Interpretive Summaries

Field Isolates of Streptomyces Differ in Pathogenicity and Virulence on Radish. Leslie A. Wanner, United States Department of Agriculture–Agricultural Research Service, Vegetable Laboratory, Beltsville, MD 20705. Plant Dis. D-2004-0601-03R, 2004 (online). Accepted for publication 22 March 2004.

Potato growers observe differences in the incidence and severity of common scab in different potato cultivars, different fields, and different years. To understand this variability in scab incidence and severity, pathogenic organisms causing the disease were isolated from scabby potato tubers and characterized. Molecular features of the disease-causing bacteria were tested, and some lacked characteristic features usually associated with disease. These organisms produced different numbers and types of scabs on radish, and some damaged or killed radish seedlings or older radish plants. Severity of disease increased as the amount of the pathogenic organisms in the soil increased. These results provide a basis for developing methods for predicting the likelihood of common scab that could be useful for growers of potato and other tuber and root crops.

Survival of Teliospores of Tilletia indica in Arizona Field Soils. M. R. Bonde and S. E. Nester, USDA-ARS, Foreign Disease-Weed Science Research Unit, Fort Detrick, MD 21702-5023; M. W. Olsen, Department of Plant Pathology, University of Arizona, P.O. Box 210036, Tucson 85721-0036; and D. K. Berner, USDA-ARS, Foreign Disease-Weed Science Research Unit, Fort Detrick, MD 21702-5023. Plant Dis. D-2004-0520-01R, 2004 (online). Accepted for publication 24 March 2004.

Karnal bunt of wheat is a minor disease, caused by a fungus, that can result in small reductions in grain quality and yield. However, because it is present only in a few countries, this disease is highly regulated. Even a few pathogen propagules in a wheat shipment can result in rejection by an importing country. Following the first discovery of Karnal bunt in the United States in Arizona and California in 1996, a study was initiated to determine how long the pathogen might survive in Arizona wheat fields. This information will be important in making critical disease control decisions. Spores of the pathogen were incorporated into soil, which was placed in very fine mesh bags. These were buried at three depths in irrigated and nonirrigated field plots. Identical bags of infested soil were retained dry in a laboratory. At 6-month intervals over 4 years, bags from the field and laboratory were tested for spore viability. Results showed that initially the pathogen spores lost viability rapidly, but after approximately 2 years the remaining propagules lost viability at a much slower rate. By the end of 2 years, viability had dropped from 56% to 9.7 and 6.7% in nonirrigated and irrigated field plots, respectively. These resilient, long-lived spores are probably responsible for the reappearance of small amounts of disease every few years in the southwestern United States.

Influence of Late Season Harvesting, Fall Grazing, and Fungicide Treatment on Verticillium Wilt Incidence, Plant Density, and Forage Yield of Alfalfa. F. A. Gray and D. W. Koch, Professors, Department of Plant Sciences, POB 3354, University of Wyoming, Laramie 82071-3354. Plant Dis. D-2004-0524-01R, 2004 (online). Accepted for publication 16 March 2004.

Studies were conducted from 1986 through 1994 in Wyoming to determine the effect of fall harvesting practices on performance of alfalfa in the presence of Verticillium wilt. The studies were conducted on the Padlock Ranch near Dayton, WY. The ranch is located in the eastern shadow of the Big Horn Mountains near the Montana border at an elevation of 3,926 ft. The ranch raises approximately 6,000 acres of alfalfa, of which 4,000 are irrigated and 2000 are rainfed. All hay is fed to their approximately 10,000 beef cattle during late fall and winter months. Rainfall at the ranch is around 18 inches annually. Verticillium wilt, a devastating disease of alfalfa, was first introduced into the United States in the mid-1970s and has since spread to most of the middle and upper states. It was first reported in Wyoming on the Padlock Ranch in 1981. Loss in Wyoming in1988 was estimated at $1.73 million annually. On the Padlock Ranch, and in most ranches in the mountainous regions of the northwestern United States, alfalfa is cut two to four times, depending on elevation, and regrowth is grazed after a killing frost. Studies were undertaken to assess the effect of late season cutting and/or grazing, in the presence of Verticillium wilt, on plant density and forage yield of both resistant and susceptible alfalfa varieties. Both studies were conducted in fields with sprinkler irrigation. In the first experiment, a late third cutting and grazing produced the lowest yield the next year in the moderately resistant Apollo II, while the best yield the following year was obtained in the two-cut treatment without grazing. In the second experiment, both the resistant variety Apollo and the susceptible variety Arrow were tested. Arrow had less Verticillium wilt, better plant stand, and higher forage yield than Apollo. Alfalfa cut twice, with or without grazing, resulted in the highest forage yield and best remaining stands. Overall results indicated that, in the presence of Verticillium wilt, growing adapted alfalfa varieties with resistance to Verticillium wilt, managing for two cuttings, and grazing after killing frost, rather than cutting fall regrowth, would prolong stand life and maximize forage production. Therefore, purchasing hay for winter feed may be more economical than harvesting a third time in the fall. Alternatively, given the generally poor hay curing conditions in the fall, grazing after a killing frost would be more practical. Newer varieties of alfalfa that are highly resistant to Verticillium wilt may now be purchased and may provide even higher yields and longer stand life in the presence of this disease.

