The value of field ratings of differential lines for pathotyping Puccinia graminis f. sp. tritici

Traditional race phenotyping of cereal rusts is based on the pathogenicity of field isolates on seedlings using a set of differential host lines. These lines are regularly planted in the field, often as part of trap nurseries. In trap nurseries the primary objective is to assess the occurrence of isolates with virulence or avirulence on key resistance genes. Although it is generally accepted that field nurseries cannot be used for race identification, the correlation between seedling and adult plant response of commonly used stem rust differential lines needs further clarification. To determine if any of the wheat stem rust differential lines in South Africa contains unknown genes for adult plant resistance, or are poorly expressed in the adult stage, entries were assessed over four seasons in a nursery inoculated with race PTKST of Puccinia graminis f. sp. tritici. A strong correlation between seedling infection type and field response occurred for differential lines carrying Sr5, 6, 7b, 8a, 8b, 9a, 9b, 9d, 9e (Vernstein), 10, 11, 24, 27, 31, 36, 38, 44, McN, Kw, and Satu. However, differential lines carrying Sr9e (Vernal), 9g (Acme), 30 (Festiguay), and Sr17 (Renown) failed to accurately predict virulence under epidemic conditions in the field. Importantly, higher than expected severity and response types were recorded for differential lines carrying SrTmp and Sr21, especially under prolonged epidemic conditions. It can be concluded that field ratings of the current set of South African stem rust differential lines do not accurately describe P. graminis f. sp. tritici race PTKST.