of which previously have been reported from Central America. The fourth virus is closely related at DNA sequence level to a tomato-infecting geminivirus from Honduras, putatively designated Tomato mild mottle virus. According to our data, this virus seems to be different from the other known American begomoviruses. Identification of the viruses involved is an important step towards the management of the disease. This study shows the complexity of geminivirus diseases of tomato in Nicaragua, which are widespread and caused by several distinct geminiviruses.

Incidence of Latent Infection of Immature Peach Fruit by Monilinia fructicola and Relationship to Brown Rot in Georgia. K. M. Emery, Department of Plant Pathology, University of Georgia, Athens 30602; T. J. Michailides, Department of Plant Pathology, University of California-Davis, Kearney Agricultural Center, Parlier 93648; and H. Scherm, Department of Plant Pathology, University of Georgia. Plant Dis. D-2000-0519-01R, 2000 (on-line). Accepted for publication 20 April 2000.

The brown rot fungus Monilinia fructicola, a pathogen of peach and other stone fruits, causes two economically important symptom types, a blight of flowers during bloom (blossom blight) and a rot of ripe fruit at harvest and postharvest. Green (immature) peach fruit generally do not exhibit symptoms or signs of M. fructicola unless infection is favored by prolonged rain or high humidity following injury. Previous work has shown, however, that even without wounding, green fruit may harbor symptomless (latent) infections. Latent infections remain asymptomatic during green fruit development and may become active as the fruit ripen, thus serving as a possible means of carryover of M. fructicola from the spring to the preharvest period. The primary objectives of this study were to monitor the seasonal dynamics of latent fruit infection by M. fructicola in peach orchards in Georgia and to determine the relationship between latent infection at different stages of fruit development and fruit rot at harvest and postharvest. From 1997 to 1999, green peach fruit were collected at 14-day intervals from orchards in middle and northern Georgia and assayed for latent infections in the laboratory. The incidence of latent infection generally was low until the final sampling date, 7 to 12 days before harvest. The incidence of latent infection on the final sampling date was associated with both the incidence of blossom blight earlier in the season and the incidence of fruit rot at harvest. There also was an association between the incidence of latent infection at the onset of pit hardening (between 7 and 10 weeks before harvest) and subsequent fruit rot incidence. The results suggest that latent infections can be a source of preharvest fruit rot in Georgia peach orchards. Despite its close association with fruit rot incidence, the potential for using latent infection as a biological indicator of disease risk at harvest may be limited; the assessment of latent infection during fruit ripening (similar to the timing of the final sampling date in this study) would not provide sufficient lead time for preharvest disease management decisions, while an earlier assessment (e.g., at the onset of pit hardening) would necessitate large sample sizes due to the low levels of latent infection during that period.

Development of Monoclonal Antibodies Reactive to a New Grapevine Leafroll-Associated Closterovirus. Judit Monis, Agritope, Inc. 16160 SW Upper Boones Ferry Road, Portland, OR 97224. Plant Dis. D-2000-0601-01R, 2000 (on-line). Accepted for publication 20 April 2000.

Grapevine leafroll is a graft transmissible disease caused by several distinct grapevine leafroll-associated viruses (GLRaVs). To aid viral detection prior to plant propagation, it is important to carry out tests, including the enzyme-linked immunosorbent assay (ELISA) and Western blot. Monoclonal antibodies (MAbs) are produced by selecting animal cells that express a specific antibody so that it will react only to a region on the surface of the virus called epitope. MAbs reactive to a previously uncharacterized GLRaV protein were developed. The novel protein was found associated with grapevine leafroll disease in a mixed virus infection. MAbs were selected that react to GLRaV-4, -5, and the newly discovered protein associated with a virus named GLRaV-8. ELISA and Western blot were developed for the sensitive and specific detection of GLRaVs in infected grapevines.

Effect of Silicon Rate and Host Resistance on Blast, Scald, and Yield of Upland Rice. K. W. Seebold, Former Graduate Research Assistant, and L. E. Datnoff, Professor of Plant Pathology, University of Florida-IFAS, Everglades Research and Education Center (EREC), Belle Glade 33430; F. J. Correa-Victoria, Research Plant Pathologist and Rice Program Leader, Centro Internacional de Agricultura Tropical, A.A. 6713, Cali, Colombia; T. A. Kucharek, Professor of Plant Pathology, Plant Pathology Department, University of Florida-IFAS, Gainesville; and G. H. Snyder, Professor of Soil & Water Science, University of Florida-IFAS, EREC, Belle Glade 33430. Plant Dis. D-2000-0616-02R, 2000 (on-line). Accepted for publication 5 May 2000.

Blast, caused by Magnaporthe grisea, is one of the most destructive diseases of upland rice. The disease is typically controlled through the planting of resistant cultivars and by the application of fungicides. However, changes in the race structure of the pathogen population in rice fields have rendered many resistant cultivars ineffective, and fungicides are often too expensive for use by many growers. Researchers have shown that the application of silicon (Si)-containing fertilizers to soils that are Si-deficient, a common problem in soils where upland rice is grown, results in increased control of rice diseases such as blast and scald (caused by Monographella albescens) and improved yield and yield quality. The purpose of this study was to evaluate the effects of Si fertilizer, applied at three rates, on the severity of leaf and neck blast for cultivars of rice with complete resistance, partial resistance, or complete susceptibility to blast. Effects on scald, yield, and yield quality also were examined. When applied at either 500 or 1,000 kg/ha, Si significantly reduced the severity of leaf and neck blast, and also scald, on blast-susceptible and partially resistant cultivars. Depending upon the location, blast severity (leaf and neck) on partially resistant and blast-susceptible cultivars that had been fertilized with Si at 500 or 1,000 kg/ha was reduced to that of resistant cultivars that had not been fertilized with Si. Yields were increased by as much as 42% where Si was applied, depending upon location and cultivar. Higher rates of Si generally reduced the number of broken grains harvested, and grain discoloration was significantly lower at the highest rate of Si. The data from this study are evidence that Si can be used to complement host resistance to blast, resulting in an effective strategy for disease control. Along with blast control, Si provides the additional benefits of controlling scald and improving yield and yield quality in upland rice.

Effect of Irrigation and Soil Water Stress on Densities of Macrophomina phaseolina in Soil and Roots of Two Soybean Cultivars. S. R. Kendig, Former Graduate Student, and J. C. Rupe, Associate Professor, Department of Plant Pathology; and H. D. Scott, Professor, Department of Crop, Soil, and Environmental Sciences, University of Arkansas, Fayetteville 72701. Plant Dis. D-2000-0613-01R, 2000 (on-line). Accepted for publication 28 April 2000.

Charcoal rot, caused by the soilborne fungus Macrophomina phaseolina, is a serious disease of many crops and is associated with drought stress. There are few effective controls for this disease. The most reliable control is irrigation, but there have been no studies in soybean determining when irrigation needs to be applied. This study compares the effect of four irrigation regimes on the development of charcoal rot in two soybean cultivars. The irrigation treatments were irrigation until flowering (TAR2), after flowering (IAR2), full season (FSI), or not at all (NI) and the soybean cultivars were Davis and Lloyd. The study was conducted over three years in Fayetteville, AR. Although no symptoms of charcoal rot developed in this study, the irrigation regimes significantly affected yields and root colonization by the fungus. The highest yields were with FSI, followed by IAR2, TAR2, and NI, in that order. Root colonization by M. phaseolina as measured by microsclerotial (MS) density was also affected by irrigation. Colonization was highest in NI and lowest in FSI. When irrigation was initiated at flowering (IAR2), the MS density decreased due to increased root growth and decreased root colonization by the fungus. Likewise, when irrigation was terminated at flowering (TAR2), root growth slowed and colonization increased, resulting in greater MS densities in the root. Root colonization at harvest was higher in Davis than Lloyd and this was associated with a trend toward lower soil densities of the fungus in plots planted with Lloyd than in those planted with Davis. These results demonstrate that irrigation strongly affects root colonization of soybean by M. phaseolina, even when irrigation is begun at flowering, and emphasizes the importance of water management in the control of charcoal rot.

Stewart’s Wilt Reactions of an International Collection of Zea mays Germ Plasm Inoculated with Erwinia stewartii. Jerald K. Pataky, Lindsey J. du Toit, and Noah D. Freeman, Department of Crop Sciences, University of Illinois, Urbana 61801. Plant Dis. 2000-0612-02R, 2000 (on-line). Accepted for publication 1 May 2000.

Stewart’s wilt is a bacterial disease of corn caused by Erwinia stewartii. Corn flea beetles transmit E. stewartii when they feed on corn plants. The disease occurs annually in the Mid-Atlantic and Ohio River Valley regions of the United States and in the central portion of the U.S. Corn Belt. Stewart’s wilt has been reported infrequently from other parts of the world. Yield of field corn is rarely affected by Stewart’s wilt because most hybrids grown where the disease occurs have adequate levels of host plant resistance. However, some sweet corn hybrids are highly susceptible and sustain severe yield reductions when infected. Identification of additional sources of resistance to this disease could diversify and improve Stewart’s wilt resistance in sweet corn. Resistance to diseases often occurs in plant germ plasm collected from the center of origin of the cultivated species. Corn was domesticated in Central America and Mexico. It is unlikely that high levels of Stewart’s wilt resistance occur in corn germ plasm collected from the center of origin of corn because the pathogen, E. stewartii, is restricted primarily to corn growing regions of the United States where the corn flea beetle is common. In this study, we evaluated nearly 2,000 different corn lines collected from every continent of the world in order to identify additional sources of Stewart’s wilt resistance and to determine if Stewart’s wilt reactions differed among corn germ plasm collected from various regions of the United States and throughout the world. The distribution of Stewart’s wilt reactions was similar for corn germ plasm collected throughout the world except for germ plasm from the Mid-Atlantic/Ohio River Valley area, the southern United States, and the northeastern United States. Germ plasm from the Mid-Atlantic/Ohio River Valley area and the southern United States was more resistant than other germ plasm. Germ plasm from the northeastern United States was more susceptible. Resistance in corn from the Mid-Atlantic/Ohio River Valley area reflects a response to persistent selection pressure due to regular epidemics of Stewart’s wilt in this area. Plants with resistance have been selected, and those with susceptibility have been eliminated either by man or by nature. Corn germ plasm from other areas of the world tends to have an average reaction to this disease. These results exemplify the result of response to selection and suggest that further exploration for diverse sources of resistance to Stewart’s wilt will most likely to be successful among germ plasm collected from areas where the disease is most prevalent. The sources of resistance identified from this research will be examined further to determine if they differ genetically and to decide if they can be used successfully to improve reactions of susceptible sweet corn.