Recovery of Anastomosis Groups of Rhizoctonia solani from Different Latitudinal Positions and Influence of Temperatures on their Growth and Survival. R. Harikrishnan and X. B. Yang, Department of Plant Pathology, Iowa State University, Ames 50011. Plant Dis. D-2004-0601-01R, 2004 (online). Accepted for publication 22 March 2004.

This study was conducted to evaluate the influence of climate on the distribution of anastomosis groups (AGs) of Rhizoctonia solani in soybean across a large geographic region. Isolates of R. solani were recovered from soil samples collected from the major soybean-growing regions of the United States. Based on the anastomosis grouping of the recovered isolates of R. solani, isolates of AG-1 predominantly was found more from the southernmost locations while isolates of AG-2-2 were found mostly in the northern locations. However, isolates of AG-4 and AG-5 were recovered from all the locations sampled. Further testing with representative isolates from each AG/location for their ability to survive saprophytically under different temperatures revealed a clear difference among isolates from northern and southern regions to withstand colder temperatures irrespective of the AG to which they belong. These results clearly indicate the influence of temperature on the distribution of AGs in R. solani and thus enhance our understanding of how weather in general, and temperature in particular, affect the distribution of isolates of R. solani, which is an important fungal pathogen of many plants.

Biological and Molecular Variability Among High Plains virus Isolates. Dallas L. Seifers, Professor, Kansas State University, Agricultural Research Center–Hays 67601-9228; Y.-M. She, Department of Physics, University of Manitoba, Winnipeg, Manitoba, Canada; Tom L. Harvey, Professor, Department of Entomology, Kansas State University, Manhattan 66506; T. J. Martin, Professor, Kansas State University, Agricultural Research Center–Hays; S. Haber, Cereal Research Centre, Agriculture & Agri-Food Canada, Winnipeg, Manitoba, Canada; W. Ens and K. G. Standing, Department of Physics, University of Manitoba, Winnipeg; and Raymond Louie, United States Department of Agriculture–Agricultural Research Service and D. T. Gordon, Professor, Emeritus, Department of Plant Pathology, The Ohio State University, Wooster 44691. Plant Dis. D-2004-0601-02R, 2004 (online). Accepted for publication 30 March 2004.

In 1993, a previously unknown pathogen was identified infecting maize (Zea mays L.) from the U. S. High Plains states of Colorado, Idaho, Kansas, and Texas. The pathogen has been named the High Plains virus (HPV). Resistance to HPV has been identified in B73 maize using a single HPV isolate. We wanted to determine the stability of this resistance using several HPV isolates obtained from wide geographic origins and, most importantly, also determine variation in biological and molecular characteristics among HPV isolates. Observations showed that variation existed among all five HPV isolates at the molecular level and that the resistance reported for B73 varies depending upon the HPV isolate. However, it appears that, for those isolates that can infect B73, that the systemic spread of HPV throughout the plant may be inhibited, indicating that this source of resistance is of value.

