ASSOCIATION OF BINUCLEATE RHIZOCTONIA WITH SOYBEAN AND MECHANISM OF BIOCONTROL OF RHIZOCTONIA-SOLANI

The association of binucleate Rhizoctonia (BNR) AG-K with soybean and the interaction of BNR, R. solani AG-4, and soybean seedlings were inv estigated to elucidate the mechanism of biocontrol of R solani by BNR. Sixty-hour-old seedlings were inoculated and incubated in a growth ch amber at 24 degrees C; plants were examined with light microscopy and with scanning and transmission electron microscopy at various times fo llowing inoculation. BNR grew over hypocotyls, roots, and root hairs, but only colonized epidermal cells. Hyphae of BNR appeared to attach t o the epidermis and 5.5 h following inoculation,began penetrating cell s by means of penetration pegs without forming distinct appressoria or infection cushions. There was evidence of cuticle degradation at the point of penetration; Infection hyphae moved to adjacent epidermal cel ls by direct penetration of epidermal radial walls. There were epiderm al and cortical cell necrosis, beginning with the fragmentation of the tonoplast and followed by the disintegration of cytoplasm organelles, and plasma membranes. Cell necrosis was also observed in adjacent cel ls where there was no evidence of BNR hyphae. Cell walls were not dest royed. After 144 h, there was no evidence of BNR hyphae in cortical ce lls. Attempted penetrations were observed, but papillae formed on the inside of cortical cell walls. Preinoculation of soybean seedlings wit h BNR 24 or 48 h before inoculation with R. solani (1 cm between inocu la) affected the growth of R. solani on soybean tissue. There were few er hyphae of R. solani, the hyphae branched sparingly, and infection c ushions were rare when compared with hyphal growth on soybean inoculat ed only with R. solani. These effects were observed before the BNR hyp hae began to intermingle with the hyphae of R. solani on the surface o f the inoculated host. Preinoculation of soybean seedlings 24 h before inoculation with R. solani significantly (P = 0.05) reduced disease i ncidence and severity caused by R.:solani AG-4. The lesions caused by R. solani always appeared distally, not proximally, to the BNR inoculu m. The interactions of intermingling hyphae of BNR and R solani were e xamined in vitro and on the surface of the host. There was no evidence of lysis, mycoparasitism, inhibition of growth, or any other form of antagonism between hyphae. The results of these studies strongly sugge st that induced resistance is the mechanism of biocontrol of R. solani on soybean by BNR. The inhibition of hyphal growth of R. solani on th e surface of soybean tissue preinoculated with BNR appears to be a nov el characteristic of induced resistance.