Fungal Leaf Spots
Viburnum is susceptible to several fungal leaf spot diseases including Cercospora, Phoma, Phyllosticta and Plasmopara viburni (downy mildew). Cercospora, Phoma and Phyllosticta leaf spot infections produce small angular spots with red to brown coloration on leaf tissue. As the diseases progress, the spots enlarge and merge. Downy mildew symptoms appear on the lower side of leaf surfaces. Symptoms first appear as light green spots and change to red and brown as the disease progresses. Prevent fungal leaf spot infection by increasing air circulation to prevent free moisture on leaf surfaces and apply fungicides to control severe infections.
Bacterial Leaf Spot
Bacterial leaf spot is a disease of Viburnum, caused by the bacterium Pseudomonas syringae. The disease initially causes sunken water-soaked leaf spots. As the disease progresses, the spots enlarge and turn brown, leaf growth becomes distorted and shoot dieback can occur. The bacterium overwinters in infected twigs and buds, so remove them to prevent perennial infection. Prune overgrown branches to increase airflow and avoid overhead irrigation to prevent infection. Copper-based bactericides help reduce the spread of new infections.
Algal Leaf Spot
Algal leaf spot is a disease caused by the parasitic algae, Cephaleuros virescens. Symptoms of infection consist of light green circular leaf spots that turn red to brown as the disease progresses. Algal infections are severe during cool, wet weather conditions and spread during windy and rainy weather conditions. Control algal leaf spot infections with copper-based fungicides as soon as symptoms appear and every 10 to 14 days thereafter.
Shoot blight is a serious fungal disease, caused by Botrytis cinerea. Symptoms initially appear as gray to brown leaf spots along the leaf margin. As the diseases progresses, the spots enlarge and spread to the entire leaf surface, leaves die and shoot dieback occurs. The fungus favors wet weather conditions for infection. Avoid freestanding moisture on leaf surfaces, increase airflow by pruning and providing adequate spacing and plant in sunny environments to prevent infection. Destroy infected plant tissue to prevent the fungus from overwintering and spreading.