The present study evaluated the antagonistic potential of different isolates of Bacillus subtilis to Colletotrichum acutatum, causal agent of post-bloom fruit drop of citrus (Citrus spp.) (PFD), under laboratory and field conditions. Sixty four isolates of B. subtilis, four isolates of Bacillus spp. and one isolate of B. thuringiensis were evaluated in relation to their ability to inhibit the development of the plant pathogen in paired cultures and their production of metabolites with antimicrobial activity. The most promising isolates were tested for the control of the disease under field conditions. All the Bacillus spp. isolates induced strong inhibition on the growth of the plant pathogen. The isolates of Bacillus spp. produced, in vitro, metabolites capable of inhibiting the mycelial growth of C. acutatum, and they maintained their activities in enough concentrations to cause the inhibition, even after submitted to 120 ºC, for 20 min. The inhibition of C. acutatum by B. subtilis was due to antibiosis. Among seven isolates of B. subtilis tested for the disease control, in natural conditions, the BCA-69 differed from the control, and was statistically similar to benomyl, presenting lower percentage of symptomatic flowers and higher average number of effective fruits. An isolate of each of the species of Trichoderma viride, T. pseudokoningii and T. aureovirde were also tested, in field conditions, for controlling PFD, and none differed from the control. The percentage of symptomatic flowers was more efficient than the average number of effective fruits (ANEF) in determining the effect of the applications of the biocontrol agents on disease control.