Assessment of Virulence of Acremonium cucurbitacearum and Monosporascus cannonballus on Cucumis melo. B. D. Bruton, United States Department of Agriculture, Agricultural Research Service, South Central Agricultural Research Laboratory, Lane, OK 74555; J. Garcia-Jimenez and J. Armengol, Unidad de Patologia Vegetal, Dpto. de Produccion Vegetal, Universidad Politecnica, C de Vera s/n, 46020 Valencia, Spain; and T. W. Popham, United States Department of Agriculture, Agricultural Research Service, Stillwater, OK 74074. Plant Dis. D-2000-0619-01R, 2000 (on-line). Accepted for publication 28 April 2000.

Vine declines can be caused by several different pathogens and are yield-limiting in many production areas around the world. Vine decline is a term used to describe diseases of muskmelon that normally appear as the fruit approach maturity. These diseases are characterized by yellowing and death of the crown leaves, which gradually radiates outward. Accurate assessment of virulence of soilborne pathogens is necessary for monitoring changes in the genetic makeup within the pathogen population and is a prerequisite for effective plant breeding programs for resistance. The purpose of this study was to develop a method for assessing virulence of two important vine decline pathogens (Acremonium cucurbitacearum and Monosporascus cannonballus) of cucurbits. The system provided good assessment of damage to the hypocotyl, stem-root junction, primary root, secondary roots, and reduction of the first two true leaves. For cucurbit root rot pathogens, reduction in leaf area of the first two true leaves may be preferable to taking root weight due to the difficulty in obtaining all the roots and the great amount of variability in root mass that is normally experienced. Alone, area of the first two true leaves is insufficient to estimate plant damage. Combining ratings of damage to the hypocotyl, stem-root junction, primary root, and secondary roots with a scaled leaf area reduction provided a useful method for assessment of plant damage and thus isolate virulence. Analysis of multiple inoculum densities established that colony forming units per gram of soil of 5, 10, 20, and 40 for M. cannonballus and 0.1 × 10(^4), 1 × 10(^4), 2 × 10(^4), and 3 × 10(^4) for A. cucurbitacearum, respectively, were adequate for virulence comparisons of isolates of either fungus in greenhouse tests on seedlings. Seed planting depth had a significant effect on disease severity index. The 4-cm depth generally resulted in more reliable disease ratings, indicating greater precision in determining isolate virulence. Perhaps this study can provide a basis for the development of a "universal testing system" to standardize methods for evaluating soilborne pathogens of cucurbits.

July, 2000

Responses of 11 Fraxinus Cultivars to Ash Yellows Phytoplasma Strains of Differing Aggressiveness. W. A. Sinclair, Department of Plant Pathology, Cornell University, Ithaca, NY 14853; M. L. Gleason, Department of Plant Pathology, Iowa State University, Ames 50011; H. M. Griffiths, Department of Plant Pathology, Cornell University; J. K. Iles, Department of Horticulture, Iowa State University; N. Zriba, D. V. Charlson, and J. C. Batzer, Department of Plant Pathology, Iowa State University; and T. H. Whitlow, Department of Floriculture and Ornamental Horticulture, Cornell University. Plant Dis. D-2000-0418-01R, 2000 (on-line). Accepted for publication 8 March 2000.

The disease ash yellows retards growth of ash trees and causes or contributes to the decline of highly susceptible ash species. Impact of this disease on shade trees could be lessened if disease-tolerant planting stock were available. Six commercial cultivars of green ash and five of white ash were tested in Iowa and also in New York for ability to grow well and maintain near-normal foliar color while infected with phytoplasmas that cause ash yellows. High-level tolerance of infection was not detected. Growth of diseased trees of each cultivar, averaged across the two test sites, was depressed at least 30% in comparison to growth of healthy trees. Cultivars reacted differentially to the Iowa and New York test sites, inasmuch as cultivar ranks for growth rate and disease response in Iowa were not correlated with ranks in New York. However, green ash cultivars Bergeson, Dakota Centennial, and Patmore and white ash cultivar Autumn Applause were above average in tolerance at both locations.

Other Natural Hosts of Potato virus T. C. Lizárraga, M. Querci, M. Santa Cruz, I. Bartolini, and L. F. Salazar, Crop Protection Department, International Potato Center (CIP), Apartado 1558, Lima 12, Peru. Plant Dis. D-2000-0508-03R, 2000 (on-line). Accepted for publication 13 March 2000.

Plant viruses can cause yield reduction, and virus-free seed is used to assure higher yields. The first step in producing virus-free seed in a crop is to identify the viruses present. Potato virus T (PVT) was found in three Andean tuber crops (ulluco, oca, and mashua) under study at the International Potato Center in Lima, Peru. PVT has only been found infecting potatoes in Peru and Bolivia. The PVT from these three crops also infected potato plants. International quarantine restrictions forbid the introduction of virus-infected planting materials such as botanical seed, tubers, cuttings, or storage roots. Because of this new information, ulluco, oca, and mashua plants exported from the Andean region should be tested for PVT. Furthermore, these species may be a source of virus to healthy potato or Andean tuber crops. PVT is important because it is one of four potato viruses that can be transmitted through botanical seed.

Petunia vein banding virus: Characterization of a New Tymovirus from Petunia × hybrida. M. A. V. Alexandre, L. M. L. Duarte, E. B. Rivas, C. M. Chagas, and M. M. Barradas, Research Plant Virologists, Instituto Biológico, Av. Cons. Rodrigues Alves, 1252, CEP 04014-002, São Paulo, Brazil; and R. Koenig, Research Plant Virologist, Institut fur Pflanzenvirologie, Mibrobiologie und Biologische Sicherheit, Messeweg 11/12, D-38104, Braunschweig, Germany. Plant Dis. D-2000-0502-01R, 2000 (on-line). Accepted for publication 10 March 2000.

Petunia is an economically important bedding and balcony ornamental, especially the new hybrids which display intensively colored flowers. In Brazil, a new tymovirus was detected in Petunia spp. showing pronounced vein banding and was named Petunia vein banding virus (PetVBV). The virus was mechanically transmitted only to petunia, Nicotiana benthamiana, and Nicandra spp., and not to tomato, tobacco, pepper, eggplant, or cucumber. Tests made to verify transmission by aphids and seeds were negative. Tymoviruses are spread by chrysomelid beetles. In general, tymoviruses are not aggressive and appear sporadically, a characteristic probably associated with ecological dispersion of the vectors. As far as is known, this virus is not economically important because it does not infect economic crops. However, the new petunia hybrids are also vegetatively propagated; therefore, appropriate sanitary measures are important to prevent epidemic spread and accumulation of virus infection.

Differential Seed Infection of Wheat Cultivars by Stagonospora nodorum. Denis A. Shah, Graduate Research Assistant, and Gary C. Bergstrom, Professor, Department of Plant Pathology, and Mark E. Sorrells, Professor, Department of Plant Breeding, Cornell University, Ithaca, NY 14853-4203. Plant Dis. D-2000-0428-01R, 2000 (on-line). Accepted for publication 17 March 2000.

Stagonospora nodorum blotch, also known as leaf and glume blotch, is a common wheat disease that reduces yield. The disease is caused by the fungus Stagonospora nodorum, also known as Septoria nodorum. The fungus infects wheat seed and can be transmitted from seed to developing seedlings. Infected seed may be the main source of the disease in some regions. Disease development may be reduced or delayed by planting seed with a low incidence of infection. This may be accomplished in part by the development of wheat varieties with resistance to seed infection by the fungus. To determine whether wheat varieties actually differ in their susceptibility to seed infection, we collected seed of soft white winter varieties from New York regional trials in 1995 and 1996 and examined them for presence of the fungus. Some varieties had lower levels of seed infection. To be sure that varietal differences we observed were actually due to differences in the susceptibility of the developing seeds, 12 varieties were sprayed with fungus spores in the greenhouse when the plants were flowering. Two cultivars, Delaware and Houser, showed lower levels of seed infection than the other cultivars. These two cultivars were also lowest in seed infection in the field trials. Thus, it was concluded that some wheat varieties possess a certain level of resistance to seed infection by this fungus. It may be possible through breeding and selection to develop wheat varieties with increased levels of resistance to seed infection. Such varieties could play an important role in the integrated control of this disease.

Seasonal Infection of the Weed Dyer’s Woad by a Puccinia sp. Rust Used for Biocontrol, and Effects of Temperature on Basidiospore Production. Karen M. Flint and Sherman V. Thomson, Department of Biology, Utah State University, Logan 84322-5305. Plant Dis. D-2000-0427-02R, 2000 (on-line). Accepted for publication 17 March 2000.

Dyer’s woad (Isatis tinctoria L., Brassicaceae) is a plant well known in antiquity as a source of blue dye, the predecessor of indigo, and valued so highly that some European economies were built around its cultivation, processing, and trade. Dyer’s woad has achieved a new and less benign notoriety in the arid West, where it has proliferated and is now considered a noxious weed in eight western states. A rust (Puccinia thlaspeos) was discovered recently that systemically infects dyer’s woad and prevents seed production. The rust is specific to dyer’s woad and is ideal for use as a biological control agent. We demonstrated that potted dyer’s woad rosettes exposed to natural rust inoculum at field sites became infected when exposed from late April through early July, depending upon the location. The latent period between exposure and symptom expression varied from 9 to 54 weeks. When naturalized stands of woad were inoculated with teliosori, either fresh or dried, the incidence of infection was 58 to 76%, compared with 2 to 7% incidence in noninoculated plants. Basidiospores were readily produced from intact teliosori when suspended over water agar, with the highest rate of production between 3 and 6 h of incubation at 10 to 20°C. The optimum temperature for basidiospore production over a 24-h period was 15°C, but spores were produced at temperatures as low as 5°C and not at 25°C. These low temperatures for spore production corroborate the field evidence that dyer’s woad rust most actively infects in springtime, when temperatures are comparatively low and rainfall is more frequent.

