Abstract
Two isolates of binucleate Rhizoctonia, previously selected for control of seedling damping-off diseases caused by Pythium spp. and Rhizoctonia solani , were tested for their ability to suppress four Phytophthora spp. Hyphal interactions in paired cultures with P. cinnamomi (two isolates), P. citricola and P. cryptogea on 1/4-strength potato dextrose agar were examined microscopically. Both binucleate Rhizoctonia isolates prevented growth of all Phytophthora isolates within 36 h of the paired cultures meeting. All fungal isolates produced parallel hyphae, hooks and coils on opposing cultures, except the binucleate Rhizoctonia isolates on P citricola. In four glasshouse experiments, however, neither the binucleate Rhizoctonia nor four bicontrol bacterial isolates, consistently suppressed diseases caused by P. cryptogea or P. nicotianae var. nicotianae in tomato seedlings grown in potting medium. Only the fungicide, metalaxyl, gave satisfactory disease control against both Phytophthora spp. In a subsequent in vitro test of five fungicides at commercial doses, only metalaxyl did not inhibit growth of the two binucleate Rhizoctonia isolates. Because of the compatability of metalaxyl with the binucleate Rhizoctonia isolates, this fungicide could be used to control Phytophtora spp. in an integrated control programme with a binucleate Rhizoctonia to control the other major damping-off fungi.
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