Biological Control of Geranium Rust byBacillus subtilis

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Abstract
Twelve strains of Bacillus were isolated from the leaves of geranium cultivars and tested for their effect on spore germination of Puccinia pelargonii-zonalis, the causal agent of geranium rust. Of these, strain 3 of B. subtilis, isolated from a rust-infected geranium leaf, inhibited spore germination and reduced the incidence of rust pustules on inoculated leaves in the greenhouse. The inhibitory agent(s) was present in the culture filtrate of strain 3 and was retained in or on washed bacterial cells. The culture filtrate was most inhibitory in decreasing the amount of pustules per leaf area, followed by the washed bacterial cell treatment. Nutrients affected the pathogen/antagonist interaction. Cells cultured and applied to leaves in nutrient broth were more effective in reducing rust development compared with a culture filtrate. Eugon broth as a culture medium enhanced the antagonistic effect compared with cultures produced in nutrient broth. When bacteria were applied for different periods before inoculation with rust spores, the antagonistic effect persisted for at least 4 days after application.