Occurrence of Cowpea aphid-borne mosaic virus in Peanut in Brazil. G. Pio-Ribeiro, Universidade Federal Rural de Pernambuco, Dois Irmãos, Recife, PE - CEP: 52 171-900, Brazil; S. S. Pappu, Department of Entomology, and H. R. Pappu, Department of Plant Pathology, University of Georgia, Coastal Plain Experiment Station, Tifton 31793; G. P. Andrade, Universidade Federal Rural de Pernambuco, Dois Irmãos, Recife, PE - CEP: 52 171-900, Brazil; and D. V. R. Reddy, International Crops Research Institute for the Semi-Arid Tropics, Patancheru, Andhra Pradesh 502 324, India. Plant Dis. D-2000-0427-01R, 2000(on-line). Accepted for publication 20 March 2000.

Peanut is an economically important crop in Brazil. An outbreak of a new virus disease was first observed in northeastern Brazil in 1995. The causal virus was identified as a strain of Cowpea aphid-borne mosaic virus (CABMV), of the genus Potyvirus and family Potyviridae based on sequence comparisons of the coat protein and the 3(prime) terminal region of the viral genome with known viruses. Under experimental conditions, the virus was transmitted by aphids Toxoptera citricidus and Aphis gossypii. Several viruses cause economically important diseases in peanut, but this is the first time that CABMV is shown to be widespread in peanut.

Plant Growth-Promoting Rhizobacterial Mediated Protection in Tomato Against Tomato mottle virus. John F. Murphy and Geoffrey W. Zehnder, Department of Entomology & Plant Pathology, Auburn University, AL 36849; David J. Schuster, University of Florida-IFAS, Bradenton 34203; Edward J. Sikora, Department of Entomology & Plant Pathology, Auburn University, AL 36849; Jane E. Polston, University of Florida-IFAS, Bradenton 34203; and Joseph W. Kloepper, Department of Entomology & Plant Pathology, Auburn University, AL 36849. Plant Dis. D-2000-0509-01R, 2000 (on-line). Accepted for publication 29 March 2000.

Whitefly-transmitted geminiviruses pose a serious threat to commercial vegetable production. Management of these viruses under natural conditions is difficult when resistant plant varieties are not available, and thus alternative strategies must be evaluated. In this study, tomato plants were treated with plant growth-promoting rhizobacteria (PGPR), organisms known to induce a plant’s natural defenses, in an effort to induce resistance against infection by Tomato mottle virus (ToMoV). The PGPR treatments were applied as an industrially formulated seed treatment, a spore preparation mixed with potting medium (referred to as powder), or as a combined seed+powder treatment and were evaluated under field conditions. Experiments were conducted in the fall of 1997 and the spring and fall of 1998 at the University of Florida’s Gulf Coast Research & Education Center, Bradenton, FL. All plants were rated for symptoms and analyzed for the presence of ToMoV DNA at 40 days after transplant (dat). Whitefly densities were determined on individual plants in each trial, and marketable fruit yields were determined at least two times during each trial. The highest level of protection occurred in the fall 1997 trial when, at 40 dat, ToMoV disease severity ratings were significantly less in all PGPR powder-based treatments than in either of the seed or control treatments. Detection of viral DNA using Southern dot blot analyses correlated with symptom severity ratings, as did fruit yields. A reduction in ToMoV symptom severity ratings and incidence of viral DNA were also observed for some PGPR treatments in the spring 1998 trial, although corresponding yield responses were not apparent. No differences in disease severity, detection of ToMoV DNA, or yields occurred among treatments in any of the trials at 80 dat. These data show that up to 40 dat under natural conditions of high levels of vector–virus pressure, some PGPR treatments resulted in reduced ToMoV incidence and disease severity and, in some cases, with a corresponding increase in fruit yield. Use of PGPR could become a component of an integrated program for management of this virus in tomato.

Preplanting Bahia Grass or Wheat Compared for Controlling Mesocriconema xenoplax and Short Life in a Young Peach Orchard. A. P. Nyczepir, Research Nematologist, United States Department of Agriculture-Agricultural Research Service, Southeastern Fruit and Tree Nut Research Laboratory, 21 Dunbar Road, Byron, GA 31008; and P. F. Bertrand, Professor, Extension Plant Pathologist, University of Georgia, Tifton 31793. Plant Dis. D-2000-0505-01R, 2000 (on-line). Accepted for publication 26 March 2000.

Ring nematodes are widely distributed throughout the world, with certain species considered to be economically important to the stone fruit industry. Probably the most studied ring nematode species on Prunus spp. is Mesocriconema xenoplax. This ring nematode is the only plant-parasitic nematode that has been associated with the peach tree short life (PTSL) disease complex in the southeastern United States. Tree loss due to PTSL in South Carolina alone was estimated at over $5 million per year. New preplant alternatives to chemical control (i.e., nematode suppressive ground covers) that are less hazardous to people and also more environmentally safe must be found to protect peach trees from this ring nematode. Pensacola and Tifton 9 bahia grass were evaluated from 1991 to 1998 as a potential preplant ground cover management strategy in suppressing the ring nematode population density and to determine the influence long-term preplant bahia grass rotations have on peach tree growth and incidence of PTSL. Results indicate that tree growth was greatest in Pensacola bahia grass killed sod and least in the unfumigated weed plots. However, PTSL tree survival in both preplant bahia grass treatments did not differ from trees planted into unfumigated soil. Additionally, preplanting wheat was as effective as preplant methyl bromide fumigation in increasing tree survival from PTSL. These data provide useful insights into the use of bahia grass as a preplant alternative to chemical control of the ring nematode. This data was also necessary for substantiating the effectiveness of wheat as a nonchemical preplant management strategy of the ring nematode on PTSL sites in the Southeast.

Particle Lengths of Whitefly-Transmitted Criniviruses. H.-Y. Liu, G. C. Wisler, and J. E. Duffus, United States Department of Agriculture-Agricultural Research Service, 1636 East Alisal Street, Salinas, CA 93905. Plant Dis. D-2000-0517-01R, 2000 (on-line). Accepted for publication 14 April 2000.

Particle length is an important criterion for classifying plant closteroviruses into genera. Published particle measurement data must be regarded with caution because different procedures for virus isolation were used. Apparent differences may actually be the result of different techniques. In this paper, an improved method for particle length measurement was used for six members of the new genus Crinivirus in the family Closteroviridae, and the method provides information for closterovirus taxonomy. In a comparison of specimen preparation methods, the leaf-dip method is more representative and reproducible than the antibody capture method or preparation from purified virions. Particle length (nm) ranges of whitefly-transmitted criniviruses are: Abutilon yellows virus (AYV), 800 to 850; Cucurbit yellow stunting disorder virus (CYSDV), 750 to 800; Lettuce chlorosis virus (LCV), 800 to 850; Lettuce infectious yellows virus (LIYV), 700 to 750; Tomato chlorosis virus (ToCV), 800 to 850; and Tomato infectious chlorosis virus (TICV), 850 to 900.

June, 2000

Effects of Gypsum Soil Amendments on Avocado Growth, Soil Drainage, and Resistance to Phytophthora cinnamomi. B. J. Messenger, J. A. Menge, and E. Pond, Department of Plant Pathology, University of California, Riverside 92521. Plant Dis. D-2000-0417-01R, 2000 (on-line). Accepted for publication 15 February 2000.

Phytophthora root rot of avocado, caused by Phytophthora cinnamomi, is a devastating disease in most avocado-growing regions of the world. Crop losses due to Phytophthora root rot can be as much as 30% of the value of the crop, despite 70 years of research into this disease. Several previous field studies showed that gypsum soil amendments decreased disease in infested avocado groves. This study examined the effects of gypsum soil amendments on severity of Phytophthora root rot, soil drainage, avocado seedling growth, root exudates, and disease resistance under controlled greenhouse conditions. The addition of 5% (wt/vol) gypsum to soil greatly reduced the percentage of rotted avocado roots. Resistance of avocado roots grown in gypsum-amended soil and challenged with zoospores of P. cinnamomi did not appear to be affected by the gypsum treatment. Similarly, total plant growth of the avocado seedlings was unaffected in uninfested gypsum-amended soil, although roots of plants grown in the 5% gypsum treatment in infested soil showed a significant increase in growth compared with plants grown in unamended soil. This can be attributed to the reduction in root rot, rather than stimulation in root growth. Zoospores of this pathogen locate roots through root exudates, and a reduction of root exudation could result in less infection. Total root exudation, as measured by amount of (^86)Rb exuded from root segments over time, was unaffected by growth in gypsum-amended soil. Phytophthora root rot of avocado is greatly exacerbated by poorly drained soils; consequently, soil infiltration studies were conducted to determine whether the gypsum amendment decreased disease by increasing water infiltration into the soil. Root infection of avocados grown in finely milled gypsum amendment was significantly reduced, although water infiltration into the soil was not statistically different from infiltration into the unamended soil. The cause of reduction of Phytophthora root rot of avocados grown in gypsum-amended soil is not clear based on this study. The effect of gypsum on growth and reproduction of the pathogen itself may be more influential in disease reduction.

Effects of Gypsum on Zoospores and Sporangia of Phytophthora cinnamomi in Field Soil. B. J. Messenger, J. A. Menge, and E. Pond, Department of Plant Pathology, University of California, Riverside 92521. Plant Dis. D-2000-0417-02R, 2000 (on-line). Accepted for publication 15 February 2000.

