Grow viburnum--Latin for "the wayfaring tree"--and experience a profusion of glossy, red berries set against dark green foliage, perfect for accenting a landscape. Though they're called cranberry bushes because of their cranberry-like fruit, viburnum are not related to the true cranberry (Vaccinium spp.). Popular varieties include the high-bush cranberry (V. edule) and American cranberry bush (V. trilobum). The plants typically reach a height of 10 to 15 feet, according to Ohio State University Extension. Though viburnum shrubs are relatively hardy, several diseases can attack them.
Powdery mildew is very common on viburnum shrubs grown in shady garden areas, according to the University of Maryland. Numerous fungi cause the disease, which creates a thin layer of gray or white dust on the shrub's foliage. It typically appears during weather periods of prolonged moisture. Pruning surrounding shrubs and foliage to increase airflow and sunlight on the shrub will help. A fungicide formulated with sulfur or potassium bicarbonate can also control the problem. Spray the shrub with fungicide as soon as mildew begins to appear.
Fungal Leaf Spot
Fungal leaf spot is rarely lethal to the shrub, according to Clemson University, but it can cause leaf drop. Symptoms include brown or red spots on the leaves, which start small but can slowly cover the entire leaf. It usually appears during warm, humid summer days when the fungi spores (Cercospora spp., Phyllosticta spp. and Phoma spp.) are stimulated to develop. Clemson recommends a fungicide formulated with myclobutanil or chlorothalonil, administered every 14 days until the leaf spots disappear.
Sooty mold begins as a strictly aesthetic problem but, if left untreated, will kill the viburnum shrub by blocking the plant's ability to absorb energy-producing sunlight. The problem starts with aphids, which secrete a sticky substance called honeydew. Honeydew is the prime growing medium for sooty mold, which slowly grows over the entire bush and leaves it looking like it was covered in soot. The viburnum varieties most susceptible to aphids are the European viburnum and the snowball viburnum. Treat aphids with a standard insecticidal soap. Any standard fungicide, such as a fixed copper spray, will kill the mold if it is already present.
The Botryosphaeria spp. fungus causes canker sores on the plant where the shrub was cut or damaged and did not heal correctly. Over time, the fungus spreads and causes the shrub's wood to rot. This can make entire branches wilt and die. The best solution is to prune off the diseased wood, according to Clemson University. Cut it back with a standard saw or pruning shears. Disinfect your tools between prunings to avoid spreading the fungus. Mix 1 part household bleach with 9 parts water, and dip the pruning tools in the solution for five minutes.
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