A JAPANESE POWDERY MILDEW ISOLATE WITH EXCEPTIONALLY LARGE INFECTION EFFICIENCY ON MLO-RESISTANT BARLEY

A Japanese field isolate (Race I) of Erysiphe graminis f,sp. hordei wa s tested on 17 barley lines carrying the mlo powdery mildew resistance gene. Race I produced many successful infections with infection type larger than or equal to 2 on six lines (M66, MC20, SR1, SR7, Atem and Totem). On the remaining 11 lines it reacted with infection type 0. Co lony numbers on the Mlo-lines were between 2.6% and 12.9% of the numbe rs on a susceptible cultivar Manchuria, These numbers were larger than , or similar to those produced by isolate HL3/5, which has the highest recorded infection efficiency on Mlo-resistant cultivars. The interac tion between isolates and lines was highly significant. The isolate GE 3, from which HL3/5 was derived by selection, gave rise to occasional colonies corresponding to less than 0.3% of the number produced on cv. Manchuria.