Phytophthora root rot of avocado, caused by Phytophthora cinnamomi, is the limiting factor in commercial avocado production in California and is a devastating disease in most avocado-growing regions of the world. Previous studies showed that gypsum soil amendments decreased avocado root rot in groves and in greenhouses. The mechanisms by which gypsum affects the causal agent of this disease were examined. We studied mycelial mats of the fungus that were buried in soil to determine the effects of gypsum on P. cinnamomi in a soil environment. Phytophthora root rot spreads from tree to tree by means of zoospores: motile, infectious spores that use chemotaxis to reach the host root. Gypsum may be able to affect this process by reducing the size and number of sporangia, the structures that produce zoospores, thereby reducing the number of zoospores. This effect was seen whether the fungus was grown in gypsum-amended soil, in extracts from gypsum-amended soil, or water with gypsum added. It appears that the calcium present in gypsum affects sporangia. We also studied the effects of gypsum on zoospore attraction to the host, passive movement through the soil, and zoospore encystment. None of these factors were significantly affected by gypsum treatments.

Distribution of Xylella fastidiosa in Citrus Rootstocks and Transmission of Citrus Variegated Chlorosis Between Sweet Orange Plants Through Natural Root Grafts. C. X. He, Department of Technology, UNESP, Jaboticabal, SP, Brazil; W. B. Li and A. J. Ayres, FUNDECITRUS, Araraquara, SP, Brazil; J. S. Hartung, USDA-ARS, Beltsville, MD 20705-2350; and V. S. Miranda and D. C. Teixeira, FUNDECITRUS, Brazil. Plant Dis. D-2000-0331-01R, 2000 (on-line). Accepted for publication 28 January 2000.

Citrus variegated chlorosis (CVC) affects up to 40% of the sweet orange trees in Brazil and can cause substantial loss due to reduced fruit size. The disease is not yet present in the United States. It is caused by the bacterium Xylella fastidiosa and is transmitted by leafhopper insects that feed in the xylem tissue where the bacterium lives in infected plants. The insect vectors for the disease are abundant in the United States. CVC is also spread through contaminated budwood used in propagation. We show here that, although the insects feed in the foliage and branches of the tree, the bacterium is also found in the root systems of 11 of 15 rootstocks tested. The mechanism used by the bacterium for systemic movement in infected plants is not known. Using potted plants in a screened greenhouse, we have also shown that the pathogen can be transmitted between plants through naturally occurring root grafts. The rate of natural root-graft formation in citrus is not known but should be more frequent at higher planting densities as are used in nurseries. Other strains of the same bacterium cause important diseases of coffee in Brazil and of grapevines, oleander, peach, plum, and several shade trees in the United States. This is the first report of root-graft transmission of this pathogen and thus has implications for the management of this pathogen in several of these hosts in addition to citrus, particularly those grown in hedgerows where extensive natural root grafting is likely. These crops include coffee in Brazil and oleander in the western United States.

Wheat yellow mosaic virus Widely Occurring in Wheat (Triticum aestivum) in China. C. Han, D. Li, Y. Xing, K. Zhu, Z. Tian, Z. Cai, J. Yu, and Y. Liu, National Laboratories for Agrobiotechnology, China Agricultural University, Beijing, China 100094. Plant Dis. D-2000-0405-01R, 2000 (on-line). Accepted for publication 2 February 2000.

Wheat yellow mosaic virus (WYMV) was identified in 25 samples from six provinces along the Yangtze and Huai Rivers in China. Wheat spindle streak mosaic virus, which occurs in North America, was not detected. Thus, contrary to previous reports, WYMV may be the only bymovirus in wheat in China. An isolate from Henan Province in China showed differences in virulence and protein composition from all other isolates and caused severe damage on wheat, including some resistant cultivars.

Evaluation of BSPcast Disease Warning System in Reduced Fungicide Use Programs for Management of Brown Spot of Pear. I. Llorente, Associate Professor, Institute of Food and Agricultural Technology-CeRTA, University of Girona, 17071 Girona (Spain); P. Vilardell, Research Agronomist, Mas Badia Agricultural Experiment Station, La Tallada, Girona (Spain); R. Bugiani, Research Agronomist, Servizio Fitosanitario-Regione Emilia-Romagna, Via di Corticella 133, Bologna (Italy); I. Gherardi, Associate Professor, Dipartimento de Produzione e Valorizacione Agraria, University Degli Studi di Bologna, Via Filippo Re 8, 40126 Bologna (Italy); and E. Montesinos, Professor, Institute of Food and Agricultural Technology-CeRTA, University of Girona, 17071 Girona (Spain). Plant Dis. D-2000-0328-02R, 2000 (on-line). Accepted for publication 28 February 2000.

Brown spot of pear (BSP) is an economically important fungal disease caused by Stemphylium vesicarium, which affects several pear growing areas of Europe. Infections occur on leaves, fruit, and twigs, mainly in the most susceptible cultivars Abate Fetel, Passe Crassane, Alexandrine, and Conference. Control of BSP is currently achieved with 7- or 15-day-interval sprays using carbamate or carboximide fungicides, and large numbers of fungicide applications are needed to maintain commercially acceptable levels of disease in affected orchards. However, some applications of fungicides may not be necessary because environmental conditions are not always suitable for infections. A model, named BSPcast, was developed from controlled environment and field experiments for prediction of BSP, which uses daily wetness duration and temperature. The model was validated for its accuracy of disease prediction in several field trials under different climatic conditions. This study evaluates BSPcast as an advisory system for reduced fungicide use in disease control programs. The results obtained during 3 years of study in 11 orchard trials performed in two different climatic regions in Spain and Italy, with three pear cultivars and three fungicides, showed consistently that the BSPcast model is a useful tool for rational control of BSP. The use of BSPcast at specific action thresholds reduced the number of fungicide sprays 20 to 70% and maintained the same levels of control as the commercial fixed-spray schedule. A brown spot warning system software now under construction provides disease risk and action threshold values, based on weather, pathogen, and host parameters, that advise fungicide application programs through warning stations.

Environmental Factors Affecting the Severity of Alternaria Brown Spot of Citrus and Their Potential Use in Timing Fungicide Applications. L. W. Timmer, Professor, H. M. Darhower and S. E. Zitko, Senior Biologists, T. L. Peever, Assistant in Plant Pathology, and A. M. Ibáñez and P. M. Bushong, Senior Biologists, University of Florida, Citrus Research and Education Center, 700 Experiment Station Road, Lake Alfred 33850. Plant Dis. D-2000-0331-02R, 2000 (on-line). Accepted for publication 28 February 2000.

Alternaria brown spot affects many tangerines and their hybrids causing lesions on leaves, twigs, and fruit, resulting in reduced yield and fruit quality. Field studies were conducted to relate environmental factors to disease severity on field trees and potted trap plants. Disease was most severe during the summer rainy season, but some infection occurred nearly every week of the year. When measured on a weekly or daily basis, disease severity was only weakly correlated with rainfall amount, duration of leaf wetness, and temperature, and was not correlated with the number of airborne spores. When disease severity values on a daily basis were categorized according to the following environmental conditions: (i) rain versus no rain, (ii) <10 h or >10 h of leaf wetness, and (iii) average daily temperature <20°C, 20 to 28°C, and >28°C, the relationship was much clearer. A point system, called the ALTER–RATER, was designed where each day is assigned a severity score based on the weather. Fungicide applications can be made after a predetermined number of points have been accumulated. The threshold used depends on the susceptibility of the cultivar and the disease history in the grove. Such a weather-based control system could reduce the number of fungicide applications and improve control of Alternaria brown spot.

Proficiency Testing in a Laboratory Accreditation Program for the Bacterial Ring Rot Pathogen of Potato. S. H. De Boer, Centre for Animal and Plant Health, Canadian Food Inspection Agency, 93 Mount Edward Road, Charlottetown, PEI, Canada C1A 5T1; and J. W. Hall, Pacific Agri-Food Research Centre, Agriculture and Agri-Food Canada, Summerland, BC, Canada V0H 1Z0. Plant Dis. D-2000-0410-01R, 2000 (on-line). Accepted for publication 29 February 2000.

Bacterial ring rot is potentially a very serious disease of potato that can cause extensive losses to infected crops. The causal agent of the disease is a bacterium which is spread from infected tubers to additional tubers during planting, harvesting, and storing of a potato crop. Although the bacterium survives well on contaminated equipment and in storage facilities, no other host plants or sources of infection are known. Control of the disease is achieved by growing potatoes only from healthy seed tubers; that is, tubers that are not infected with the ring rot pathogen. Ring rot infections, however, may be latent and not show any visible symptoms of disease. The elimination of any seed lots which harbor the bacterium, particularly as latent infections, is of utmost importance for eradicating the disease. A number of methods have been developed to detect the presence of latent ring rot infections in consignments of seed potatoes. The most important of these methods involves taking a random sample of tubers from a consignment and testing these by enzyme-linked immunosorbent assay and immunofluorescence. Both of these procedures involve the use of antibodies that react specifically with the ring rot bacterium. In Canada, testing of seed lots for the possible presence of the ring rot bacterium is now done in private, accredited laboratories. In this study, we evaluated how reproducible these tests are when conducted in private laboratories. In other words, do all laboratories provide the same result? While the consistency of laboratory results has long been confirmed in medical and veterinary laboratories, such testing for plant pathogens is relatively new. Testing plants for the presence of a pathogenic bacterium is complicated by the fact that plants in the soil environment are contaminated by large populations of harmless soil microorganisms that are difficult to distinguish from plant pathogens. Therefore, in this study, we measured the expected variation in results from individual analysts repeatedly testing the same sample, and the variation in results among many analysts testing the same sample. We described the variation that occurred and used the expected variation to monitor the consistency of individual analysts by determining whether the results tended to be higher or lower than the actual value and whether this variation was within or beyond the acceptable limit. Although this study had its application for the bacterial ring rot disease, the analytical approach should be applicable to other laboratory tests for plant pathogens and may become increasingly important as countries need to accept one another’s laboratory result for the safe movement and trade of propagative material for food crops.

Biosynthesis of Deoxynivalenol in Spikelets of Barley Inoculated with Macroconidia of Fusarium graminearum. C. K. Evans and W. Xie, Research Associates, R. Dill-Macky, Assistant Professor, and C. J. Mirocha, Professor Emeritus, Department of Plant Pathology, University of Minnesota, St. Paul 55108. Plant Dis. D-2000-0323-02R, 2000 (on-line). Accepted for publication 29 February 2000.

