Downy Mildew of Viburnum

Disease Information

Downy Mildew

Symptoms and damage

Downy mildew on viburnum is caused by the fungus, Plasmoparaviburni. This foliar disease occurs and spreads rapidly during cool to warm weather conditions coupled with periods of leaf wetness. Initially, this disease appears as light green spots on the upper leaf surfaces. The spots enlarge to form angular patches between the leaf veins. On the lower leaf surfaces downy grayish white fungal growth appears.

Downy mildew disease is different from powdery mildew in that the fungal growth is observed on the lower rather than on the upper leaf surface. The infected areas redden and then turn brown as the leaf tissue dies. Infections of the foliage in the spring can result from splashing spores produced by the diseased foliage remaining on the ground from the previous year.


 As with other foliar diseases, downy mildew can be prevented by keeping the foliage as dry as possible. When planting viburnum or other nearby plants, allow for adequate plant spacing. Prune back adjacent shrubs or overhanging tree limbs. These steps will improve air circulation around the plants and aid in the drying of foliage. Rake up and destroy infected fallen leaves.

If fungicides are necessary, sprays should adequately cover both the upper and lower leaf surfaces. Fungicides labeled to control downy mildew include mancozeb, chlorothalonil, or copper fungicides .


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