INTERACTION BETWEEN POWDERY MILDEW AND BARLEY WITH MLO5 MILDEW RESISTANCE

Powdery mildew infection of barley with the mlo5 barley powdery mildew resistance gene was examined, using near-isogenic barley lines, with and without mlo5 resistance, and two near-isogenic powdery mildew isol ates, HL3/5 and GE3 with high (virulent) or low (avirulent) penetratio n efficiency on the resistant barley line. in all isolate-host combina tions (except GE3 on the resistant barley line), frequency of haustori um formation increased significantly from zero at 11 h after inoculati on to a maximum by 13 h, and there was no subsequent increase up to 24 h. In the susceptible barley line, 27% of appressoria from both isola tes formed haustoria. Although this was significantly higher than the frequency of haustorium formation (18%) of HL3/5 on the resistant barl ey line, HL3/5 was much more successful than GE3 (frequency of haustor ium formation less than 1%). The fact that HL3/5 did not possess a gen erally higher ability to penetrate successfully to form haustoria on t he susceptible barley line, indicates that HL3/5 did not overcome the mlo5 resistance by being generally more vigorous. Ln the resistant bar ley line, papillae were larger than in the susceptible line; however, both isolates were associated with papillae of the same diameter at th e time of penetration. We suggest that the mlo5 resistant barley line confers two different forms of resistance: isolate-specific and isolat e-nonspecific.