Fusarium head blight or scab of wheat and barley in the United States is primarily caused by the fungal pathogen Gibberella zeae/Fusarium graminearum. The pathogen not only reduces grain weight, but also reduces grain quality because of toxins it produces. The mycotoxins threaten the health of humans and livestock that consume scabby grain. Previous research demonstrated that many fungi, including F. graminearum, have a toxin(s) in or on their fungal spores. This investigation demonstrates a method of culturing and harvesting spores of F. graminearum that have no detectable toxins in or on them. The toxin-free spores of F. graminearum were used to inoculate barley to determine how soon the pathogen produces detectable amounts of the mycotoxins deoxynivalenol (DON) and 15-acetyldeoxynivalenol (15-ADON) in the developing seeds of barley. The pathogen produced detectable amounts of the two toxins 36 h after inoculation. Establishing when the toxins are produced serves as a reference point to study the effects of fungicides, biological control organisms, and new genetic resistance mechanisms. It is important to know whether any of these disease management tools will prevent or delay the production of toxins in barley and other cereal species. We also inoculated 31 barley cultivars and lines with toxin-free spores, then sampled plants 72 h after inoculation. The amount of DON in inoculated seeds ranged from 0.14 to 1.26 µg. The range of toxin accumulation we observed among these cultivars and lines demonstrates that we may be able to develop cultivars that accumulate less toxin.

Influence of Soil Saturation and Temperature on Erwinia chrysanthemi Soft Rot of Carrot. J. J. Farrar, Department of Plant Pathology, University of California, Davis 95616; J. J. Nunez, University of California Cooperative Extension, Kern County; and R. M. Davis, Department of Plant Pathology, University of California, Davis. Plant Dis. D-2000-0414-01R, 2000 (on-line). Accepted for publication 29 February 2000.

Soft rots occur on the fleshy parts of many vegetable crops and are caused by several species of bacteria. In California, carrot soft rot is generally a minor problem and the causal agent is Erwinia carotovora subsp. carotovora. In the late summer and early fall of 1998, California carrot growers and packers noticed a high incidence of carrot with soft rot. We isolated and identified E. chrysanthemi from carrots from several fields. The 1998 outbreak of carrot soft rot appeared to be related to unusually high soil temperatures and consequent increases in irrigation. We examined environmental conditions necessary for soft rot caused by E. chrysanthemi. Soil saturation was required for disease development. Soft rot severity and incidence increased with increasing soil temperature from 20 to 35°C and duration of soil saturation. We also compared the ability of E. chrysanthemi and E. carotovora subsp. carotovora to rot carrot tissue at several temperatures. Both species rotted a moderate amount of carrot tissue at 20 and 25°C. E. chrysanthemi rotted a large amount of carrot tissue at 30 and 35°C. E. carotovora subsp. carotovora was not able to rot carrot tissue at 35°C. These results support the hypothesis that high soil temperature was the reason the 1998 carrot soft rot outbreak was due to E. chrysanthemi.

Effect of Chloride and Soybean Cultivar on Yield and the Development of Sudden Death Syndrome, Soybean Cyst Nematode, and Southern Blight. J. C. Rupe, Department of Plant Pathology, University of Arkansas, Fayetteville; J. D. Widick, College of Agriculture, Arkansas State University, Jonesboro; W. E. Sabbe, Agronomy Department, University of Arkansas, Fayetteville; and R. T. Robbins and C. B. Becton, Department of Plant Pathology, University of Arkansas, Fayetteville. Plant Dis. D-2000-0417-03R, 2000 (on-line). Accepted for publication 1 March 2000.

Yields of irrigated soybean in Arkansas are threatened by two problems: chloride toxicity and sudden death syndrome (SDS). Soybeans are sensitive to chloride, which accumulates in the upper soil profile when water with high salt content is used for irrigation. Sudden death syndrome is a soilborne disease caused by the fungus Fusarium solani f. sp. glycines and is often associated with well-irrigated fields and the soybean cyst nematode, Heterodera glycines. Even though these problems both affect irrigated soybeans, there are no reports on the effect of chloride toxicity on SDS. To determine if there is an effect of chloride toxicity on SDS, four cultivars were subjected to three levels of soil chloride (low Cl, moderate Cl, or high Cl) in a field at the University of Arkansas, Cotton Branch Station, Marianna, AR, in 1995 and 1996. The cultivars were either susceptible to SDS (S) or resistant to SDS (R) and either translocated chloride to the leaves (includer, I) or confined chloride in the roots (excluder, E). The cultivars tested were Hartz 6686 (SE), Terra Vig 6653 (SI), Hartz 6200 (RE), and Asgrow 6297 (RI). Soil chloride concentrations were adjusted by the addition of KCl at the four-leaf growth stage. Leaf chloride concentrations were influenced by both the chloride treatment and the cultivar: elevated concentrations occurred with the includer cultivars in the moderate and the high Cl treatments. Soil concentrations of chloride reflected the chloride treatments in 1995, but not in 1996. Soil populations of F. solani did not respond consistently to either chloride treatment or cultivar; however, H. glycines egg densities increased with increased soil chloride treatments in Hartz 6686 (SE) and Terra Vig 6653 (SI) at flowering, but not at harvest. Increased soil chloride treatments increased SDS in both years with Hartz 6686 (SE), but did not affect this disease in the other cultivars. Higher soil chloride treatments decreased yield in all cultivars except H6200 (RE) in 1996. Although Terra Vig 6653 (SI) did not develop severe levels of SDS foliar symptoms, it did have increased lodging and significant increases in southern blight with the moderate and high soil chloride treatments. These results indicate that growers with fields that have both elevated concentrations of soil chloride and SDS should select SDS-resistant excluder cultivars to minimize yield losses due to both problems.

High Frequency of Brown Stem Rot Resistance in Soybean Germ Plasm from Central and Southern China. M. S. Bachman, Graduate Research Fellow, and C. D. Nickell, Professor of Plant Breeding, Department of Crop Sciences, University of Illinois, Urbana 61801. Plant Dis. D-2000-0411-02R, 2000 (on-line). Accepted for publication 9 March 2000.

Brown stem rot is a fungal disease of soybean characterized by browning of the internal stem tissue and premature death of interveinal leaf tissue. The disease is prevalent in northern soybean production regions of the United States, where it causes significant yield losses. Genetic resistance to brown stem rot has been identified and utilized in the production of many soybean cultivars. Although current sources of resistance provide adequate protection against brown stem rot in most cases, there have been reports of this disease on resistant cultivars. Continuous use of current sources of brown stem rot resistance in cultivars may lead to selection for new strains of the causal fungus and eventual uselessness of these resistance sources. This study evaluated hundreds of soybean types from China in an attempt to identify sources of resistance to brown stem rot. Over two hundred soybean types were identified with resistance to three strains of the fungus responsible for brown stem rot. Further evaluation will determine if the source of resistance in the Chinese types is different from sources of resistance in U.S. cultivars. If unique sources are identified, these sources can be incorporated into cultivars for use by producers.

Cross-Compatibility and Distribution of Mating Type Alleles of the Rice Blast Fungus Magnaporthe grisea in India. B. V. Dayakar and N. N. Narayanan, Research Fellows, and S. S. Gnanamanickam, Professor, Center for Advanced Studies in Botany, University of Madras, Chennai 600025, India. Plant Dis. D-2000-0419-01R, 2000 (on-line). Accepted for publication 1 March 2000.

Blast disease caused by the fungus Magnaporthe grisea is a devastating rice disease. It is a constraint to rice production in most of the large rice-growing regions of India. The genetic diversity and pathotype organization of the pathogen populations are not completely characterized to assist in disease resistance breeding efforts. In previous studies, we have characterized the pathogen populations prevalent in southern states of India through DNA fingerprinting and pathotyping on near-isogenic rice lines, each of which carry a single gene for blast resistance. Other researchers have characterized the M. grisea populations in the Central Himalayan region of northern India and have suggested that the pathogen is sexually recombinant. In the present study, we have examined the mating type distributions and genetic diversities in pathogen populations assembled from important rice-growing regions in the states of Andaman Islands, Andhra Pradesh, Himachal Pradesh, Haryana, Karnataka, Meghalaya, and Punjab. Perhaps for the first time, in this study we have identified high levels of fertility (24 to 53%) in rice isolates of M. grisea in India. The mating type MAT1-1 was detected widely except in Meghalaya and Himachal Pradesh, where the pathogen populations had both MAT1-1 and MAT1-2 types; the frequency of MAT1-2 was very small. Twenty-three Magnaporthe grisea repeat (MGR)-restriction fragment length polymorphism lineages were identified from these pathogen populations. Meghalaya populations showed maximal lineage diversity with nine lineages, indicating that, in this region, the pathogen and the host have coevolved for a long time. On the basis of mating type distributions, lineage diversity, fertility, and pathogenicity of M. grisea presented in this study, we think sexual recombination is of minor occurrence in fields of large rice-growing regions in different states of India.

May, 2000

Occurrence, Distribution, and Relative Incidence of Five Viruses Infecting Cucurbits in the State of São Paulo, Brazil. V. A. Yuki, Centro de Fitossanidade, Instituto Agronômico, 13020-902 Campinas, SP, Brazil; J. A. M. Rezende and E. W. Kitajima, Dept. de Fitopatologia, ESALQ/USP, 13418-900 Piracicaba, SP, Brazil; P. A. V. Barroso and H. Kuniyuki, Centro de Fitossanidade, Instituto Agronômico, 13020-902 Campinas, SP; G. A. Groppo, DEXTRU/CATI, 13073-001 Campinas, SP, Brazil; and M. A. Pavan, Dept. de Defesa Fitossanitária, FCA/UNESP, 18603-970 Botucatu, SP, Brazil. Plant Dis. D-2000-0316-01R, 2000 (on-line). Accepted for publication 22 December 1999.

