Rph20: adult plant resistance gene to barley leaf rust can be detected at early growth stages

Rph20 is the only designated gene conferring adult plant resistance to leaf rust in barley, and to date, it has not displayed race specificity. It is not known at what particular growth stage the onset of Rph20 resistance occurs, or if the expression of the gene could be observed in controlled greenhouse assays. The present studies demonstrated that Rph20 can be identified reliably under greenhouse conditions in 5-week-old seedlings under normal temperature conditions (22 +/- 2 A degrees C), although the resistance becomes effective at 4 weeks in certain genetic backgrounds. The resistance conferred by Rph20 was strongest and best expressed at cooler temperatures (18 A +/- 2 A degrees C). The effectiveness of Rph20 at cooler temperatures may have important implications in the epidemiology of barley leaf rust under field conditions, by preventing the carryover of inoculum through winter and early spring, leading to a delayed onset of rusting and subsequently lower, if any, yield losses. The detection of Rph20 in early growth stages could help simplifying the selection of the gene in breeding programs and in monitoring Puccinia hordei populations for pathogenicity.