Viburnums are tough, long-lived landscaping shrubs that produce showy spring blossoms, bright fruit and colourful fall foliage. They usually require little maintenance and suffer few diseases. Insects are probably the cause of curling leaves on a viburnum. In small numbers, they require no treatment. If infestations are severe, though, treat them with pesticides, following package directions carefully.
Small grey or green aphids suck the juice from viburnums, causing curled leaves and distorted growth. These insects are most likely to attack snowball viburnums or European cranberry bush varieties. The damage is usually insufficient to warrant treatment, but in severe cases, spray the leaves with horticultural oil or insecticidal soap. Spray both the tops and bottoms of the leaves, thoroughly coating aphids. Make three applications at five- to seven-day day intervals. Do not spray when temperatures are above 29.4 degrees C to prevent plant damage.
Flower thrips are tiny, fast-moving insects difficult to see with the naked eye. They also suck the juice from viburnum leaves and stems, causing curled leaves, distorted blossoms and silver streaking on the leaves. Plant flowering annuals and perennials, such as trumpet vine, catmint or petunias in your yard. These nectar-producing plants attract beneficial insects that feed on thrips. Remove weeds and debris from around the viburnum, as they act as a breeding ground and habitat for the insects. Spray thrips with insecticidal soap, coating leaves completely.
Powdery mildew is a fungal disease that affects viburnums during warm, humid weather. It is more prevalent on bushes planted in the shade and reaches its peak in late summer. The disease can cause leaf curling or distortion, but its main symptom is a fuzzy, white or grey film on the leaves. Plant viburnum away from walls and spaced so air can circulate freely around them. Remove dead and diseased leaves promptly. Spray viburnums with a fungicide labelled to treat powdery mildew, such as fungicides containing myclobutanil, triadimefon, propiconazole, thiophanate-methyl, triforine or horticultural oil. Do not use sulphur-based fungicides, as these harm viburnum.
Healthy viburnum plants are better able to fend off insect and disease infestations. Plant viburnums in well-drained soil, in full sun or partial shade. Water viburnum as needed to keep the soil slightly moist. Prune out old, dead and diseased wood or branches that rub against each other.