Cucurbit virus diseases are a worldwide problem, and in the state of São Paulo they represent one of the most limiting factors for growers. Despite the importance of cucurbit crops, only limited information is available about the incidence of viral diseases and their effect on the yield. A 2-year survey was carried out to estimate the incidence of Cucumber mosaic virus (CMV), Papaya ringspot virus–type W (PRSV-W), Watermelon mosaic virus-2 (WMV-2), Zucchini lethal chlorosis virus (ZLCV), and Zucchini yellow mosaic virus (ZYMV). PRSV-W and ZYMV were the most frequently found viruses, accounting for 49.1 and 24.8% of 605 samples tested, respectively. ZLCV, CMV, and WMV-2 were detected in 7.8, 6.0, and 4.5% of 612, 497, and 423 samples tested, respectively. Double infection was found in 97 samples, and triple infection in 10 samples. The survey also showed that the presence of cultivated and wild cucurbit species in São Paulo during the entire year, plus favorable weather for the occurrence of insect vectors, provide the combination that makes virus diseases one of the most important problems for growers in the state. The following strategies are recommended for control of virus diseases on cucurbits in São Paulo: (i) destruction of old crops before starting new plantings; (ii) control of wild species of Cucurbitaceae in the vicinity of cucurbit crops to reduce viruses sources; (iii) control of weed species that harbor insect vectors; and (iv) use of resistant or tolerant cultivars and/or mild strain protection whenever available. Mild strain protection has been successfully applied for control of PRSV-W on zucchini squash in São Paulo since 1997.

Influence of Gibberellic Acid on Carrot Growth and Severity of Alternaria Leaf Blight. P. Santos, Graduate Student, Department of Plant Pathology, University of California, Davis 95616; J. J. Nunez, University of California Cooperative Extension, Kern County 93307; and R. M. Davis, Cooperative Extension Specialist, Department of Plant Pathology, University of California, Davis 95616. Plant Dis. D-2000-0313-06R, 2000 (on-line). Accepted for publication 31 January 2000.

Alternaria leaf blight is one of the most common and damaging diseases of carrots. The disease is difficult to manage when the crop is exposed to prolonged periods of leaf wetness and warm temperatures. Fungicides are a primary control strategy, but as the crop matures and the leaf canopy becomes increasingly dense, good coverage is difficult to obtain. Applications of gibberellic acid (GA), a plant hormone, to carrot foliage consistently reduced the relative severity of Alternaria leaf blight. The reduction of visual symptoms of blight with two applications of GA was similar to the reduction achieved with regular applications of the fungicide iprodione. Applications of GA usually resulted in plants with longer leaves, wider petioles, and a more upright growth habit. At very high rates (250 mg/liter), growth of carrot foliage was increased at the expense of root yield. However, at lower rates (20 to 40 mg/liter), root yield and quality were not compromised, yet disease was reduced, possibly due to improve air movement through the canopy, which reduces leaf wetness, a necessary condition for disease development. The majority of carrots destined for the fresh market are harvested with self-propelled, multirow harvesters that undercut and lift the roots by their tops using a system of belts. Thus, an increase in shoot length and petiole diameter resulting from GA applications may provide the additional benefit of improving the harvestability of the crop.

Sensitive Method for Testing Peanut Seed Lots for Peanut stripe and Peanut mottle viruses by Immunocapture-Reverse Transcription-Polymerase Chain Reaction. A. G. Gillaspie, Jr., R. N. Pittman, and D. L. Pinnow, USDA-ARS, Plant Genetic Resources Conservation Unit, Griffin, GA 30223-1797; and B. G. Cassidy, Samuel Roberts Noble Foundation, Inc., Ardmore, OK 73402. Plant Dis. D-2000-0313-03R, 2000 (on-line). Accepted for publication 24 January 2000.

An improved method was developed for detection of two plant viruses in peanut seeds. This method, called immunocapture-reverse transcription-polymerase chain reaction (IC-RT-PCR), is much more sensitive than the currently used serological method for detecting Peanut stripe virus and Peanut mottle virus in seed. This new method allows large numbers of seeds to be processed more rapidly than with the currently used serological test and means that there will be less chance of viruses being brought into the United States or distributed to peanut-growing areas.

Mechanism of Suppression of Meloidogyne hapla and its Damage by a Green Manure of Sudan Grass. T. L. Widmer and G. S. Abawi, Department of Plant Pathology, New York State Agricultural Experiment Station, Geneva 14456. Plant Dis. D-2000-0321-01R, 2000 (on-line). Accepted for publication 10 January 2000.

Plant parasitic nematodes are a problem on many crops throughout the world. The northern root-knot nematode, Meloidogyne hapla, is a continuing problem on vegetables in the state of New York. Control of this nematode can be achieved through the use of chemicals; however, alternative control measures are being examined as a result of the increased environmental concerns and government regulations. A sustainable management option which has been proven to be effective is the use of cover crops and green manures. Specifically, Sudan grass has been demonstrated to suppress infection and damage to susceptible vegetables caused by this nematode when incorporated as a green manure. A number of Sudan-grass cultivars contain a cyanogenic compound (dhurrin) that is degraded to hydrogen cyanide and other secondary metabolites during tissue decomposition in soil. Nematode eggs go through several stages of development before hatching into juveniles which are motile in the soil and capable of penetrating into the roots. Exposing M. hapla eggs to a Sudan-grass extract resulted in a 55% reduction in the number of juveniles that penetrated lettuce roots. Juveniles were less sensitive to exposure of Sudan-grass extracts. Sudan-grass extract affected egg maturation by delaying development, but it did not affect hatching. Exposing eggs to a low concentration of CN(^–) solution (0.1 ppm) reduced the number of nematodes that penetrated the roots by 48%, while exposing juveniles to the same concentration reduced nematode infection by only 4%. When the Sudan-grass-extract compounds were separated, only the fractions collected that contained cyanide suppressed infection of lettuce roots by M. hapla juveniles when eggs were exposed to these fractions. These results suggest that CN(^–) is the primary factor involved in the suppression of M. hapla by a green manure of Sudan grass.

Rice blast, caused by the fungal pathogen Pyricularia grisea, is one of the most destructive diseases of rice worldwide and can cause significant reductions in yield. Rice blast management strategies in Arkansas incorporate and stress the use of disease resistance and cultural practices to minimize the use of fungicides. However, management strategies, particularly N fertilization, water management and the use of susceptible cultivars, have profound effects on rice blast development. Currently, multiple applications of specific amounts of nitrogen are recommended to meet the needs of specific cultivars; however, some growers may make a single application of the entire recommended amount early in the growing season in an effort to reduce production cost. Also, single applications of N fertilizer are being considered for some of the newer cultivars that do not produce higher yields in response to midseason applications. The objectives of our study were to: (i) quantify disease progress of rice blast on commercial rice cultivars differing in blast susceptibility, (ii) determine the effects of nitrogen rate and timing on rice blast development, and (iii) determine if rice cultivars responded differently to nitrogen treatments in regards to leaf blast development. Our results indicate that, in Arkansas, the disease developed, regardless of N treatments, in a unimodal fashion; that is, disease incidence and total lesion area per plant reached a maximum near midseason (when panicles were being formed) and then gradually declined. The decline in the disease was attributed to adult-plant resistance, leaf senescence and therefore the loss of some lesions, and to the development of new and healthy leaves during a part of the season unfavorable to the increase of the disease itself. Furthermore, the effects on incidence of disease and the total lesion area per plant were different for each cultivar and nitrogen treatment. For example, increasing the amount of nitrogen applied in one season and/or applying it all at one time had a greater effect on the incidence and severity of rice blast by increasing the amount of disease on the more susceptible cultivars than it did on cultivars more resistant to rice blast. Since our results indicate a differential cultivar response to nitrogen, selecting cultivars that are resistant or moderately resistant to rice blast may be more desirable (less risky) than selecting the higher yielding cultivars that become increasingly susceptible with increased N levels.

Incidence of Tomato spotted wilt virus (Bunyaviridae) and Tobacco Thrips in Virginia-Type Peanuts in North Carolina. L. E. Garcia, Former Graduate Student, and R. L. Brandenburg, Extension Entomologist, Department of Entomology, North Carolina State University, Raleigh 27695; and J. E. Bailey, Extension Plant Pathologist, North Carolina State University, Raleigh 27695. Plant Dis. D-2000-0216-01R, 2000 (on-line). Accepted for publication 27 December 1999.

Tomato spotted wilt virus (TSWV) has become a significant disease of peanuts in Texas and Georgia. This virus is spread by small insects called thrips. Tobacco thrips is the principal vector in North Carolina; however, two other species, the onion thrips and the western flower thrips, also vector this virus and can be found occasionally in North Carolina peanut fields. Disease management strategies such as cultivar selection, field history, planting date, and cultural practices have been successful in reducing the incidence of this disease in Texas and Georgia. This study was undertaken to identify specific management practices to minimize future TSWV impact in North Carolina. Each of three Virginia-type peanut cultivars, NC-9, NC-V11, and NC-12C, was sown in a different location in North Carolina in 1996. The plants were rated weekly for TSWV symptoms, and leaf samples were collected weekly to determine thrips populations. The greatest single factor influencing TSWV incidence was field location. Test plots where peanuts are grown every year had a TSWV incidence of 11%; whereas peanuts at two areas away from densely aggregated peanut fields had TSWV incidence of 3.1 and 3.6%. Of the three tested cultivars, disease incidence was highest in NC-9, but only during the middle of the season. Weekly thrips counts were highest in NC-V11, followed by NC-9 and NC-12C. These results indicate field location and cultivar selection are factors that can affect incidence of TSWV among Virginia-type peanut cultivars.

Detection of Acidovorax avenae subsp. citrulli in Watermelon Seeds Using Immunomagnetic Separation and the Polymerase Chain Reaction. R. R. Walcott and R. D. Gitaitis, University of Georgia, Coastal Plain Experiment Station, Tifton 31793. Plant Dis. D-2000-0218-01R, 2000 (on-line). Accepted for publication 5 January 2000.

