Pathogenic Fitness of Pyrenophora tritici-repentis Isolates Causing Tan Spot of Winter Wheat in Oklahoma

Tan spot of wheat (Triticum aestivum L.) caused by the fungus Pyrenophora tritici-repentis can be a significant disease, and thus developing resistant varieties using fit and virulent isolates is critical to wheat breeding programs.  Hence, variability in fitness characters and virulence was determined for P. tritici-repentis isolates collected in three different decades (1980s, 1990s, and 2000s) from Oklahoma wheat fields. Growth (hyphal extension on media), conidia production on media, and conidia production on wheat leaves of a susceptible (TAM 105), an intermediate (Deliver) and a resistant (Red Chief) variety were determined for 17 isolates. Isolates also were evaluated for pseudothecia production and ascospore maturity on wheat straw, and isolate virulence was determined on all three wheat varieties. Isolates differed significantly (p<0.01) for growth on media, and for conidia production on agar and on wheat leaves. Isolates varied significantly in pseudothecia production and ascospore maturity on wheat straw. Isolates differed significantly (p<0.01) in virulence (percent leaf area infection) on the three wheat cultivars. Isolates collected from the 2000s were more virulent than those collected in the 1980s and 1990s, and also were deemed to be more fit than isolates collected in the 1980s and 1990s in terms of traits such as growth, conidia production, pseudothecia formation, mature ascospore production, and virulence. This fitness could affect the competitiveness of these isolates in the field, as well as facilitate selection of isolates used to test wheat germplasm lines for reaction to tan spot.