Fusarium Head Blight and Deoxynivalenol Accumulation of Barley in Eastern Canada: Cultivar Response and Correlation Analysis. T. M. Choo, Eastern Cereal and Oilseed Research Centre, Agriculture and Agri-Food Canada, Ottawa, Ontario, Canada K1A 0C6; R. A. Martin, Crops and Livestock Research Centre, Agriculture and Agri-Food Canada, Charlottetown, PEI, Canada C1A 4N6; K. M. Ho, Eastern Cereal and Oilseed Research Centre, Agriculture and Agri-Food Canada, Ottawa, Ontario, Canada K1A 0C6; Q. Shen, Zhejiang Academy of Agricultural Sciences, 48 Shiqiao Road, Hangzhou, China 310021; G. Fedak, M. Savard, and H. Voldeng, Eastern Cereal and Oilseed Research Centre, Agriculture and Agri-Food Canada, Ottawa, Ontario, Canada K1A 0C6; D. E. Falk, Department of Plant Agriculture, University of Guelph, Guelph, Ontario, Canada N1G 2W1; M. Etienne, W. G. Thompson and Sons Ltd., Nairn Research Lab., R. R. #1, Ailsa Craig, Ontario, Canada N0M 1A0; and E. Sparry, C & M Seeds, R. R. #3, Palmerston, Ontario, Canada N0G 2P0. Plant Dis. D-2004-0604-01R, 2004 (online). Accepted for publication 24 March 2004.

Fusarium head blight of barley (Hordeum vulgare) is a devastating disease in many countries. A study was undertaken to identify barley cultivars, if any, that are resistant to Fusarium head blight and deoxynivalenol (DON) accumulation and to determine if DON concentration is correlated with other plant traits in Eastern Canada and China. Barley cultivars were grown in the field under artificial inoculation conditions at two locations (Charlottetown and Ottawa) in Canada during two summers and at Hangzhou in China during two winters. Seed samples were collected for DON analysis from the Barley Performance Trial at five locations in Ontario in 2000. None of the 64 barley cultivars were immune to Fusarium head blight infection. Two-row cultivars, however, were significantly more resistant to Fusarium head blight infection and DON accumulation than six-row cultivars. Two commercial cultivars, Island and AC Alberte, were found to be most resistant as they were consistently low in Fusarium head blight incidence and DON concentration in both Eastern Canada and China. These cultivars could be useful as a means to reduce DON contamination and as parents for barley breeding programs that aim at Fusarium head blight resistance. In six-row barley, DON concentration was correlated positively with Fusarium head blight incidence at both Charlottetown and Ottawa, and it was negatively correlated with plant height at Ottawa. DON concentration and heading date were not consistently correlated. Barley yellow dwarf and powdery mildew appeared to have very little effect on Fusarium head blight infection. Susceptibility to DON accumulation did not result in low yield under natural infection conditions in Ontario.

Development of a Robust Screening Method for Pathogenicity of Colletotrichum spp. on Strawberry Seedlings Enabling Forward Genetic Studies. Sigal Horowitz, Department of Plant Pathology and Microbiology, Faculty of Agricultural, Food and Environmental Quality Sciences, The Hebrew University of Jerusalem, Rehovot 76100, and Department of Plant Pathology, ARO, The Volcani Center, Bet Dagan 50250, Israel; Oded Yarden, Department of Plant Pathology and Microbiology, Faculty of Agricultural, Food and Environmental Quality Sciences, The Hebrew University of Jerusalem; and Aida Zveibil and Stanley Freeman, Department of Plant Pathology, ARO, The Volcani Center. Plant Dis. D-2004-0609-01R, 2004 (online). Accepted for publication 31 March 2004.

