Relationship between anIn Vitroand Greenhouse Assay for Biological Control ofBipolaris sorokiniana-Induced Seedling Blight of Wheat

Bioassays were conducted under greenhouse conditions to test the efficacy of selected bacterial and fungal antagonists applied to wheat seeds for protection against seedling blight caused by Bipolaris sorokiniana. Initially, 120 wheat rhizobacteria from soils collected at five wheat fields in Argentina were screened by two different paired culture methods. Seven isolates of Trichoderma spp. that previously showed biocontrol activity against necrotrophic pathogens of wheat were also evaluated. The in vitro tests, based on the ability of the antagonists to inhibit mycelial growth of the pathogen, were used to select the most effective isolates for further testing in a plant bioassay conducted on wheat seedlings. Disease severity, disease incidence and dry weight were estimated in the greenhouse 3 weeks after sowing. Out of 120 bacteria, 33 showed moderate to strong inhibition to the mycelial growth of B. sorokiniana on potato dextrose agar in Petri dishes. Thereafter, dual cultures in flasks containing nutrient broth showed that four strains of Bacillus cereus and one isolate of Stenotrophomonas maltophilia exhibited the greatest inhibition (> 77%) to the fungal biomass. All Trichoderma spp. inhibited the mycelial growth of the pathogen by between 51 and 71%. Seed microbiolization failed to supress seedling blight under greenhouse conditions. Nevertheless, Trichoderma isolate Th2 as well as guazatine, significantly (p = 0.05) increased plant dry weight compared with untreated pathogen-infested seeds. Suitability of in vitro methods to evaluate potential efficacy of antagonists as biocontrol agents is discussed.