Black leaf streak disease (BLSD; also known as black Sigatoka) caused by Mycosphaerella fijiensis Morelet is one of the main diseases affecting banana trees worldwide. Associated microorganisms in the phylloplane have been previously studied for their control. An exhaustive understanding of biocontrol mechanisms will potentiate the biological control agent. Therefore, strain Bacillus pumilus CCIBP-C5, isolated from banana leaves surface was evaluated for in vitro antagonism against M. fijiensis and the effect of culture filtrate on fungal growth and disease progress was determined. Bacterial cell and its diffused metabolites inhibited M. fijiensis growth. The culture filtrate of B. pumilus affected the mycelial growth, with antifungal effect on hyphal tip, membrane permeability and integrity. Also, a reduction of number of BLSD necrotic lesions and disease progression were found after application of the B. pumilus CCIBP-5 culture filtrate in banana plants. Results indicate that CCIBP-C5 has potential for biological control of BLSD.

The effect of antifungal activity of culture filtrate (CF) of strain CCIBP-C5, an isolate from a phyllosphere of banana () leaves, was determined on challenged banana plants. The CF was shown to decrease the fungal biomass and induce changes in banana plant. In this sense, at 70 days post inoculation (dpi), a lower infection index as well as a decrease in fungal biomass after 6 dpi was obtained in treated plants with respect to control ones. At the same time, changes in the activities of several enzymes related to plant defense responses, such as phenylalanine ammonia lyase, chitinases, β-1,3-glucanases and peroxidases were observed. These results indicate that CCIBP-C5 has a potential role for biological control of possibly due to the production of antifungal metabolites.

The metabolites of Bacillus subtilis CCIBP-M27 were evaluated as an antagonist of Pseudocercospora fijiensis. The culture filtrate did not inhibit ascospore germination but significantly reduced conidial germination and mycelial growth. Through microscopic analysis, deformations were observed as vacuolization and swelling in P. fijiensis mycelia when exposed to culture filtrate during 48 h. A similar response was induced by peptide-type compounds found on Bacillus subtilis CCIBP-M27 culture filtrate. The results obtained suggest that the in vitro antifungal effect of the strain CCIBP-M27 against P. fijiensis is related to the action of diffused metabolites such as proteins or peptide substances.

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