ToxA and ToxB have minimal effect on tan spot epidemics of spring wheat in the Nordics

Tan spot, caused by the fungus Pyrenophora tritici-repentis (Ptr), is one of the major diseases of spring wheat (Triticum aestivum L.) in the Nordic countries. However, little is known about the role of Ptr necrotrophic effectors (NEs), ToxA, ToxB and ToxC, behind tan spot epidemics in the region. A total of 217 Nordic Ptr isolates were screened for ToxA and ToxB genes using PCR. ToxA was found to be prevalent (64.5%), whereas ToxB was absent from the population. A subset of 25 Ptr isolates was further tested on a set of wheat differentials. The majority (n = 21) of these isolates induced susceptible reactions on ToxC sensitive genotype ‘6B365’, indicating the presence of the ToxC effector. The symptoms observed on ToxA and ToxB differentials were not completely in line with the PCR results, suggesting the presence of ‘atypical’ Ptr isolates. In a collection of 197 spring wheat genotypes, ToxA and ToxB sensitivities were determined and found in 46.2% and 23.9% of the lines, respectively. Additionally, 179 of the wheat accessions were tested in the field over three years at two locations in Finland, and NE sensitivities were found to explain very little of the variation in tan spot susceptibility. Interestingly, ToxB sensitivity was found to have a counter-intuitive effect in European spring wheat germplasm, as ToxB-sensitive genotypes were less susceptible to tan spot disease compared to the insensitive ones. The results presented here provide the first comprehensive overview of the importance of Ptr NEs in tan spot epidemics in the Nordic countries.