Interpretive Summaries

Assessment of a Regional Site-Specific Sorghum Ergot Severity Potential Using Radar-Rainfall Measurement. F. Workneh, Texas Agricultural Experiment Station, Bushland 79012; B. Narasimhan and R. Srinivasan, Department of Forest Science, Spatial Statistics Laboratory, Texas A & M University, College Station 77843; and C. M. Rush, Texas Agricultural Experiment Station, Bushland 79012. Plant Dis. DOI: 10.1094/PD-90-0704. Accepted for publication 19 January 2006.

Sorghum ergot, caused by Claviceps africana, generally requires cool weather and humid conditions for optimum infection. Rainfall reportedly is not required for infection as long as relative humidity is high. However, occurrence of high humidity in the Texas Panhandle during the summer is usually associated with rain showers, and ergot incidence in the region has been observed to be associated with rain events. These events are often irregular and can vary within a small area both in incidence and intensity. Existing ground weather stations are too far apart to provide accurate representation of localized rainfall events. Radar-based precipitation measurements have a resolution of 4 × 4 km out to 230 km from the radar location. In the present study, radar rainfall measurements were used to assess regional site-specific sorghum ergot potential in the Texas Panhandle. The results have a potential for development of a web-based ergot risk assessment system in which growers can enter the GPS locations of their fields and determine whether management actions are necessary.

Evaluation of Virulence of Phakopsora pachyrhizi and P. meibomiae Isolates. M. R. Bonde, S. E. Nester, C. N. Austin, C. L. Stone, and R. D. Frederick, USDA-ARS, Foreign Disease-Weed Science Research Unit, Fort Detrick, MD 21702-5023; G. L. Hartman, USDA-ARS and Dept. of Crop Sciences, University of Illinois, Urbana 61801; and M. R. Miles, USDA-ARS, University of Illinois, Urbana 61801. Plant Dis. DOI: 10.1094/PD-90-0708. Accepted for publication 11 January 2006.

Soybean rust, discovered for the first time in the United States in nine southeastern states in 2004, has been of concern to the U.S. Department of Agriculture (USDA) for more than 30 years. Breeding for resistance to this fungus-caused disease has been hampered by a lack of resistance to soybean rust and inadequate methods to detect and quantitate resistance. In the USDA-ARS plant pathogen containment facility at the Foreign Disease–Weed Science Research Unit, Fort Detrick, MD, soybean accessions, some possessing previously described single genes for resistance to rust, were inoculated separately with several isolates of the pathogen collected from different parts of the world over three decades. The soybean accessions then were evaluated for disease symptoms. Leaf pieces from inoculated plants were stained so that propagule-producing structures, called uredinia, developing in the leaves could be observed. Some combinations of soybean accessions and rust isolates yielded a resistant response, while others produced a susceptible response. The numbers and sizes of the uredinia were measured, and correlations were observed with few, small uredinia and resistance. This method should prove useful in detecting different types of resistance to soybean rust, including durable types usually requiring quantitative measurements.

Weather Factors Associated with Development of Sorghum Ergot in the Texas Panhandle. F. Workneh and C. M. Rush, Texas Agricultural Experiment Station, Bushland 79012. Plant Dis. DOI: 10.1094/PD-90-0717. Accepted for publication 20 January 2006.

The United States produces approximately 25% of the world’s grain sorghum, and Texas is the second leading state in production following Kansas. However, about 95% of the U.S. sorghum seed supply is produced in the Texas Panhandle. Sorghum ergot, caused by the fungus Claviceps africana, was detected in the United States for the first time in 1997. Since its introduction, the disease has periodically caused widespread damage in seed production fields in the region. Inoculation experiments were conducted in which the pathogen was introduced to flowers of male-sterile sorghum at various weather conditions to characterize weather factors associated with ergot development. The results showed that temperature and relative humidity were the predominant factors that affect ergot development. Two models, containing temperature and relative humidity, then were developed and evaluated using independent data sets. The models have a potential to forecast the degree of sorghum ergot severity 18 or 24 hours after the onset of favorable weather conditions. However, further field validation is required before they are used for spray advisory programs.

Widespread Occurrence of Wheat spindle streak mosaic virus in Belgium. C. Vaïanopoulos, A. Legrève, C. Lorca, and V. Moreau, Unité de Phytopathologie, Université catholique de Louvain (UCL), Louvain-la-Neuve 1348, Belgium; S. Steyer, Département Lutte biologique et Ressources phytogénétiques, Centre wallon de recherches agronomiques, CRA-W, Gembloux 5030, Belgium; and H. Maraite and C. Bragard, Unité de Phytopathologie, UCL, Belgium. Plant Dis. DOI: 10.1094/PD-90-0723. Accepted for publication 2 January 2006.

