Both bacterial and fungal diseases can affect euonymus shrubs. Over-watering and improper spacing may contribute to the development of fungal diseases and following good cultural practices can often prevent outbreaks. When plants begin to show signs of infection, you may need to use chemicals to fight the disease. Identifying the exact disease will help you determine the appropriate course of action.
Powdery mildew causes the growth of white or gray fungus on the upper sides of the leaves of infected euonymus shrubs. Leaves sometimes turn yellow and drop from the shrub. Severe infections may cause new leaves to curl. To prevent powdery mildew, plant euonymus in a sunny location and avoid watering the foliage. Provide plenty of room between plants to improve air circulation.
Spray shrubs with fungicides containing sulfur, thiophanate-methyl or myclobutanil to prevent outbreaks of the disease. Prune back and discard infected branches. Sterilize pruning shears with rubbing alcohol between each cut to prevent spreading the disease. If one fungicide is used repeatedly, powdery mildew may build up a resistance to it. Use different fungicides to prevent this resistance.
A soil-borne bacterium called Agrobacterium tumefaciens causes outbreaks of crown gall. Round, abnormal growths called galls form on the stems and roots of infected plants. Crown galls weaken euonymus shrubs and may cause branches to die. These growths inhibit the passage of water and nutrients through the plant. No cure is available, but shrubs with crown galls may survive for several years.
Cut back branches below the galls and sterilize your tool between cuttings. This bacteria survives in the soil for two to three years, so avoid planting other susceptible plants in the same area.
Cercospora Leaf Spot
Cercospora leaf spot causes brown spots to form on leaves. These spots may merge and cause the whole leaf to turn brown but this disease rarely leads to the death of an infected shrub. Clean up and discard of fallen leaves. Apply fungicides containing thiophanate-methyl according to the directions on the label.
Small brown spots form on the leaves and twigs of euonymus shrubs infected with anthracnose. This fungal disease begins during wet, cool weather and may cause significant defoliation. Spray shrubs with chlorothalonil or copper fungicides to prevent anthracnose. These fungicides will not control the disease once infection occurs. Clean up fallen foliage and prune back diseased branches.
The fungal disease scab causes small, grayish spots with orange edges on both leaves and stems. Large spots may contain a darker, raised center. Remove fallen leaves and discard of them away from the garden. Apply thiophanate-methyl to the shrub to control scab.