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Resistance

Interactions of Pythium ultimum with Germinating Resistant or Susceptible Pea Seeds. T. E. Stasz, Graduate research assistant, Department of Seed and Vegetable Sciences, New York State Agricultural Experiment Station, Geneva 14456; G. E. Harman, associate professor, Department of Seed and Vegetable Sciences, New York State Agricultural Experiment Station, Geneva 14456. Phytopathology 70:27-31. Accepted for publication 4 July 1979. Copyright 1980 The American Phytopathological Society. DOI: 10.1094/Phyto-70-27.

Variously treated seeds of the susceptible pea line Dark Skin Perfection (DSP) and resistant accession, PI 257593, were planted in soil artificially infested with a range of densities of Pythium ultimum oospores. After 10 days, both the proportion of diseased plants and disease severity were measured. The resulting curves representing the proportion of diseased plants or disease severity versus inoculum density were similarly shaped for the two lines regardless of seed condition or soil fungistasis level. However, the curves differed markedly in position as reflected by ED₅₀ values. The ED₅₀ value for PI 257593 was greatly reduced by damaging the seed coats whereas that of DSP was only slightly reduced by similar treatment. Aging seeds prior to planting, which reduces seed vigor and increases exudation during germination, reduced the ED₅₀ values of both lines. ED₅₀ values also differed in two different soil mixtures that were tested, but the level of resistance of PI 257593 relative to that of DSP was not altered. In all cases, however, curves plotted for disease severity versus the proportion of diseased plants were similar in both shape and position. Levels of resistance or susceptibility as determined by disease severity are therefore due to differences in numbers of infections occurring at a given inoculum density; ie, are relative to the proportion of oospores which successfully infect the host, and not to differences in host-pathogen interactions following infection. Comparison of these data obtained by generation of proportion of diseased plants versus inoculum density curves suggest that only the oospores in the spermoplane are capable of infecting the host.