Wheat spindle streak mosaic virus (WSSMV) is a soilborne virus causing mosaic symptoms on wheat characterized by chlorotic spindle-shaped streaks on leaves. This virus is transmitted by soilborne vector Polymyxa graminis Ledingham, ensuring its persistence in the field over long periods. WSSMV occurs worldwide in wheat-growing areas and causes yield losses that often are underestimated. The presence of the disease as well as the resistance of wheat to the virus currently is evaluated by enzyme-linked immunosorbent assay, an antibody-based technology. In this study, molecular tools have been developed for the detection and quantification of WSSMV. Using these methods, the presence of the virus was evidenced in wheat plants showing strong yellow mosaic symptoms collected in Belgian fields. WSSMV incidence in Belgium also was evaluated in bait plants (wheat cvs. Cezanne and Savannah and rye cv. Halo) grown in 104 Belgian soil samples, and WSSMV was detected in plants grown in 32% of these soils. Thus, the presence of WSSMV was reported in regions where it has not hitherto been reported, indicating the presence of a new potential threat for Belgian and other European cereal growers. The tools proposed here also can be used for determining the impact of WSSMV on yield as well as for evaluating wheat cultivar resistance to the virus, with a view of developing an integrated management approach for the control of this noxious and persistent disease.

An Improved Method for Infecting Tomato Leaves or Seedlings with Oospores of Phytophthora infestans Used to Investigate F1 Progeny. Evgenia Rubin and Yigal Cohen, Faculty of Life Sciences, Bar-Ilan University, Ramat-Gan, Israel. Plant Dis. DOI: 10.1094/PD-90-0741. Accepted for publication 3 January 2006.

Phytophthora infestans is an algal-like fungus attacking potato and tomato crops and causing late blight, with an annual damage of billions of dollars worldwide. During the epidemic phase, this pathogen is reproduced by asexual spores (sporangia). Toward the end of the season, it might generate sexual spores (oospores) that can withstand harsh environmental conditions in the soil and initiate the disease in the next season. Oospores are produced by mating two kinds of isolates, A1 and A2. The oospores, therefore, are recombinants of their two parents. Genetic recombination may result in the appearance of new offspring progeny with altered pathogenic traits that might make the epidemics more severe and control of the disease more difficult. Most studies with oospores of P. infestans were done in vitro. A method was developed to assess the infectivity of oospores produces in vivo to tomato leaves and seedlings. Oospores were produced in tomato leaves by inoculation with sporangia of two parents: A1, metalaxyl sensitive, and A2, metalaxyl resistant (metalaxyl is a major fungicide used to control late blight). Oospores were produced in the leaves. The leaves containing the oospores were crushed in water and exposed to two cycles of drying and wetting in order to kill the sporangia and mycelia but let the oospores survive. The oospores then were incorporated into wet perlite and healthy leaves or seed of tomato were placed on top as bait. Oosporic infections were observed in about 12% of the leaves and 1% of the seedlings developed. The pathogen was isolated from the infections and the genetic traits of the offspring progeny were determined. The progeny isolates were moderately resistant to metalaxyl and segregated in mating type, indicating their meiotic origin. Also, isolates with a new, unusual mating type A1A2 appeared. Other analyses (not published) indicated that some of the offspring isolates were more aggressive to potato and tomato compared with their parents. The results showed that oospores are infective to tomato and support the hypothesis that sexual recombinants of P. infestans produce offspring progeny different from their parents, with some more difficult to control.

Mutation at beta-Tubulin Codon 200 Indicated Thiabendazole Resistance in Penicillium digitatum Collected from California Citrus Packinghouses. Leigh S. Schmidt, Jennifer M. Ghosoph, Dennis A. Margosan, and Joseph L. Smilanick, USDA-ARS, San Joaquin Valley Agricultural Sciences Center, 9611 South Riverbend Avenue, Parlier CA 93648. Plant Dis. DOI: 10.1094/PD-90-0765. Accepted for publication 19 January 2006.

