Evaluation of quantitative resistance to yellow rust (Puccinia striiformis f. sp. hordei) in the ICARDA/CIMMYT barley breeding programme

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Abstract
The resistance to yellow rust (Puccinia striiformis f. sp. hordei) of 500 advanced barley lines from the ICARDA/CIMMYT breeding programme in Mexico was evaluated on seedlings in the greenhouse and on adult plants in the field. A high frequency of advanced lines (85.8%) showed a susceptible reaction (infection type ≥ 7) on seedlings after inoculation with isolate Mex‐1, representing a Mexican variant of race 24. This indicates the absence of effective hypersensitive resistance. In addition, the same advanced lines showed a large variation in disease severity in the field, ranging from 0 to 95%. More than 76% of the advanced lines with a susceptible reaction in the seedling stage demonstrated low disease severity (10% or less in the adult plant). Consequently, these advanced lines possess high levels of quantitative resistance.Two aspects in the ICARDA/CIMMYT barley breeding programme may explain the large number of advanced lines with high levels of quantitative resistance. First, a recurrent selection approach is applied when advanced (F5) lines reaching homozygosity are intercrossed. Second, low levels of disease are accepted in the selection process instead of selecting the ultimate green plant. Both aspects combined allow the accumulation of quantitative resistance. Certain cultivars released from South‐American national programmes in the late 1970s and early 1980s in Peru (UNA‐80), Bolivia (IBTA 80) and Ecuador (Teran) are still resistant, demonstrating the durable nature of quantitative resistance to yellow rust.