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.
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.
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.
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 .