Genetic studies on the resistance to tan spot of wheat and genetic similarity among isolates of Pyrenophora tritici-repentis

Tan spot of wheat, caused by the fungus 'Pyrenophora tritici-repentis ' (Died.) Dreches., has become a major component of the leaf spot disease complex in the prairie region in recent years. On susceptible host cultivars 'P. tritici-repentis' induces two distinct symptoms, tan necrosis and extensive chlorosis. To date, five races of 'P. tritici-repentis ' have been identified based on their reaction on a set of wheat differential cultivars. The major objective of this study was to investigate the genetics of resistance to tan spot in several diverse hexaploid wheat cultivars to races 1, 2, and 3, and to the culture filtrate of 'P. tritici-repentis ', race 2. The F1 and F2 generations and F 2:3 and F2:8 families of crosses between the seven selected genotypes were tested with each race in growth room experiments. Two independently inherited genes controlled resistance to tan spot of wheat. A single dominant gene controls resistance to chlorosis induced by races 1 and 3, while a single recessive gene controlsresistance to necrosis induced by races 1 and 2. The same recessive gene controls resistance to necrosis induced by spore-inoculation or culture filtrate infiltration of 'P. tritici-repentis', race 2. This indicated that infiltration with the culture filtrate could replace spore-inoculation when testing for resistance to the necrosis component of tan spot. No segregation was observed in crosses among the resistant cultivars for both the necrosis and the chlorosis components, indicating that the resistant cultivars carried the same resistant genes. The second objective of this study was to assess the genetic similarity among different isolates of 'P. tritici-repentis' and to determine if this similarity is related to race classification or geographic origin of the isolates. Thirty-eight different isolates including other ' Pyrenophora' species were studied with 30 RAPD primers. Cluster analysis and AMOVA indicated that high genetic variability existed among the isolates of 'P. tritici-repentis', but this variability was independent of race classification or geographic origin of the isolates. The last objective of this study was to identify molecular markers linked to the resistance gene controlling the necrosis component of tan spot. No RAPD marker linked to gene controlling resistance to the necrosis component was identified