Seven microorganisms were evaluated for their biocontrol potential against Monilinia vaccinii-corymbosi which causes mummy berry disease through stigmatic infection of blueberry flowers: the bacteria Bacillus subtilis, B. mojavensis, B. mycoides, and Pantoea agglomerans; the yeast Wickerhamiella australiensis; and the filamentous fungi Trichoderma harzianum and Gliocladium roseum. All organisms were tested for antibiosis using dual cultures and for nutrient competition (niche overlap) using Biolog microplates. Their population dynamics on detached blueberry flowers were investigated to estimate epiphytic fitness, and such flowers, co-inoculated with M. vaccinii-corymbosi, were used to determine efficacy in reducing pathogen ingress into the styles. The most promising candidates were B. subtilis, which showed strong antibiotic activity and considerably reduced pathogen ingress, despite having limited epiphytic fitness; G. roseum, which exhibited complete niche overlap with the pathogen and consistently suppressed stylar penetration; and P. agglomerans, which showed favorable population dynamics but produced more variable results in reducing stylar infection.