Studies on the pathogenicity of Puccinia graminis f. sp. Tritici and the nature of host resistance in wheat.
This study was undertaken to investigate the pathogenic variation of Puccinia graminis f.sp. tritici in South Africa and to determine the nature of host resistance to this fungus. Seventeen pathotypes of wheat stem rust were identified in 1981 and only one during 1982 and 1983. The 1984 and 1985 surveys provided six and nine pathotypes respectively. These included two new pathotypes 2SA100 and 2SA101, both virulent for Sr24. No avirulence was detected for wheat stocks having Sr7a, 8a,10,11,14,16,18,19,20 and 28. No virulence was found on wheats with Sr8b,13,21,22,25,26,27,29,31,32,35,dp2 and Tt3. Data suggested that the pathogenicity pattern in South Africa is strongly influenced by the resistance genes present in wheat cultivars. Pathogenic studies of pathotypes 2SA100 and 2SA101 cultivars demonstrated that both possessed increased virulence for Sr24 and appeared to be mutations of earlier types. Seedling and adult plant response studies indicated genetic vulnerability in 60% of the locally cultivated cultivars. Nine of the 23 recommended cultivars possess Sr24, either alone or in combination with other resistance factors. Pathotypes 2SA100 and 2SA101, or races similar to these, constitute a major threat to wheat production in Southern Africa since they combine increased virulence with aggressiveness and good survival ability. Genetic studies showed that resistance was conferred by one dominant gene in each of Belinda (Sr5), SST33 (Sr9e), SST66 (Sr9e), SST102(Sr24), Gamka (Sr24), Gamtoos (Sr31), Karee and Betta. Two partially dominant genes were identified in Wilge, Gouritz (Sr36+) and SST23 (Sr24+). Resistance in Palmiet was conferred by one dominant, (Sr24) and one recessive gene, whereas one recessive and one dominant gene operated in Molen. Tugela carried three partially dominant genes, one of which was positively identified as Sr5. Palala also has three unidentified dominant genes. The study determined that inheritance of resistance in 15 South African cultivars was relatively simple with little genetic diversity. Three spring wheat cultivars, SST44, Palmiet and Gamtoos were used to study the quantitative aspects of specific resistance to P. graminis tritici. Averaged over three cultivars, latent period was extended by 57% and uredinium density was reduced by 29% relative to the susceptible check Morocco. Latent period appeared to be a race-nonspecific resistant component of all three cultivars. This study indicated that the variation in uredinium density associated with specific resistance was similar to that reported by other workers for compatible host pathogen interactions.
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