Nature of Protection of Bean Seedlings from Rhizoctonia Root Rot by a BinucleateRhizoctonia-likeFungus

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Abstract
Protection of bean [Phaseolus vulgaris] seedlings from Rhizoctonia root rot by a binucleate Rhizoctonia-like fungus (BNR) was investigated in laboratory and greenhouse studies. BRN failed to show antagonistic interaction when grown in dual culture on agar media with Rhizoctonia solani; also, filtrates from 10-day-old cultures of BNR did not inhibit R. solani. Histologic sections of hypocotyls and roots of BNR-treated seedlings showed that BRN did not penetrate beyond the epidermal cells, but it extensively colonized the rhizosphere and rhizoplane of bean seedlings. Root exudates from 10-day-old BNR-treated seedlings inhibited hyphal growth and sclerotial germination of R. solani in vitro. Treatment of bean seedlings with BNR before inoculation with R. solani inhibited formation of infection cushions by R. solani. Surface sterilization with either 1% sodium hypochlorite or 70% ethanol for 30 sec completely eradicated BRN from bean roots and hypocotyls. When seedlings were replanted and subsequently inoculated with the pathogen, however, the protective capability against R. solani was maintained. These results suggest that the main mechanism of protection in this system involves a BNR-induced metabolic response by bean seedlings that suppresses R. solani at the infection site.

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