In this study, we evaluated the efficiency of six isolates of Saccharomyces cerevisiae in controlling Colletotrichum acutatum, the causal agent of postbloom fruit drop that occur in pre-harvest citrus. We analyzed the mechanisms of action involved in biological control such as: production of antifungal compounds, nutrient competition, detection of killer activity, and production of hydrolytic enzymes of the isolates of S. cerevisiae on C. acutatum and their efficiency in controlling postbloom fruit drop on detached citrus flowers. Our results showed that all six S. cerevisiae isolates produced antifungal compounds, competed for nutrients, inhibited pathogen germination, and produced killer activity and hydrolytic enzymes when in contact with the fungus wall. The isolates were able to control the disease when detached flowers were artificially inoculated, both preventively and curatively. In this work we identified a novel potential biological control agent for C. acutatum during pre-harvest. This is the first report of yeast efficiency for the biocontrol of postbloom fruit drop, which represents an important contribution to the field of biocontrol of diseases affecting citrus populations worldwide.

Isolates of Trichoderma spp. (ACB-14, ACB-33, ACB-37, and ACB-40) and Bacillus subtilis (ACB-66, ACB-69, ACB-77, and ACB-83) were tested separately or in mixtures for suppression of postbloom fruit drop in citrus, caused by Colletotrichum acutatum. This work aimed at: (i) determining the incubation time and temperature for production of cells of biocontrol agents; (ii) determining the effect of the isolates, separately or in mixture on the germination conidia of C. acutatum; (iii) evaluating the efficiency of antagonistic isolates on detached citrus flowers and under field conditions. The results of the interactions in vitro showed that there was little differentiation in cell production among the species, and the optimum temperature was 27°C. The best time for multiplication of bacterial cells was 36 hours, whereas for Trichoderma, the production of conidia continued to increase up to 120 hours of incubation. The mixtures of the ACB-77 plus ACB-66, ACB-33, or ACB-37 inhibited pathogen germination from 84% to 89%. Studies with detached citrus flowers showed that ACB-69 alone gave 99% control. The use of mixture ACB-69 plus ACB-37 proved to be viable in the control of disease under field conditions, but the efficiency of the control was lowest than the obtained by applications of ACB-69 alone.