Viburnum opulus, also known as the snowball bush, is an ornamental shrub that produces white flowers and small red berries. Fungal diseases and insect infestations can cause leaf wilt of the snowball bush. Identification and prevention of known pathogens and insect pests are the first line of defence for controlling leaf wilt.
The snowball aphid (Ceruraphis viburnicola) is a small insect pest that sucks nutrients from the foliage of the snowball bush. The grey to blue tint identifies the snowball aphid from other aphid species, which have a green to white appearance. Aphid eggs overwinter on viburnum and feed on foliage growth during the spring. Symptoms of infestation consist of visible insect movement, leaf wilt, leaf curl, distorted new leaf growth and twisted branches. Mild infestations rarely cause severe damage. However, Washington State University recommends systemic pesticide application every seven to 10 days as soon as leaf buds open to prevent severe infestations.
Botryosphaeria Dieback and Canker
Botryosphaeria dieback and canker is a fungal disease of the snowball bush, caused by several botryosphaeria fungi. The disease infects the snowball bush through open wounds, which can occur from insect or pruning damage. Trees suffering from stress, disease or insect infestations are most susceptible to infection. Botryosphaeria dieback and canker causes leaf wilt, branch dieback, brown to red wood under bark and cankers. The Virginia Tech Cooperative Extension recommends increasing vigour with regular watering and fertilisation, avoiding mechanical injuries from pruning and disinfecting pruning equipment to prevent infection.
Identified by its dark green to black colour, the bean aphid is an insect pest of the snowball bush that can cause substantial foliage damage. Like the snowball aphid, the bean aphid uses tiny mouthparts to suck nutrients from leaves and new shoot tip growth. Symptoms of heavy infestation consist of leaf wilt, leaf yellowing, distorted leaves and honeydew residue. Aphids secrete the byproduct honeydew when feeding on plant foliage. This can lead to sooty mould growth, which is a fungal disease that favours honeydew residue for infection. Natural insect predators are the most common method of control for mild to moderate bean aphid populations. Apply horticulture oil with water over the entire plant to prevent aphid infestations. However, chemical control is effective for heavy aphid populations.
Verticillium wilt is a serious fungal disease of the snowball bush. The disease causes leaf wilt, leaf curl, leaf yellowing and eventual leaf death. Verticillium fungi are soil borne and infect trees through the root system. Once infected, tree death may occur rapidly or for a year or longer, depending on the severity of the infection. Soil tests from private or university-based labs can determine if verticillium fungi are present in soil. However, there is no cure once infection occurs.