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 su rvey collections made across Canada between 1957 and 1991. These isola tes were compared based on the disease reactions on seedlings of 24 li nes of Triticum aestivum containing different stem rust resistance gen es. Comparisons also were made based on length variability of the inte rgenic spacer region (IGR) of the ribosomal DNA (rDNA) repeat unit. Va riability within the IGR was investigated using Southern blots of MspI restricted genomic DNA and using polymerase chain reaction (PCR) ampl ification of the region. Isolates of race QCC obtained after 1988 were virulent to barley lines containing the Rpg1 gene for stem rust resis tance but were avirulent to gene Sr8b in wheat. Isolates collected pri or to 1986 and avirulent to barley with gene Rpg1 were virulent to gen e Sr8b. The barley-virulent isolates were found to have a length varia nt within the IGR that was absent from the other isolates. Both lines of evidence indicate that genetic variability exists within ''race'' Q CC. The length variants within the IGR provided genetic markers, in ad dition to virulence phenotypes, that distinguished between isolates of P. graminis f. sp tritici.