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.