Alternaria Leaf Spot (Alternaria tenuissima) occurs primarily in the spring during prolonged periods of cool wet weather, when spores are produced in abundance. In most cases only lower leaves are affected; however, instances do occur when severe infection completely defoliates the plant. Because of its importance as a postharvest fruit decay organism, an increase of the fungus on blueberry leaves in May can result in considerable damage to fruit after harvest. Leaf symptoms appear as circular to irregular-shaped brown to gray lesions surrounded by a red border. Prolonged periods of high humidity favor disease development.
Bill Cline, North Carolina State University
Cline, Bill. Leaf Diseases of Blueberry. Retrieved 01 July 2010.