Effect of Bacillus subtilis and chitosan applications on green mold (Penicilium digitatum Sacc.) decay in citrus fruit

• Families of iturin A, fengycin and surfactin are the major CLP antibiotics present in crude extracts of B. subtilis ABS-S14 culture medium.
• Fengycin and iturin A were responsible for the inhibitory activity of B. subtilis against P. digitatum.
• Antibiosis is the dominant mechanism mediating the reduction of P. digitatum induced fruit decay in mandarin fruit together with elicitation of induced systemic resistance.
• Systemic resistance was detected via increases of POX and PAL activity, after treatment by the antagonistic bacteria, its crude antibiotics and chitosan.

The suppression of green mold postharvest caused by Penicilium digitatum, was evaluated in mandarin fruit after application of Bacillus subtilis ABS-S14 endospores, a crude extract from its culture medium, cyclic lipopeptide antibiotics (CLPs) and chitosan. Their abilities to elicit the accumulation of the plant defense related enzymes; peroxidase (POX) and l-phenylalanine ammonia-lyase (PAL) were also tested. Both B. subtilis ABS-S14 endospores and a crude extract from its growth medium showed strong antagonistic activities against P. digitatum. The HPLC profiles of the crude extract indicated that the antifungal compounds produced by this strain of B. subtilis consisted of three clusters of the iturin A, fengycin and surfactin families of CLPs. The in vitro and in vivo inhibitory effects of the partially purified iturins and fengycins on retardation of the growth of P. digitatum were demonstrated, whereas the surfactins had no direct effect. Efficacy tests, using bacterial endospores, the crude extract and chitosan showed a significant reduction of fruit decay compared to those found in the presence of individual CLPs, and it was clearly demonstrated that the B. subtilis ABS-S14, itself and its crude extract and chitosan each induced the activities of POX and PAL in the infected flavedo tissues of mandarin fruit. They also co-enhanced the protection of fruit from the green mold pathogen P. digitatum.