Anthracnose has become a major constraint in worldwide strawberry (Fragaria × ananassa Duchesne) production. The dominant species causing disease symptoms on strawberry are Colletotrichum acutatum J. H. Simmonds, C. fragariae Brooks, and C. gloeosporioides (Penz.) Penz. & Sacc. in Penz. (teleomorph Glomerella cingulata (Stoneman) Spauld. & H. Schrenk). The ability of these pathogens to attack different plant parts, and the fact that several pathogen species are involved, adds to the complex nature of strawberry anthracnose. To advance our understanding of disease mechanisms, pathogenicity genes can be identified by utilizing a large-scale forward genetic approach for screening for reduced or impaired virulence mutants. However, in the past, pathogenicity assays of Colletotrichum spp. were designed especially for breeding programs for anthracnose resistance and are not suitable for large-scale screening for reduced pathogenicity mutants. This study reports on the development of a rapid in vitro bioassay enabling large-scale screening and isolation of nonpathogenic mutants of C. gloeosporioides and C. acutatum on strawberry seedlings. The effects of incubation temperature, seedling age, inoculation technique, and isolate were evaluated to determine the conditions best suited for this purpose. A second objective was to prove the efficacy of the proposed screening method by isolating reduced-pathogenicity mutants for future studies. This method also was reliable in determining pathogenicity of the cucurbit-specific C. magna that did not cause disease symptoms on strawberry by either inoculation method. The proposed method enabled screening of more than 980 restriction enzyme-mediated integration mutants resulting in a selection of five reduced-virulence isolates. Initial characterization of some of these mutants revealed large differences in germination and appressorial formation compared with pathogenic isolates. Application of this procedure may have potential for screening additional pathogens on strawberry, such as Rhizoctonia spp., and for assessing potential biocontrol agents.

Tolerance to Citrus mosaic virus in Transgenic Trifoliate Orange Lines Harboring Capsid Polyprotein Gene. Toru Iwanami and Tokurou Shimizu, Department of Citrus Research, Okitsu, National Institute of Fruit Tree Science, Shimizu-Okitsu-Nakacho, Shizuoka, Shizuoka 424-0292, Japan; Takao Ito, Department of Citrus Research, Kuchinotsu, National Institute of Fruit Tree Science, Kuchinotsu, Nagasaki 859-2501, Japan; and Toshio Hirabayashi, Department of Citrus Research, Okitsu, National Institute of Fruit Tree Science. Plant Dis. D-2004-0608-02R, 2004 (online). Accepted for publication 9 April 2004.

Trifoliate orange plants (Poncirus trifoliata), major rootstocks in Japan, were transformed with a binary vector containing the capsid polyprotein (pCP) gene of Citrus mosaic virus (CiMV) via Agrobacterium tumefaciens LBA4404. Transformation was performed on the epicotyl segments obtained from a young seedling, which was grown in the dark. Southern blot hybridization analysis showed that the transgenes were stable in the transgenic lines after regeneration and propagation by grafting. Transgenic lines were screened for tolerance to CiMV by mechanical inoculation. Infection was monitored on up to 120 days after inoculation by reverse transcription–polymerase chain reaction and when applicable, by enzyme-linked immnosorbent assay. The transgenic line 24 had lowest infection rate (7.1%) at 60 days after inoculation, in contrast to that of nontransgenic plants (65.1%). The response of other lines to inoculation ranged from susceptibility to moderate tolerance. The results suggested that some transgenic lines might be used as rootstock to prevent the spread of CiMV through soil.

A Comparison of Methods Used to Estimate the Maturity and Release of Ascospores of Venturia inaequalis. David M. Gadoury and Robert C. Seem, Department of Plant Pathology, Cornell University, New York State Agricultural Experiment Station, Geneva 14456; William E. MacHardy, Department of Plant Biology, University of New Hampshire, Durham 03824; Wayne F. Wilcox and David A. Rosenberger, Department of Plant Pathology, Cornell University, New York State Agricultural Experiment Station, Geneva 14456; and Arne Stensvand, Department of Plant Pathology, Plant Protection Centre, The Norwegian Crop Research Institute, N-1432 Ås, Norway. Plant Dis. D-2004-0607-03R, 2004 (online). Accepted for publication 12 April 2004.

Apple scab is a destructive fruit disease that can only be controlled in commercial apple production through the use of fungicides. A key factor driving the timing and intensity of spraying is the supply of spores of the pathogen, which overwinters in leaf litter on the orchard floor. The spores are discharged into the air during rain, and the supply is eventually exhausted after several rain events. Knowing exactly when the supply is exhausted, however, is a snag in many disease management programs. Direct microscopic assessments of spore maturity can be made, but are time-consuming and only apply to a single site. There may be hundreds of orchards that need an accurate answer to the question: how many spores are left? Over 20 years ago, a simple degree-day model was developed to answer this question. However, model forecasts often deviated from estimates obtained by various direct methods of assessment, and it was always assumed that the model forecast was wrong. We now know that the direct assessments that were in error were due to an inherent bias in the techniques. Through a detailed analysis of 14 years of data on pathogen development, the degree-day model was validated and can now be used with confidence to forecast the depletion of the pathogen’s spore supply, and thereby reduce the intensity of spraying for apple scab.

