Characterization of Tan Spot Races in Kazakhstan

Tan spot disease, which is caused by Pyrenophora tritici-repentis ( Ptr ), is one of the most significant wheat diseases in Kazakhstan, an important wheat-growing region in Central Asia. In this study, we aimed to investigate the race composition of Ptr responsible for tan spot in Kazakhstan through the phenotypic and genotypic characterization. During 2019–2020, samples of Ptr isolates were collected for analysis in six regions of the Republic of Kazakhstan from commercial and experimental fields of bread and durum wheat affected by tan spot disease. Race classification was based on inoculation bioassay of 167 isolates to four corresponding differential wheat genotypes, with a PCR assay used to identify the effector genes. The characterization of these isolates showed they belonged to four different known races (Races 1–4) in addition to one potential atypical race that does not fit into the current race system. We identified two races, Races 1 and 2, as critical for wheat production in Kazakhstan, as they occur throughout the study area. Most isolates exhibited amplification of the ToxA gene, a necrosis-causing effector, which is consistent with the inoculation results; only Races 3 and 4 did not show amplification of the ToxA gene. Inoculating wheat with the sixteen isolates of Ptr resulted in disease, the classification of which was similar to that caused by Race 8, according to the current wheat differential set. However, according to the genetic characterization, these isolates did not possess the associated effector gene expected for Race 8 assignment; therefore, we designated them as isolates of an atypical race. According to our results, race classification should be based on both phenotypic and genotypic analyses, where possible, to adequately capture the breadth of physiological variation among Ptr isolates, in addition to the possible expansion of the differential set.