Powdery mildew epidemics in the nursery: role of chasmothecia in epidemic initiation and fungicide regimes to limit chasmothecia formation

Abstract

Powdery mildew caused by Podosphaera cerasi is a detrimental fungal disease for the sweet cherry industry. Epidemics in orchards are initialized by ascospores that are released from chasmothecia generated in the fall of the previous year, however, the initiation of epidemics in nurseries has not been explored. The current study aimed to evaluate the origin of disease epidemics in Washington State cherry tree nurseries and to evaluate the efficacy of different fungicide regimes to suppress chasmothecia numbers and ascospore viability. Greenhouse and lab assays in the present study showed that chasmothecia from the nursery are not contributing to powdery mildew epidemics and the disease is originating from an external source such as wind-blown conidia from neighboring orchards. Additionally, intensive synthetic fungicide regimes and late-season JMS Stylet-Oil applications were found to be viable options to inhibit chasmothecia formation. As chasmothecia with a nursery origin are not the initial inoculum, late season fungicide applications to limit chasmothcia formation should only be applied in an orchard setting. Orchard managers can limit epidemics by implementing late-season fungicide applications into their IPM plan that target chasmothecia. Future research should evaluate the dispersal of conidia throughout the cherry tree growing industry of Washington and their role in powdery mildew epidemics in nurseries. Moreover, the physiological and genetic differences between chasmothecia in the nursery and the orchard should be evaluated.