Effect of desiccants and herbicides on germination of pseudosclerotia and development of Apothecia of Monilinia vaccinii-corymbosi

Pseudosclerotia (infected, mummified fruit) are the only source of primary inoculum of Monilinia vaccinii-corymbosi, the causal agent of mummy berry disease of blueberry. Laboratory applications of potential inhibitors of carpogenic germination were made to pseudosclerotia at three distinct developmental stages, i.e., ungerminated pseudosclerotia, pseudosclerotia with emerging stipes, and those with mature apothecia. Potential inhibitors evaluated included soybean oil and ammonium thiosulfate (two desiccants used experimentally as bloom thinners in fruit crops) and diuron and simazine (two commonly used herbicides), each applied in an aqueous suspension with 3% Latron B-1956 surfactant. Various aspects of carpogenic germination including the percentage of pseudosclerotia that produced stipes or apothecia, the number of stipes or apothecia per pseudosclerotium, the percentage of stipes that developed into apothecia, longevity of stipes and apothecia, and ascospore numbers were assessed. Compared with water, application of ammonium thiosulfate (2%) and diuron (2%) reduced stipe and apothecium production when sprayed on ungerminated pseudosclerotia, but these reductions were generally not significantly different from those achieved with Latron B applied alone (P > 0.05). The two compounds, however, completely inhibited the development of stipes into apothecia when applied to pseudosclerotia with stipes and caused a >3-fold reduction in apothecium longevity when applied to pseudosclerotia with mature apothecia. Application of simazine (2%) before germination or at stipe emergence resulted in the development of malformed apothecia from which no ascospores were recovered; stipe and apothecium longevity were also reduced. Soybean oil (15%) and Latron B applied alone had weak or inconsistent effects on most aspects of carpogenic germination of pseudosclerotia, although both compounds, when applied at stipe emergence, significantly reduced ascospore numbers in subsequently formed apothecia. The results suggest that diuron and simazine applied for weed control in commercial blueberry plantings may have beneficial side effects in reducing carpogenic germination of pseudosclerotia. The strong inhibitory effect of ammonium thiosulfate on all aspects of carpogenic germination, along with its value as a nitrogen fertilizer and ancillary herbicide, warrants further evaluation of this compound's performance and economics in the field.