Acidovorax avenae subsp. citrulli is the bacterial pathogen responsible for watermelon fruit blotch disease. This pathogen is naturally seedborne and seed transmitted, and seed plays an important role as a primary source of inoculum. No effective chemical controls are available for this disease and the most effective strategy has been to test seedlots for the pathogen. The seed-detection assays currently available for this pathogen include seedling grow-out, enzyme-linked immunosorbent assay (ELISA), semi-selective media, and polymerase chain reaction (PCR); however, the detection efficiency and sensitivity of these assays can be significantly improved by immunomagnetic separation (IMS) and PCR. This technique combines the specificity of the antibody-antigen interactions with the PCR and results in a highly sensitive, specific, and efficient technique for detecting A. avenae subsp. citrulli in watermelon seed. IMS increases the sensitivity of PCR by concentrating target bacteria, while simultaneously eliminating non-target cells and seed compounds that inhibit PCR. It eliminates the need for time-consuming cetyldimethylethylammonium bromide-DNA extraction, which involves the use of potentially harmful chemicals (phenol and chloroform). IMS recovered 10-fold more A. avenae subsp. citrulli cells than direct spread plating and it increased the sensitivity of PCR 100-fold. Finally, IMS-PCR allowed larger volumes of samples to be tested than regular PCR and ELISA and, as such, overcame another major limitation of other seed-detection assays. IMS-PCR significantly improved the efficiency of PCR detection of seedborne A. avenae subsp. citrulli and may be applicable for detecting all seedborne bacteria.

Molecular and Biological Characterization of a Trackable Illinois Isolate of Barley yellow dwarf virus-PAV. J. S. Moon and R. G. Allen, Department of Crop Sciences, University of Illinois, Urbana 61801; L. L. Domier, Department of Crop Sciences, University of Illinois, and United States Department of Agriculture-Agricultural Research Service (USDA-ARS) Crop Protection Research Unit, Urbana 61801; and A. D. Hewings, USDA-ARS Crop Protection Research Unit. Plant Dis. D-2000-0218-03R, 2000 (on-line). Accepted for publication 21 December 1999.

Barley yellow dwarf viruses (BYDVs) cause the most damaging virus disease of cereal crops. These viruses are transmitted from infected to healthy plants through the feeding of aphids. The dependence of the BYDVs on aphids for their spread makes them vulnerable to control by treatments that interfere with aphid feeding or migration. However, little is known about the specifics of how aphids move viruses within or among fields of susceptible plants. This is partially due to the fact that aphids carrying BYDVs are very common in the small-grain-growing regions of the United States, which makes tracking the spread of a single BYDV infection nearly impossible. To overcome this problem, we have identified a relatively rare isolate of BYDV that can be identified by molecular technologies. This rare virus can be differentiated from the more common isolates by a laboratory assay. The use of this traceable BYDV isolate will allow us to establish infections at known locations and monitor the spread of the virus from that site over time. This information will be useful to researchers who are interested in developing virus control strategies by studying the dynamics of the spread of BYDVs in areas of high natural infection.

White mold, caused by the soil fungus Sclerotinia sclerotiorum, is an important disease of cabbage in eastern North Carolina and throughout the world. The fungus causes a watery rot of mature cabbage heads in the field and during postharvest handling and storage. Because white mold can cause serious economic losses to cabbage growers, a better understanding of the biology of the fungus may provide important information for developing improved approaches for managing this disease. White mold is caused by airborne sexual spores, called ascospores, that infect cabbage during periods of cool, wet weather. The ascospores do not usually infect healthy cabbage leaves unless they have been wounded. Wounds provide a nutrient source for growth of the fungus and can be caused by bruising (sustained during cultivation or harvesting), cutting (caused by insect feeding), agricultural chemicals (associated with fertilizer and/or pesticide application), and adverse weather (freezing and/or frost damage). In this study, experiments were conducted in commercial cabbage fields to monitor the occurrence of ascospores and subsequent development of white mold. This information was used to evaluate the role of wounding in the infection of cabbage by ascospores of S. sclerotiorum in controlled environmental chambers. Results suggest that freezing and bruising injuries are important factors associated with the infection of cabbage by S. sclerotiorum. This is the first time freezing injury has been experimentally demonstrated to be an important factor in the development of white mold disease of cabbage.

Evaluation of Graft-Transmissible Isolates From Dwarfed Citrus Trees as Dwarfing Agents. S. P. van Vuuren, Agricultural Research Council-Institute for Tropical and Subtropical Crops, Private Bag X11208, Nelspruit 1200, South Africa, and J. V. da Graça, Department of Microbiology and Plant Pathology, University of Natal, Private Bag X01, Scottsville, Pietermaritzburg 3209, South Africa. Plant Dis. D-2000-0118-02R, 2000 (on-line). Accepted for publication 10 November 1999.

High-density planting of citrus has several advantages but it is important to control tree size. Dwarfing characteristics of isolates from dwarfed citrus trees were evaluated. Healthy trees were bud inoculated prior to planting in the field and, 5 years later, some isolates reduced canopy volumes by 60% without any detrimental effects. Fruit yield was calculated according to tree size and the yield efficiency of the dwarfed trees was equal to uninoculated trees. Citrus tristeza virus was the only pathogen detected in all isolates and, in countries where the virus is endemic, it can be utilized to the benefit of the industry. One isolate also gave protection against a citrus bacterial disease.

Effect of Botanical Extracts on the Population Density of Fusarium oxysporum in Soil and Control of Fusarium Wilt in the Greenhouse. J. H. Bowers and J. C. Locke, USDA, ARS, U.S. National Arboretum, Floral & Nursery Plants Research Unit, Rm. 238, B-010A, BARC-W, Beltsville, MD 20705. Plant Dis. D-2000-0114-01R, 2000 (on-line). Accepted for publication 22 November 1999.

Fusarium wilts, caused by the fungal pathogen Fusarium oxysporum, are some of the most widespread and destructive diseases of many major ornamental and horticultural crops. This soilborne pathogen causes vascular wilts by infecting plants through the roots and growing internally through the vascular tissues. The entire plant usually wilts and dies as the pathogen moves into the stem. Presently, preplant soil fumigation and fungicide applications are used to control wilt diseases. However, methyl bromide, the major fumigant used, is scheduled to be phased out because it was defined by the Montreal Protocol of 1991 as a chemical that contributes to the depletion of the ozone layer. Due to the environmental and safety concerns associated with pesticides, this study investigated the effect of several formulated plant extracts and essential oils on soil populations of Fusarium oxysporum and disease control in the greenhouse as an alternative component in integrated control strategies. Treatment of the soil with 10% aqueous emulsions of the formulated extracts of a chili pepper extract and essential oil of mustard mixture, a cassia tree extract, and clove oil reduced populations of Fusarium 99.9, 96.1, and 97.5%, respectively, 3 days after soil treatment. These same formulations also suppressed disease development in the greenhouse and resulted in 80 to 100% plant stand after 6 weeks. The observed reductions in the pathogen population in soil and the increase in plant stand in the greenhouse indicates that these natural plant products may have important roles in biologically based management strategies for control of Fusarium wilt diseases.

Identification and Etiology of Visible Quiescent Infections of Monilinia fructicola and Botrytis cinerea in Sweet Cherry. J. E. Adaskaveg, Assistant Professor, H. Förster, Staff Research Associate, and D. F. Thompson, Post-Graduate Researcher, Department of Plant Pathology, University of California, Riverside 92521. Plant Dis. D-2000-0120-01R, 2000 (on-line). Accepted for publication 29 November 1999.

Brown rot and gray mold caused by the fungi Monilinia fructicola and Botrytis cinerea, respectively, can result in extensive losses in production of sweet cherry and other stone-fruit crops worldwide. Damage from these fungi may result in blossom blight, green fruit rot, preharvest fruit rot, and postharvest fruit decay. The often rapid increase in brown-rot incidence of mature fruit, regardless of protectant fungicide treatments or surface disinfestation, have suggested the existence of symptomless internal infections in stone-fruit crops. Furthermore, on sweet cherry, the common occurrence of necrotic flecks on green fruit or small reddish lesions on fruit of yellow-red cultivars shortly after rains have also suggested visible quiescent (dormant) fruit infections. Both visible and non-visible infections that are established when environmental conditions or host physiology are conducive for fungal penetration, but not for active growth of the pathogen, have been defined as quiescent infections. Isolation frequencies of fungi from necrotic flecks on green cv. Bing fruit or from reddish halos on immature yellow-pink cv. Rainier fruit over a four-year period indicated that M. fructicola was more common in two of the four years and was equal to B. cinerea in the other years. Using immature Bing cherry fruit, inoculation studies in the laboratory indicated that significantly more visible quiescent infections than active decays were produced in 6-, 9-, or 12-h than 18- or 24-h wetness periods after inoculation. The existence of non-visible quiescent infections of M. fructicola and B. cinerea in immature Bing and Rainier fruit collected 2 weeks before harvest was also substantiated using the postharvest paraquat technique. Using this method, surface sterilization with sodium hypochlorite, followed by treatment with the herbicide paraquat, disinfests fruit surfaces and kills fruit tissue, respectively. This procedure allows fungi within the fruit to grow on the dead host tissue and allows for their detection. In summary, the etiological agents of visible and non-visible quiescent infections in cherry fruit collected from commercial orchards in California were determined to be M. fructicola and B. cinerea. Furthermore, visible quiescent infections of M. fructicola were produced in the laboratory under defined, semi-conducive environments on immature fruit. This research further elucidates the disease cycles and epidemiology of brown rot and gray mold on sweet cherry fruit and provides insight and explanations to the difficulty in managing these diseases.

February, 2000

Detection and Partial Characterization of Tenuiviruses from Black Spruce. J. D. Castello, Professor, S. O. Rogers, Associate Professor, G. D. Bachand and R. C. Fillhart, former Graduate Research Assistants, J. S. Murray, K. Weidemann, M. Bachand, and M. A. Almond, former Undergraduate Research Assistants, State University of New York, College of Environmental Science and Forestry, 1 Forestry Drive, Syracuse 13210-2788. Plant Dis. D-1999-1130-01R, 2000 (on-line). Accepted for publication 5 October 1999.

