A New Pathotype of Puccinia striiformis f. sp. tritici on Wheat in South Africa.

Abstract

Following the detection of Puccinia striiformis f. sp. tritici for the first time on wheat (Triticum aestivum) in the Western Cape in August 1996, stripe rust has spread to all the important wheat production areas in South Africa. Only the introduced pathotype (pt. 6E16) was detected in surveys of these areas during 1996 and 1997. In 1998, a severe stripe rust epidemic occurred in the eastern Free State on the extensively grown cultivars Hugenoot and Carina, both which are resistant to pt. 6E16. Stripe rust severities of 100% were common on flag and lower leaves, and widespread applications of fungicides were necessary. Avirulence/virulence characteristics of P. striifomis f. sp. tritici isolates collected from Hugenoot and Carina were determined on 17 standard stripe rust differential wheat lines and 11 supplementary testers. The latter testers included the wheat lines TP981 and TP1295 (supplied by R. Johnson, Cambridge, UK), both of which have a major resistance gene in common with the differentials Heines Peko, Reichersberg 42, Strubes Dickkopf, Clement, and Heines VII (1). Isolates obtained from Hugenoot and Carina differed from pt. 6E16 based on virulence to Reichersberg 42 (Yr7,25), Heines Peko (Yr2,6,25), TP981 (Yr25), and TP1295 (Yr25). The new variant, designated as 6E22, was also identified in collections from the province KwaZulu-Natal. Seedling tests with 6E16 and 6E22 have shown that Hugenoot, Carina, and Tugela-DN are the only local cultivars affected by the new pathotype. The occurrence of pt. 6E22, which appears to be a single-step adaptation from 6E16 adding virulence to Yr25, emphasizes the vulnerability of monogenic resistance to this disease. Reference: (1) R. A. McIntosh et al. Wheat Inform. Serv. 85:56, 1997.