INDUCED RESISTANCE OF CUCUMBER SEEDLINGS CAUSED BY SOME NONPATHOGENICRHIZOCTONIA (NP-R) ISOLATES

Among 153 isolates of Rhizoctonia spp. obtained from 95 soil samples c ollected from different fields in the USA, 42 (27.5%) isolates were hy povirulent or non-pathogenic on cabbage (tested on tap water agar plus 250 mu g/ml chloramphenicol plates). Of these, 14 (33.3% of the np-R) isolates protected >60% of the cabbage seedlings against R. solani, a nd the best eight isolates protected 73-95% of the cucumber seedlings. The np-R isolates RU56-8 (AG-P) and RU89-1 [AG-B(o)] induced the high est resistance against hypocotyl challenge inoculation with virulent R . solani (38.3-85.7%), whereas most of the challenged control seedling s (85-100%) collapsed. Similarly, isolates RU56-8 and RU89-1 induced t he highest resistance (22.2-87.5%) against hypocotyl challenge inocula tion with Pythium aphanidermatum, whereas most of the challenged contr ol seedlings collapsed (90-100%). Isolates RU56-8 and RU89-1 significa ntly reduced the lesion numbers and area/leaf (to 8.9-42.0% of the con trol) caused by challenge inoculation of the first true leaves with Ps eudomonas syringae pv. lachrymans. No np-R isolate could be recovered from the upper hypocotyls or from the leaves, indicating that there wa s no contact between the inducer and the pathogen. Root colonization w ith some np-R increased seedling tolerance to low soil moisture levels .