Two viruses were detected in black spruce trees in New York State. This report is important for several reasons. First, this is one of only a few reports of virus infection of a coniferous forest tree species. The impact of virus infection on the growth and development of coniferous trees is unknown and needs to be investigated. Second, the viruses detected are of the genus Tenuivirus, previously detected only in grass hosts in the tropics. Thus, this is the first report of a tenuivirus in a nongrass host. One of the viruses detected in spruce is similar in the sequence of one of its genes to Maize stripe tenuivirus (MStpV), which is a very economically important virus of corn in some tropical regions of the world. These results raise some interesting questions: Is one of the viruses detected in spruce an isolate of MStpV? If so, does it cause disease of corn in New York? If not, why not? Or are the viruses from spruce new, previously undescribed tenuiviruses? Are conifers potential reservoirs of agriculturally important tenuiviruses? How do these viruses spread? Do these viruses infect other conifer species? What is the impact of virus infection of spruce and other conifers?

Phytophthora Brown Rot of Citrus: Temperature and Moisture Effects on Infection, Sporangium Production, and Dispersal. L. W. Timmer, Professor, and S. E. Zitko, Senior Biologist, University of Florida, Citrus Research and Education Center, Lake Alfred 33850; T. R. Gottwald, Research Plant Pathologist, USDA, ARS, Orlando, FL 32803; and J. H. Graham, Professor, University of Florida, Citrus Research and Education Center, Lake Alfred 33850. Plant Dis. D-1999-1122-01R, 2000 (on-line). Accepted for publication 20 October 1999.

Phytophthora brown rot of citrus fruit in orchards in Florida is caused by Phytophthora palmivora and P. nicotianae. We determined the most favorable temperature and moisture conditions for infection and sporulation of these organisms. Only 3 h of fruit wetness was needed for maximum infection at favorable temperatures with P. palmivora. The optimum temperature for infection and disease development was 27 to 30°C, with no brown rot development below 22°C. In contrast, sporulation of P. palmivora in the laboratory and on the fruit surface was best at 24°C. Sporulation of P. palmivora on the fruit surface was profuse, and single water droplets could disperse propagules of the organism from 350 to 450 mm laterally and up to 30 to 45 mm vertically. P. nicotianae sporulated poorly and was splash-dispersed only short distances. Neither species was dispersed by air currents alone. Brown rot in Florida, caused by P. palmivora, is most likely to occur in late summer and early fall, when temperatures are high and frequent storms readily disperse sporangia. Storage of harvested fruit at cool temperatures, less than 20°C, will stop disease development.

Cytology of Fibrous Roots From Citrus Blight–Affected Trees. A. G. C. Lindbeck, Assistant Professor, Department of Biology, University of Central Florida, Orlando 32816; and R. H. Brlansky, Professor, University of Florida, IFAS, Citrus Research and Education Center, 700 Experiment Station Road, Lake Alfred 33850. Plant Dis. D-1999-1215-01R, 2000 (on-line). Accepted for publication 5 November 1999.

Blight is an economically important disease of citrus. Tree losses in Florida are estimated at $67 million annually. The disease is found in many citrus growing areas of the world, including Florida, Brazil, Argentina, Australia, South Africa, Uruguay, and Cuba. The cause of this important disease is unknown, but it has been transmitted by grafting roots of affected trees to healthy trees. Symptoms of the disease include a permanent wilt of the canopy and eventual irreversible decline. The wilt is caused by plugging of the xylem vessels of the trunk, major scaffold limbs, and major roots by an amorphous material. We found that amorphous plugging also occurs in the fibrous roots of affected trees. The plugging is present in the first 1 to 2 cm of the root, which represents the zone of differentiation and elongation and is the major site of water uptake. Therefore, the initial area of reduced water conductivity may be in the fibrous roots. Identification of the causal agent is important in controlling this disease.

Occurrence of a Strain of Texas pepper virus in Tabasco and Habanero Pepper in Costa Rica. Pongtharin Lotrakul, Rodrigo A. Valverde, Rodolfo De La Torre, and Jeonggu Sim, Department of Plant Pathology and Crop Physiology, Louisiana Agricultural Experiment Station, Louisiana State University Agricultural Center, Baton Rouge 70803; and Alvaro Gomez, Inversiones Agroindustriales PEMACA S.A., Apartado 1161-7050, Cartago, Costa Rica. Plant Dis. D-1999-1217-01R, 2000 (on-line). Accepted for publication 9 November 1999.

A viral disease causing severe leaf malformation and yellow mottle on Tabasco and Habanero peppers in Costa Rica was determined to be caused by a whitefly-transmitted geminivirus. Based on the biological and molecular properties of this virus, we concluded that it is a distinct strain of Texas pepper virus (TPV). This virus was found also in Honduras; therefore, the presence of TPV is a new threat for pepper farmers of Central America. Incidence as high as 75% was found in some farms.

Survival and Growth of Listeria monocytogenes on Fresh-cut Apple Slices and its Interaction with Glomerella cingulata and Penicillium expansum. W. S. Conway, B. Leverentz, and R. A. Saftner, Horticultural Crops Quality Laboratory, Beltsville Agricultural Research Center, United States Department of Agriculture-Agricultural Research Service (USDA-ARS), Beltsville, MD 20705; W. J. Janisiewicz, Appalachian Fruit Research Station, USDA-ARS, 45 Wiltshire Road, Kearneysville, WV 25430; C. E. Sams, Department of Plant and Soil Sciences, The University of Tennessee, Knoxville, TN 37901; and E. Leblanc, Biometrical Consulting Service, USDA-ARS-BA-OD, Beltsville, MD 20705. Plant Dis. D-1999-1215-02R, 2000 (on-line). Accepted for publication 9 November 1999.

Fresh-cut produce is a newly emerging but rapidly developing industry which offers the consumer both convenient and nutritious food. Along with the development of this industry, new problems may arise in the area of food safety. There is very little knowledge concerning the microbial contamination of fresh-cut fruits and vegetables. This is especially true in the fresh-cut fruit industry, which has not developed as quickly as that for fresh-cut vegetables. Therefore, it is necessary to obtain information on the fate and control of food-borne pathogens on fresh-cut fruit. Listeria monocytogenes is a bacterium that has been associated with a number of outbreaks of the food-borne illness listeriosis in recent years. Our research has shown that this bacterium can survive and increase in population on apple slices when fruit are stored at 20 or 10°C but cannot multiply when grown at 5°C. It also does not grow under very acidic conditions. Populations of the bacterium inoculated into decayed apple tissue increased on apple fruit decayed by Glomerella cingulata. This may be due to the ability of the fungus to decrease the acidity of the apple tissue. Therefore, proper storage temperature, as well as the absence of postharvest pathogens such as G. cingulata, is important for maintaining the safety of fresh-cut apples. This information will be useful to the fresh-cut industry in reducing potential health hazards.

Effect of Fenarimol Application Timing on Fruit Shape of Bartlett Pear. D. Sugar, Oregon State University, Southern Oregon Research and Extension Center, Medford 97502; R. A. Spotts, Oregon State University, Mid-Columbia Agricultural Research and Extension Center, Hood River 97031; W. D. Gubler, University of California, Department of Plant Pathology, Davis 95616; and G. T. McGourty, University of California, Mendocino County Cooperative Extension Service, Ukiah 95482. Plant Dis. D-1999-1217-02R, 2000 (on-line). Accepted for publication 10 November 1999.

The shape of pear fruit, like other characteristics, is an important component of the economic value of the fruit. Certain fungicides among those known as ergosterol biosynthesis inhibitors have been shown previously to affect fruit set, yield, and subsequent-year flowering in tree fruit crops when applied in the spring for control of fungal diseases of fruit and foliage. Following the observation of relatively flattened cv. Bartlett pear fruit in some commercial orchards in 1992, plots were established in four locations in California and Oregon to investigate the association of the fungicide fenarimol (Rubigan 1 EC) with Bartlett pear fruit shape. Various spray application schedules were evaluated, with the earliest application at bud burst and the latest at full bloom. No effect on fruit shape was observed with applications at bud burst, but all other timings and combinations of timings resulted in relatively flattened fruit in at least one test location. The strongest effect was associated with fenarimol applications at both white bud stage and full bloom. No effects on fruit weight were found, but fruit stem length was reduced by most application timings at the two locations where stems were measured. The results support the change in the Rubigan 1 EC label beginning in 1993, which limits applications in pear to the petal-fall stage or later.

January, 2000

Chemiluminescent and Colorimetric Detection of Erwinia amylovora by Immunoenzymatic Determination of PCR Amplicons from Plasmid pEA29. M. Merighi, Department of Plant Pathology, The Ohio State University, Columbus 43210; A. Sandrini, S. Landini, S. Ghini, and S. Girotti, UCI/SCRM Institute of Chemical Sciences, University of Bologna, Italy; and S. Malaguti and C. Bazzi, UCI/STAA Institute of Plant Pathology, University of Bologna, Italy. Plant Dis. D-1999-1108-02R, 2000 (on-line). Accepted for publication 16 September 1999.

Erwinia amylovora is the causal agent of fire blight, a bacterial disease of apple, pear, and other ornamentals. The detection of this pathogen presents special problems (e.g., presence of an epiphytic phase on dormant organs, low inoculum concentrations, and culture characteristics similar to Pseudomonas syringae) that may be critical during the screening of nursery stocks and plant propagation materials. Moreover, the aggressiveness and wide host range of E. amylovora require a fast and reliable method for prompt diagnosis and sensitive detection in multiple samples. In this study, we present a novel analytical method (PCR enzyme-linked immunosorbent assay [ELISA]) applied to the fast, specific, sensitive, and automated detection of E. amylovora DNA sequences in multiple samples. The use of previously published PCR primers and of a new internal biotinylated oligonucleotide probe allowed us to amplify and trap by liquid hybridization fragments of E. amylovora DNA in microtiter wells. After immunological detection of the molecular hybrids, PCR-ELISA showed high specificity and sensitivity (at least 10 times better than standard PCR and gel electrophoresis analysis when carried out using a chemiluminescent protocol). The assay could be performed in 6 h and as many as 96 samples could be analyzed at a time. Epidemiologists and state extension services committed to plant certification or diagnostics could greatly benefit from the proposed automated detection protocol.