Evolution of virulence in Puccinia striiformis f. sp. tritici (Pst) on genes for adult-plant resistance in soft and hard winter wheat cultivars
E. A. MILUS (1), D. E. Moon (1). (1) Department of Plant Pathology, University of Arkansas, Fayetteville, AR, U.S.A.
Since Stakman first described races of cereal rusts in 1914, races were based on reactions of seedlings with all-stage resistance (ASR). However, most soft and hard red winter wheats in the U.S. were susceptible as seedlings to stripe rust caused by Pst; sources of adult-plant resistance (APR) were race specific; and races based on seedling differentials did not explain stripe rust differences in the field. A susceptible check and 9 cultivars with APR were inoculated at boot stage with 17 isolates representing the pathogen population in south central U.S. from 1990-2012, and reactions on flag leaves were recorded. Five races were identified. Isolates from the 1990s (Race 1) were virulent on 7 differentials including 26R22 and Mason that was resistant to other races. Isolates from the new aggressive strain in 2000 with virulence on ASR gene <i>Yr9</i> (Race 2) were virulent on 2 differentials. Isolates from 2010 and 2012 with virulence on ASR gene <i>Yr17</i> (Race 3) were virulent on 1 differential (the susceptible check). Isolates from 2012 (Races 3,4,5) were virulent on 1 to 8 differentials. The population before 2000 was virulent on at least 6 sources of APR that provided resistance to 2 new introduced populations. In 2012, the population evolved virulence to at least 3 combinations of APR genes. This is the first report of races based on reactions of adult plants with APR genes and explains the reactions of cultivars in the field more accurately than races based on ASR genes.
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