Interpretive Summaries

Genetic Diversity of Ascochyta rabiei in Canada. G. Chongo, B. D. Gossen, L. Buchwaldt, T. Adhikari, and S. R. Rimmer, Agriculture and Agri-Food Canada Research Centre, Saskatoon, SK S7N 0X2, Canada. Plant Dis. D-2003-1030-01R, 2004 (online). Accepted for publication 17 August 2003.

Ascochyta blight, caused by Ascochyta rabiei, is an important factor limiting chickpea production in Canada. Cultivars (both kabuli and desi types) with moderate resistance are available, but severe epidemics of Ascochyta blight develop on these cultivars under favorable weather conditions. We collected local isolates of the pathogen and used them to assess the population variability for virulence on a set of eight differential host lines or cultivars. We also performed DNA fingerprinting using random amplified polymorphic DNA (RAPD) and compared results with isolates collected around the world. Virulence tests on 40 local isolates revealed 14 pathotype groups. These groups were compared with the clusters of isolates from RAPD analysis, but only a weak association was observed. The variability in virulence and DNA among isolates was high, and encompassed almost all of the diversity of the international isolates. This shows that the population of A. rabiei in Canada is genetically diverse. Production and distribution of airborne spores, which occur in some years under Canadian conditions, may help to maintain this diversity. This diversity, in turn, contributes to severe epidemics in chickpea fields.

Soilborne Oospores of Phytophthora infestans in Central Mexico Survive Winter Fallow and Infect Potato Plants in the Field. S. P. Fernández-Pavía, Instituto de Investigaciones Agropecuarias y Forestales, Universidad Michoacana de San Nicolás de Hidalgo, Morelia, Mexico 58240; N. J. Grünwald, United States Department of Agriculture-Agricultural Research Service, Prosser, WA 99350; M. Díaz-Valasis and M. Cadena-Hinojosa, Campo Experimental Valle de Mexico, CIR-CENTRO INIFAP, Chapingo, Mexico 56230; and W. E. Fry, Department of Plant Pathology, Cornell University, Ithaca, NY 14853. Plant Dis. D-2003-1112-01R, 2004 (online). Accepted for publication 14 August 2003.

Oospores are sexual, reproductive structures produced by oomycetes such as the potato late blight pathogen Phytophthora infestans. Oospores can serve as a means of survival of these organisms over a winter because they have thick, double walls that protect the spores. Survival and infectivity of oospores in soils naturally infested with P. infestans oospores were studied in central Mexico. Oospore concentration, viability, and infectivity varied among soils collected during the intercropping period in different locations of central Mexico. In some soils, oospores were infective regardless of the time at which they were collected during the intercropping period. However, oospore viability and infectivity decreased following 2 years of intercropping. This study confirms that oospores can survive over the winter and infect plants in the following growing season in the central highlands of Mexico.

Survival of Teliospores of Tilletia indica in Soil. M. Babadoost, Department of Crop Sciences, University of Illinois, Urbana 61801; D. E. Mathre and R. H. Johnston, Department of Plant Sciences and Plant Pathology, Montana State University, Bozeman 59717; and M. R. Bonde, United States Department of Agriculture-Agriculture Research Service, Foreign Disease and Weed Science Research Unit, Fort Detrick, MD 21702. Plant Dis. D-2003-1113-01R, 2004 (online). Accepted for publication 11 September 2003.

Karnal bunt or partial bunt of wheat (Triticum aestivum L.), caused by the fungus Tilletia indica Mitra (= Neovossia indica (Mitra) Mundkur), is one of the important quarantine diseases of crops in the world. The disease was reported initially from India in 1931 and since has been recorded in Afghanistan, Iran, Iraq, Mexico, Nepal, and Pakistan. In 1996, Karnal bunt was found in the United States for the first time. The pathogen survives from one season to another season as teliospores. Teliospores of T. indica are introduced into the soil at harvest and may persist there for months to several years. Teliospores on the soil surface germinate and produce primary and secondary sporidia that, under conducive environmental conditions, infect plants at flowering. This study was conducted to assess survival of T. indica teliospores in a location in the northern United States. Soils differing in texture and other characteristics were collected from four locations, equilibrated to -0.3 MPa, and infested with teliospores of T. indica to give a density of 10(^3) teliospores per gram of dry soil. Samples (22 g) of the infested soil were placed in 20-µm mesh polyester bags, which were sealed and placed at 2-, 10-, and 25-cm depths in polyvinyl chloride tubes containing the same field soil as the infested bags. Tubes were buried vertically in the ground at Bozeman, MT in October 1997. Soil samples were assayed for recovery and germination of T. indica teliospores 1 day and 8, 20, and 32 months after incorporation of teliospores into soil. The rates of teliospores recovered from soil samples were 90.2, 18.7, 16.1, and 13.3% after 1 day and 8, 20, and 32 months after incorporation of teliospores into soil, respectively, and was significantly (P < 0.01) affected by soil source. The percentage of teliospore recovery from soil was the greatest in loam soil and lowest from a silt loam soil. The rate of teliospores recovered from soil was not significantly affected by depth of burial and the soil source-depth interaction during the 32-month period. The mean percentage of teliospore germination at 1 day and 8, 20, and 32 months after incorporation into soils was 51.3, 15.1, 16.4, and 16.5%, respectively. In another experiment, samples of silty clay loam soil with 5 × 10(^3) teliospores of T. indica per gram of soil were stored at different temperatures in the laboratory. After 37 months of incubation at 22, 4, -5, and -18°C, teliospore recovery was 1.6, 2.0, 5.7, and 11.3%, respectively. The percentage of spore germination from soil samples was highest at -5°C. Microscopy studies revealed that disintegration of teliospores begin after breakdown of the sheath covering teliospore. The results of the field study showed that T. indica teliospores survived over 32 months in Montana. However, survival of T. indica teliospores in soil in Montana neither proves nor rejects the possibility of Karnal bunt development in Montana or any other areas in the Great Plains and Pacific Northwest. We were not allowed to work with live teliospores of T. indica in wheat fields in Montana because of quarantine concerns; therefore, we could not study Karnal bunt disease development under field conditions.