Differences Among Isolates of Pyrenophora tritici-repentis in Production of Conidia on Wheat Leaves. R. W. Rodriguez, Former Graduate Student, Kansas State University, Manhattan 66506-5502. W. W. Bockus, Professor, Department of Plant Pathology, Throckmorton Hall, Kansas State University, Manhattan 66506-5502. Plant Dis. 80:478. Accepted for publication 5 February 1996. Copyright 1996 The American Phytopathological Society. DOI: 10.1094/PD-80-0478.

Isolates of Pyrenophora tritici-repentis were .compared for their ability lo produce conidia on host tissue. In the first set of experiments, four isolates were compared on the tan spot-susceptible cultivar TAM 105. In the second set of experiments, three isolates were compared on four wheat cultivars differing in their necrotic and chlorotic reactions to tan spot. Number of conidia and percentage of disease severity were regressed against time after inoculation to compare slopes and estimates of the intercepts of linear response curves. Comparison of slopes for conidia production on the susceptible cultivar TAM 105 indicated that isolate MCR-6 had a significantly higher slope (up to 12-fold) than the other isolates. Additionally, the estimates of the intercepts for the other isolates were all different from each other (up to 5.7-fold). Comparisons of slopes and estimates of the intercepts for conidia production on cultivars differing in resistance showed that MCR-6 had a significantly higher slope than isolates MCS-I and PTF on all cultivars, and that the estimates of the intercepts of MCS-I were higher than those of PTF on all cultivars except on the highly resistant Karl 92. Thus, large differences in sporulation ability occurred within the population of P. tritici-repentis, and the ranking of an isolate remained the same on all cultivars. Differences in conidial production among isolates were not explained by differences in aggressiveness (disease severity). The large differences in sporulation capacity of strains of P. tritici-repentis, if shown to occur in the field, have implications for their rate of spread and would be determining factors in the time required for a new strain to attain a significant frequency in the fungal population.