Occurrence and pathogenicity of Puccinia graminis f. sp. tritici on wheat in South Africa during 2007 and 2008

Abstract

Surveys were conducted in 2007 and 2008 to determine the distribution and frequency of pathotypes of Puccinia graminis Pers.:Pers f. sp. tritici Eriks. E. Henn. in South Africa. Stem and/or leaf samples of wheat and triticale (X Triticosecale) infected with stem rust were collected from commercial farms and trap nurseries established across the main wheat growing areas. The pathogenicity of stem rust isolates was determined on differential lines. In both 2007 and 2008 stem rust was observed at most localities surveyed in the Western Cape but infrequently in Free State, KwaZulu-Natal and Lesotho. The severities of stem rust were generally lower during 2008 than in 2007. Six P. graminis f. sp. tritici pathotypes were identified from 92 and 112 isolates in 2007 and 2008, respectively. Pathotypes 2SA105 (35.3%) and 2SA88 (32.8%) were predominant during both seasons. Pathotype 2SA88 was detected at all localities whereas 2SA 105 was detected mostly in samples collected from the Western Cape. Other pathotypes detected during the two seasons in decreasing order of frequency were 2SA 104, 2SA 106, 2SA 102 and 2SA55. An important change in the pathogen population during the present study was the emergence of a new pathotype virulent on Sr24. The new pathotype (detected in 2007) was designated as 2SA106. Seedlings of several lines possessing different stem rust resistance genes were tested for their reaction to 2SA106. Genes Sr13, 21, 22, 25, 26, 27, 29, 31, 32, 33, 35, 36, 37, 39, 42, 43, 44 and Tmp were effective against this pathotype. Except for Sr27 and Sr36, virulence for these genes has not been reported in South Africa.