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 d isease of blueberry. Laboratory applications of potential inhibitors of car pogenic germination were made to pseudosclerotia at three distinct developm ental stages, i.e., ungerminated pseudosclerotia, pseudosclerotia with emer ging stipes, and those with mature apothecia. Potential inhibitors evaluate d included soybean oil and ammonium thiosulfate (two desiccants used experi mentally as bloom thinners in fruit crops) and diuron and simazine (two com monly used herbicides), each applied in an aqueous suspension with 3% Latro n B-1956 surfactant, Various aspects of carpogenic germination including th e percentage of pseudosclerotia that produced stipes or apothecia, the numb er of stipes or apothecia per pseudoscIerotium, the percentage of stipes th at developed into apothecia, longevity of stipes and apothecia, and ascospo re numbers were assessed. Compared with water, application of ammonium thio sulfate (2%) and diuron (2%) reduced stipe and apothecium production when s prayed on ungerminated pseudosclerotia, but these reductions were generally not significantly different from those achieved with Latron B applied alon e (P > 0.05). The two compounds, however, completely inhibited the developm ent of stipes into apothecia when applied to pseudosclerotia with stipes an d caused a >3-fold reduction in apothecium longevity when applied to pseudo sclerotia with mature apothecia. Application of simazine (2%) before germin ation or at stipe emergence resulted in the development of malformed apothe cia from which no ascospores were recovered; stipe and apothecium longevity were also reduced. Soybean oil (15%) and Latron B applied alone had weak o r inconsistent effects on most aspects of carpogenic germination of pseudos clerotia, although bath compounds, when applied at stipe emergence, signifi cantly reduced ascospore numbers in subsequently formed apothecia. The resu lts suggest that diuron and simazine applied for weed control in commercial blueberry plantings may have beneficial side effects in reducing carpogeni c 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 evaluati on of this compound's performance and economics in the field.