Interactions between damping-off fungi, antagonists and Capsicum seedlings

Abstract

Three isolates of rhizosphere bacteria and two of binucleate Rhizoctonia, all of which control damping-off diseases of bedding plants, were tested for some potential mechanisms of antagonism against the pathogenic fungi, Rhizoctonia solani anastomosis groups (AG) 4 and 8 and Pythium ultimum var. sporangiiferum. Bacillus amyloliquefaciens Fukurnoto NA31 and two strains of Pseudomonas putida (Trevisan) Migula (NA40 and NA41) showed antibiosis against multinucleate R. solani AGs 4 and 8, but not against P. u. sporangiiferum or the two non-pathogenic isolates of binucleate Rhizoctonia on various agar media. Both pseudomonads produced fluorescent pigments, but siderophores were not involved in the antibiosis. The binucleate Rhizoctonia isolates did not show antibiosis in vitro against the three phytopathogenic fungi. Both isolates of binucleate Rhizoctonia grew faster than the two R. solani pathogens, but slower than P. u. sporangiiferum, on both rich and minimal agar media at 25°C. In sterile potting medium, the binucleate Rhizoctonia isolates grew faster than any of the three phytopathogenic fungi at 25°C, 30°C and at variable temperatures (mean 25°C). The fastest growth rates for all five fungi in sterile potting medium were at 25°C and at variable temperatures, but growth at 4°C and 37°C was negligible. In two agar media in sealed vials, both non-pathogenic binucleate Rhizoctonia isolates colonized seedling tissues of Capsicum annuum more densely than any of the pathogenic fungi. Sequential inoculations of Capsicum seedlings growing in water agar showed that prior inoculation with either isolate of binucleate Rhizoctonia reduced the subsequent hyphal density of each of the pathogens on various tissues, and reduced damping-off. Conversely, prior inoculation with either R. solani pathogen reduced hyphal densities of either binucleate Rhizoctonia. One isolate of binucleate Rhizoctonia (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 BNRI was added at least one day before P. u. sporangiiferum and seeds. Damping-off was also partially suppressed when BNR1 was allowed to interact with P. u. sporangiiferum in the potting medium for at least two days before sowing.