Induced resistance of cucumber seedlings caused by some non-pathogenic Rhizoctonia (np-R) isolates

Among 153 isolates of Rhizoctonia spp. obtained from 95 soil samples collected from different fields in the USA, 42 (27.5%) isolates were hypovirulent or non-pathogenic on cabbage (tested on tap water agar plus 250/μg/ml chloramphenicol plates). Of these, 14 (33.3% of the np-R) isolates protected >60% of the cabbage seedlings against R. solani, and the best eight isolates protected 73-95% of the cucumber seedlings. The np-R isolates RU56-8 (AG-P) and RUB9-1 [AG-B(o)] induced the highest resistance against hypocotyl challenge inoculation with virulent R. solani (38.3-85.7%), whereas most of the challenged control seedlings (85-100%) collapsed. Similarly, isolates RU56-8 and RU89-1 induced the highest resistance (22.2-87.5%) against hypocotyl challenge inoculation with Pythium aphanidermatum, whereas most of the challenged control seedlings collapsed (90-100%). Isolates RU56-8 and RU89-1 significantly reduced the lesion numbers and area/leaf (to 8.9-42.0% of the control) caused by challenge inoculation of the first true leaves with Pseudomonas syringae pv. lachrymans. No np-R isolate could be recovered from the upper hypocotyis or from the leaves, indicating that there was no contact between the inducer and the pathogen. Root colonization with some np-R increased seedling tolerance to low soil moisture levels.