Biocontrol of Phyllosticta citricarpa by Bacillus spp.: biological and chemical aspects of the microbial interaction

Citrus fruits are the most produced fruits in the world, but they are threatened by several pathogens, including the fungus Phyllosticta citricarpa, the causal agent of citrus black spot (CBS). The fungus affects most citrus species and the infection results in economic losses in citrus-producing areas. This disease causes the aesthetic depreciation of fresh fruit, impairing its commercialization. As an alternative to the use of synthetic fungicides to control the pathogen, the biological control, using bacteria of the genus Bacillus, is highlighted. Such microorganisms enable biocontrol by the production of volatile organic compounds (VOC) or non-volatile. Therefore, this work aimed to evaluate the production of VOC by isolates of Bacillus spp. grown in different culture media; to evaluate the effects of these compounds on the evolution of CBS lesions in orange fruits; to study the effects of VOC on resistance induction in orange fruits; to evaluate the effects of VOC on P. citricarpa morphology in CBS lesions, and to identify the produced VOC. Tryptone soya agar (TSA) and tryptone soya broth (TSB) media used to culture the bacterium resulted in up to 73% pathogen inhibition by VOC. Volatile compounds from Bacillus spp. ACB-65 and Bacillus spp. ACB-73 when cultured in TSB culture medium provided 86% inhibition of freckles that evolved to hard spots. The volatile fractions produced by the bacteria were identified as alcohols, ketones, amines, ethers, aldehydes and carboxylic acids that can serve as arsenal against the phytopathogen. The present work demonstrated the potential of VOC produced by Bacillus spp. in the control of P. citricarpa.