Symptoms on blueberries are different from those on most other plants, and could be mistaken for a virus or bacterial disease. The powdery mildew organism, Microsphaera vacinii, at first causes a yellow mottling on the upper leaf surfaces (Photo 1), but eventually the mottled areas develop into red spots with a lighter margin; both of these symptoms could be mistaken for a virus. Symptoms on the lower leaf surfaces consist of water-soaked areas (Photo 2) that turn reddish; these symptoms might make one think that the plants have a bacterial disease. The typical "powdery" patches as seen on other crops may be present, but often are not.
Photo 1: Early powdery mildew symptoms on upper surface of blueberry leaf. Credit: Tracey Olson, PA Dept. of Agriculture
Photo 2: Powdery mildew symptoms on lower surface of blueberry leaf. Credit: Tracey Olson, PA Dept. of Agriculture
Since disease development is favored by warm, dry weather, symptoms start to appear in mid-summer. The fungus overwinters in dormant buds, so inoculum can build up over time if not managed. In most cases, this disease has minor impacts on growth and fruit production, primarily causing infected leaves fall off prematurely. In rare cases impacts on growth can be severe.
Generally powdery mildew incidence on blueberries is not sufficiently severe to warrant a fungicide spray. However, if leaves are severely affected, fungicides such as Orbit, Tilt or Quash (all in activity group 3), or Pristine (activity groups 7 and 11) may be used. Be sure to follow label directions to avoid development of resistant fungal strains. Cultivars vary in resistance, but information on this subject is limited.
Thanks to Tracey Olson at the Pennsylvania Department of Agriculture for bringing the presence of this disease to our attention and for providing photos for this article.