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