INTERACTIONS BETWEEN DAMPING-OFF FUNGI, ANTAGONISTS AND CAPSICUM SEEDLINGS

Three isolates of rhizosphere bacteria and two of binucleate Rhizocton ia, all of which control damping-off diseases of bedding plants, were tested for some potential mechanisms of antagonism against the pathoge nic fungi, Rhizoctonia solani anastomosis groups (AG) 4 and 8 and Pyth ium ultimum var. sporangiiferum. Bacillus amyloliquefaciens Fukumoto N A31 and two strains of Pseudomonas putida (Trevisan) Migula (NA40 and NA41) showed antibiosis against multinucleate R. solani AGs 4 and 8, b ut not against P. u. sporangiiferum or the two non-pathogenic isolates of binucleate Rhizoctonia on various agar media. Both pseudomonads pr oduced fluorescent pigments, but siderophores were not involved in the antibiosis. The binucleate Rhizoctonia isolates did not show antibios is in vitro against the three phytopathogenic fungi. Both isolates of binucleate Rhizoctonia grew faster than the two R. solani pathogens, b ut slower than P. u. sporangiiferum, on both rich and minimal agar med ia at 25 degrees C, In sterile potting medium, the binucleate Rhizocto nia isolates grew faster than any of the three phytopathogenic fungi a t 25 degrees C, 30 degrees C and at variable temperatures (mean 25 deg rees C). The fastest growth rates for all five fungi in sterile pottin g medium were at 25 degrees C and at variable temperatures, hut growth at 4 degrees C and 37 degrees C was negligible, In two agar media in sealed vials, both non-pathogenic binucleate Rhizoctonia isolates colo nized seedling tissues of Capsicum annuum more densely than any of the pathogenic fungi. Sequential inoculations of Capsicum seedlings growi ng in water agar showed that prior inoculation with either isolate of binucleate Rhizoctonia reduced the subsequent hyphal density of each o f the pathogens on various tissues, and reduced damping-off, Conversel y, prior inoculation with either R. solani pathogen reduced hyphal den sities of either binucleate Rhizoctonia. One isolate of binucleate Rhi zoctonia (BNR1) was also inoculated at various time intervals before P . u. sporangiiferum and/or sowing with Capsicum in pasteurized potting medium in a glasshouse. Biological control was greatest when BNR1 was added at least one day before P. u. sporangiiferum and seeds. Damping -off was also partially suppressed when BNR1 was allowed to interact w ith P. u. sporangiiferum in the potting medium for at least two days b efore sowing. (C) 1997 Elsevier Science B.V.