Use of virulence and length variability within the rDNA repeat unit to distinguish isolates of Puccinia graminis f sp tritici race QCC

Eleven isolates of race QCC and single isolates of races TPM, QFC, and MCC of Puccinia graminis f. sp. tritici were obtained from disease survey collections made across Canada between 1957 and 1991. These isolates were compared based on the disease reactions on seedlings of 24 lines of Triticum aestivum containing different stem rust resistance genes. Comparisons also were made based on length variability of the intergenic spacer region (IGR) of the ribosomal DNA (rDNA) repeat unit. Variability within the IGR was investigated using Southern blots of MspI restricted genomic DNA and using polymerase chain reaction (PCR) amplification of the region. Isolates of race QCC obtained after 1988 were virulent to barley lines containing the Rpg1 gene for stem rust resistance but were avirulent to gene Sr8b in wheat. Isolates collected prior to 1986 and avirulent to barley with gene Rpg1 were virulent to gene Sr8b. The barley-virulent isolates were found to have a length variant within the IGR that was absent from the other isolates. Both lines of evidence indicate that genetic variability exists within ''race'' QCC. The length variants within the IGR provided genetic markers, in addition to virulence phenotypes, that distinguished between isolates of P. graminis f. sp tritici.