Relationship Among Concentrations of Sphaerotheca macularis Conidia in the Air, Environmental Conditions, and the Incidence of Powdery Mildew in Strawberry. C. Blanco, B. de los Santos, C. Barrau, F. T. Arroyo, M. Porras, and F. Romero. C.I.F.A. Las Torres – Tomejil. Apdo. de Correos Oficial, Alcalá del Río. 41200 Sevilla, Spain. Plant Dis, D-2004-0607-01R, 2004 (online). Accepted for publication 12 April 2004.

Strawberry powdery mildew is one of the most common diseases on the aerial parts of the plant and it has been reported worldwide. The disease is caused by the obligate parasite Sphaerotheca macularis (Wall. ex Fries) Jacz f.sp. fragariae (Peries). Control of powdery mildew relies on the application of protectant and systemic fungicides. Aerobiological studies could lead to a better understanding of the biology and epidemiology of S. macularis, and could contribute to the development of disease management. Atmospheric concentrations of S. macularis conidia were monitored for 2 years on a strawberry crop in Huelva (southweestern Spain) using a Burkard volumetric spore sampler. The presence of conidia was related to temperature, relative humidity, and rainfall, with a positive correlation for the first factor and a negative correlation with the other two. The presence of conidia in the air was positively correlated with incidence of powdery mildew on fruits. A diurnal pattern of conidia release was observed. The current work could be the basis for development of a predictive model of powdery mildew occurrence in strawberry production, which would be a tool to improve crop management and the efficiency of fungicide applications.

Wheat Stripe Rust Epidemic and Virulence of Puccinia striiformis f. sp. tritici in China in 2002. Anmin Wan, Institute of Plant Protection, Chinese Academy of Agricultural Sciences, Beijing 100094, China; Zhonghua Zhao, Division of Crop Disease Control, National Agro-Technique Extension and Service Center, Ministry of Agriculture, Beijing 100026, China; Xianming Chen, USDA-ARS, Wheat Genetics, Physiology, Quality, and Disease Research Unit, and Department of Plant Pathology, Washington State University, Pullman 99164-6430, USA; Zhonghu He, Beijing Office of CIMMYT, Beijing 100081, China; Shelin Jin and Qiuzhen Jia, Gansu Institute of Plant Protection, Lanzhou 730070, China; Ge Yao and Jiaxiu Yang, Sichuan Institute of Plant Protection, Chengdu 610066, China; Baotong Wang and Gaobao Li, Northwestern University of Agricultural and Forestry Science and Technology, Yangling 712100, China; Yunqing Bi, Yunnan Institute of Plant Protection, Yunnan 650200, China; and Zongying Yuan, Shanxi Institute of Plant Protection, Taiyuan 030031, China. Plant Dis. D-2004-0610-01S, 2004 (online). Accepted for publication 1 April 2004.

Stripe rust is one of the most important diseases of wheat in China. In the 2001–2002 growing season, a stripe rust epidemic affected about 6.6 million hectares in 11 provinces and autonomous regions of China. The epidemic was the most severe and widespread since 1990. The wide use of fungicides limited yield losses due to stripe rust to 1.3 million metric tons. The epidemic could be attributed to relatively warm weather from November 2001 to March 2002, high frequencies of highly virulent races, and widely grown susceptible cultivars. Virulences of the rust population were identified using a set of 17 wheat genotypes that have been used to differentiate Chinese races of the pathogen. More than 45 virulence patterns and their frequencies were determined from 926 stripe rust samples collected throughout the regions where disease occurred. Race CYR32, which was virulent on 16 of the 17 Chinese differential genotypes, had the highest frequency and was predominant throughout the epidemic regions. CYR32 also was virulent on many wheat genotypes with reported stripe rust resistance genes. Resistance genes in wheat genotypes Zhong 4, Triticum spelta album, and Moro were still effective against all races identified thus far. Integrated control strategies were recommended to include development and use of cultivars with effective and durable resistance, application of effective fungicides when necessary, and appropriate crop management.