Viburnum is a flowering shrub that is widely used in landscaping throughout the United States. The viburnum genus includes more than 150 species, most of which are shrubs but some that are tree-sized. Most species are relatively resistant to pests and disease, but there are a few of the latter that might cause problems. Such diseases include fungal leaf spots, algal leaf spots, powdery mildew, downy mildew, Botryosphaeria dieback/canker and armillaria root rot.
Fungal Leaf Spots
Fungal leaf spotting on virburnum, usually caused by one of the species in the Cercospora, Spora and Phyllosticta fungus families, shows up most often as irregularly shaped, depressed spots that are black in colour. Although unsightly, they do not usually threaten the life of the plant, according to Clemson University's Home & Garden Information Center. Minimise this type of leaf spotting by keeping foliage as dry as possible. Give plants plenty of breathing room, water them at the roots and not over the foliage and hand-remove affected leaves.
Algal Leaf Spots
Caused by a species of algae called Cephaleuros virescens, algal leaf spotting on viburnum most often shows up as circular lesions that are pale green or red in colour, according to the Alabama Cooperative Extension System (ACES). Plants are most susceptible to infection when conditions are cool and moist. Treat with copper fungicide as soon as symptoms appear to control this type of leaf spotting.
This disease, caused by the Erysiphe sparsa fungus, usually targets young plants and shoots and is most commonly seen during the late summer growing period, according to Oregon State University Extension Service. A powdery growth may be found on affected plant tissue. To avoid this disease, plant viburnum in full sun, remove fallen leaves from beneath the plant and give plants plenty of breathing room. To prevent this disease, spray plants with fungicide before symptoms appear.
Caused by the Plasmopara viburni fungus, downy mildew first appears as light green spots on the upper surface of leaves while the underside of leaves may show a light grey fungal growth. The susceptibility of plants to mildew infection can be minimised by keeping the plant's foliage as dry as possible. Chemical preventive methods include thorough spraying of leaves---both top and bottom surfaces---with fungicides.
This disease is caused by various species of the Botryosphaeria fungus and is most likely to infect plants that are weakened from water stress or plant wounds. The infection first appears as small lesions on branches and other plant tissue but gradually spreads. Its spread may be slowed by pruning off affected areas of the plant.
Armillaria Root Rot
Also known as mushroom root rot, this fungal infection causes a yellowing and thinning of foliage and is caused by various species of the Armillaria fungus. The University of Florida's Institute of Food and Agricultural Sciences says that bark removal at the base of the plant will reveal white, mat-like fungal growth (mycelium). Clusters of honey-coloured mushrooms can sometimes be observed emerging from the trunk or roots nearby. Once this infection has taken hold, remove and destroy the entire plant. Recondition the soil before replanting in this area.