Harvested citrus fruit often rot after harvest due to infection by the fungus Penicillium digitatum, the citrus green mold pathogen. In most years, growers lose 2 to 4% of their fruit, and more are lost during marketing and after sale of the fruit to consumers. Among other means, the fungicide thiabendazole (TBZ) has been commonly used in citrus packinghouses, but not in groves, to control citrus green mold since about 1970. It interferes with tubulin polymerization and inhibits the growth of the fungus. The effectiveness of thiabendazole has declined because of the development of isolates of the fungus resistant to this fungicide. We assembled a large collection of isolates of the green mold pathogen from many geographically diverse locations in California. Those from commercial groves and residential trees were all sensitive to TBZ, while about 30% of those collected within packinghouses were resistant to it. Random amplified polymorphic DNA analysis indicated that the isolates were genetically distinct and differed from each other. All TBZ-resistant isolates had a point mutation in the beta-tubulin gene sequence corresponding to amino acid codon position 200. Thymine was replaced by adenine (TTC--TAC), which changed the phenylalanine (F) to tyrosine (Y). In contrast, for 49 TBZ-sensitive isolates that were sequenced, no mutations at this or any other codon positions were found. All of the isolates of P. digitatum resistant to TBZ appeared to have the same point mutation conferring thiabendazole resistance. This information can be used to make molecular probes to rapidly identify resistant isolates of the fungus, so other means to control them can be employed to manage decay losses.

Seed Transmission of Cephalosporium gramineum in Winter Wheat. Timothy D. Murray, Professor, Department of Plant Pathology, Washington State University, Pullman, WA 99164-6430. Plant Dis. DOI: 10.1094/PD-90-0803. Accepted for publication 2 February 2006.

Cephalosporium stripe is a vascular wilt disease of winter wheat and other cereal crops as well as several grasses. In fields where the disease is established, the pathogen survives a few years between susceptible crops in colonized plant residue in soil and produces spores that are washed into soil and infect plant roots. Because this pathogen was isolated from seed when the disease was first described, it was assumed that the pathogen was also seed transmitted (i.e., able to survive in seed and infect the plant that developed), but conclusive experimental evidence for this was never published. Seed transmission could provide a source of inoculum for the establishment of the pathogen in areas where it doesn’t occur and possibly for the initiation of epidemics. Consequently, this research tried to answer the question “Can this pathogen be transmitted from infected seed to plants grown from that seed, and if so, what might be the potential impact?” Seed was collected from plants with Cephalosporium stripe from three different field plots, two in Washington State and one in Idaho, and evaluated for the amount of seed infection and whether the pathogen was transmitted to plants grown from the seed. Seed lots known to have infected seed were planted in pots with a potting mix in which the pathogen did not occur, and a small percentage of the plants that grew developed disease, thereby demonstrating conclusively that the pathogen can be seed transmitted. Although the rate of transmission was too low to initiate an epidemic in the first year of introduction, it was high enough to establish the pathogen in fields where it was not present and produce enough infected plants to initiate an epidemic in subsequent crops.

Vector Specificity, Host Range, and Genetic Diversity of Tomato chlorosis virus. William M. Wintermantel, United States Department of Agriculture–Agricultural Research Service, Salinas, CA; and Gail C. Wisler, Department of Plant Pathology, University of Florida, Gainesville. Plant Dis. DOI: 10.1094/PD-90-0814. Accepted for publication 10 February 2006.

Tomato chlorosis virus (ToCV), family Closteroviridae, genus Crinivirus, causes interveinal chlorosis, leaf brittleness, and limited necrotic flecking or leaf bronzing on tomato leaves. ToCV can cause a decline in plant vigor and reduce fruit yield. It is emerging as a serious production problem for field and greenhouse tomato growers, and has been increasing in prevalence in many parts of the world. The virus is unique among known whitefly-transmitted viruses, due to its ability to be transmitted by four whitefly vectors from two genera. Studies demonstrated that transmission efficiency and virus persistence in the vector varies significantly among the different whitefly vectors. Trialeurodes abutilonea and Bemisia tabaci biotype B are highly efficient vectors of ToCV. B. tabaci biotype A and T. vaporariorum are less efficient vectors, but are fully capable of transmission. ToCV persists for up to 5 days in T. abutilonea, 2 days in B. tabaci biotype B, and only 1 day in B. tabaci biotype A and T. vaporariorum. ToCV has a moderately wide host range, infecting 24 host plant species in seven families. A portion of the coat protein coding region of five geographically diverse ToCV isolates was compared and found to be highly conserved. This information, coupled with existing information on conservation within the heat shock protein 70 homologue coding region, suggests that many ToCV isolates throughout the world are related very closely, and may have been distributed on plant material.