By David Strickland and Kerik Cox
Cornell AgriTech
Disease Snapshot: Blueberry Powdery Mildew
Causal pathogen: Microsphaera vaccinii
When to watch for it: July through season’s end
First line of defense: Use of resistant cultivars
Summary:
The signs of M. vaccinii infection do not typically appear until midsummer, when white fungal growth appears on the upper leaf surface. Leaf reddening may occur as chlorotic spots with reddish borders, and the leaf surface may become puckered. This reddening may be mistaken for symptoms of red ringspot virus (RRSV) infection but can be distinguished by the presence of water-soaked areas on the leaf’s lower surface opposite the chlorotic areas. If left uncontrolled, blueberry powdery mildew may lead to premature defoliation and a long-term reduction in bush productivity.
The planting of resistant cultivars is the best option for management of powdery mildew. Fungicide applications should begin just after petal fall, which targets primary inoculum. If these infections are managed early, an epidemic can be avoided. Suggested products are oils (e.g. stylet oils) or sterol biosynthesis inhibiting fungicides such as propiconazole (on 14-day intervals). Remember that fungicide products containing sulfur should not be applied within a 2-week period of an oil product application due to the potential for phytotoxic effects. Postharvest burning of infected leaf matter has not been shown to decrease the incidence of disease in subsequent years.