Abstract
Tan spot, caused by the fungal pathogen Pyrenophora tritici-repentis, is an important foliar disease of wheat. Eight races of the fungus are presently known based on their virulence on a wheat differential set. In 2001, approximately 80 wheat fields in Azerbaijan, Kyrghyzstan, Kazakhstan, Uzbekistan, and Syria were surveyed for the occurrence of tan spot. These countries lie on the historic Silk Road, which encompasses part of the wheat center of diversity. Single-spore isolates of P. tritici-repentis were recovered from 52 fields, and the virulence of 253 of these isolates was assessed on the differential hosts. The greatest diversity was observed in Azerbaijan, where races 1, 2, 3, 5, 7, and 8 of the pathogen were identified, and in Syria, where races 1, 3, 5, 7, and 8 were detected. However, little variation was found in the virulence of isolates from Kazakhstan, Kyrghyzstan, and Uzbekistan, with only one or two races identified in each of these countries. No isolate of races 4 and 6 was detected. Race 1 was predominant in all countries except Syria, where race 3 was the most common. Isolates of races 1 and 2 were collected mainly from hexaploid wheats, whereas isolates of races 3, 5, 7, and 8 were found predominantly or exclusively on tetraploid wheats. No new virulence pattern was identified, but as races are currently defined on a very limited set of differentials, the complete range of diversity in P. tritici-repentis is likely not being fully assessed.