Race Structure of Pyrenophora tritici-repentis in the Kansas Wheat Pathogen Population

Tan spot, caused by Pyrenophora tritici-repentis (Ptr), carries any of at least three identified necrotrophic effectors (NEs) as part of its host invasion repertoire (Ptr ToxA, Ptr ToxB, and Ptr ToxC). The eight possible combinations of these genes, or their absence, are described using a race system. These races characterize the disease phenotype observed on a wheat differential set. The primary NE, Ptr ToxA, enables pathogenesis through an interaction with the wheat susceptibility gene, Tsn1. This study aimed to determine the race structure of the Ptr population in Kansas. To complete this objective, a survey was conducted to collect isolates of the pathogen from the Kansas wheat production region. In addition, 16 historic isolates were also included. The isolates were screened against the wheat differential set and molecularly characterized for presence of the genes ToxA and ToxB and a nonfunctioning ToxB haplotype called toxb. The survey identified 63 Ptr isolates. Our results revealed the predominance of race 1, which contains ToxA and ToxC. However, we also identified isolates of races 2, 3, and 4. The results of molecular assays matched the screening results for Ptr ToxA and Ptr ToxB in most cases. Some isolates caused atypical symptoms that did not match the current race structure. The prevalence of race 1 Ptr in the Kansas population suggests that breeding for resistance in Kansas should focus on selecting against Tsn1, which encodes susceptibility to Ptr ToxA. This research provides critical insights that will inform breeders and pathologists for improved control of Ptr.