Pathotypes of Puccinia graminis f. sp. tritici with Increased Virulence for Sr24. J. Le Roux, Senior Agricultural Research Officer, Department of Agriculture, Grain Crops Research Institute, Bethlehem, 9700, South Africa. F. H. J. Rijkenberg. Professor, Department of Microbiology and Plant Pathology, University of Natal, Pietermaritzburg, 3200, South Africa. Plant Dis. 71:1115-1119. Accepted for publication 21 April 1987. Copyright 1987 The American Phytopathological Society. DOI: 10.1094/PD-71-1115.

Pathogenic studies of two pathotypes of Puccinia graminis f. sp. tritici first isolated during 1984 on the previously resistant cultivars SST44 and Gamka demonstrated that both possessed increased virulence for Sr24. These pathotypes, designated 2SA100 and 2SA101, which appeared to be mutations of earlier types, became widespread throughout the South African wheat production areas within two seasons. Seedling and adult-plant response studies indicated genetic vulnerability in 60% of the cultivars. Nine of the 23 recommended cultivars possess Sr24, either alone or in combination with other resistance factors. Susceptibility ratings, measured in terms of latent period and uredinium density, separated cultivars into three distinctive groups. Differences in the degree of susceptibility in lines/cultivars having Sr24 as a major stem rust resistance gene were evident. Pathotypes 2SA100 and 2SA101 or races similar to these constitute a major threat to wheat production in Southern Africa and other regions where Sr24 is deployed, because they combine increased virulence with aggressiveness and good survival ability.