James A. Chatfield, Ohio State University Extension, Northeast District/Horticulture and Crop Science; Nancy A. Taylor, Plant Pathology, C. Wayne Ellett Plant and Pest Diagnostic Clinic, Ohio State University; Erik A. Draper, Ohio State University Extension, Geauga County; and Joseph F. Boggs, Ohio State University Extension, Hamilton County/Southwest District.
In many years, this disease is first seen by early July and intensifies in August and September. With this year's hot, dry summer, we had to wait until late summer for downy mildew to show up. Symptoms were much less severe than normal, illustrating the key importance of moisture for development of many foliar diseases.
On upper leaf surfaces, symptoms start out as grayish to brown spots that are somewhat angular and margined by leaf veins. Eventually these lesions resemble leaf scorch and can coalesce into large areas and result in leaf drop. On the underside of the leaves, beneath the areas of upper leaf discoloration, the signs of whitish to grayish downy fungal growth of the pathogen (Plasmopara viburni) are seen.
Keeping the leaves as dry as possible and cleaning up infested leaves where the overwintering stages of the fungus will survive to next year are recommended for control. Fungicides are probably not warranted in most sites during dry summers such as 2002.