Biological control of infection of blueberry flowers caused by Monilinia vaccinii-corymbosi

Monilinia vaccinii-corymbosi, the fungus that causes mummy berry disease, infects open blueberry flowers via the stigma–style pathway, followed by internal colonization and subsequent mummification of the developing fruit. The potential for use of the commercial biocontrol products Serenade (Bacillus subtilis QRD137) and BlightBan (Pseudomonas fluorescens A506) to suppress floral infection in this pathosystem was investigated. B. subtilis exhibited marked antibiosis against the pathogen in vitro, reducing radial growth rates of M. vaccinii-corymbosi to less than one-half of those of the untreated control after 1 week of co-culture on potato-dextrose agar (P=0.0029). In contrast, P. fluorescens did not reduce fungal growth compared with the untreated control (P=0.3182). When Serenade was applied to stigmatic surfaces of detached blueberry flowers in the laboratory, population densities of B. subtilis, determined by dilution-plating, were variable 1 and 2 days after application, followed by a consistent drop in bacterial numbers after 3 and 4 days. In contrast, population densities of P. fluorescens increased exponentially 1 and 2 days after application of BlightBan and remained constant thereafter. The greatest mean population densities observed were 1.6 × 105 and 1.5 × 105 colony-forming units per stigma for B. subtilis and P. fluorescens, respectively. In a separate experiment, conidia of M. vaccinii-corymbosi were applied to detached flowers 24 h before or after application of the biocontrol products to flower stigmas. Serenade significantly reduced the number and growth rates of hyphae penetrating the stylar canal to less than one-fifth of those measured in the untreated control, whereas fungal suppression following application of BlightBan was less pronounced. Reductions in fungal growth rate in flowers treated with B. subtilis isolated from Serenade and reformulated by freeze-drying in skim milk were intermediate between the untreated control and the Serenade treatment, suggesting that part of the observed activity of Serenade was due to the formulation of that product. The results of these experiments identify Serenade as a promising candidate for further testing against mummy berry disease.