Evaluation of Bacillus strains isolated from solanaceous phylloplane for biocontrol of Alternaria early blight of tomato

• Ninety-three phylloplane bacteria were screened for antagonism against Alternaria alternata.
• Four Bacillus strains capable of control early blight of tomato were selected.
• Antibiosis and fungi-stasis can be involved in antagonistic mechanisms.
• Hyphal structures damaged by bacterial secretion were observed in light microscope.

Bacillus biological control agents are powerful alternatives to chemical fungicides for suppressing plant diseases caused by fungi. In this work, 93 strains of spore-forming bacteria isolated from solanaceous phylloplane, were screened for in vitro antibiotic activity against Alternaria alternata, causal agent of tomato early blight. The twenty most active strains were characterized for their morphological and physiological traits (Gram reaction, production of endospores, antifungal volatile compounds and siderophores) and by M13-PCR DNA fingerprinting. Planta bioassays performed with four selected strains, were capable to decrease severity of Alternaria disease on tomato. These antagonistic bacteria were identified by 16S-rRNA partial gene sequencing, and results assigned them to strains related to Bacillus species. The evident inhibition zone observed in dual culture plates, suggested an antibiosis-like mechanism. While, API-ZYM enzymatic profiles indicated that strains could be potential ecological competitors. Consistently, light microscopy revealed the occurrence of Bacillus-induced malformations in the fungus’s vegetative structures, probably caused by secreted compounds.

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