Powdery Mildew (fungi – Oidium euonymi-japonici and Microsphaera alni): Powdery mildew is the most common and possibly the most difficult disease to control on Euonymous. It forms a flat, whitish growth on the leaves which can be easily rubbed off. Leaves may yellow slightly and drop off, but heavy defoliation is not a characteristic symptom. The fungus only invades the epidermal cells and, thus, leaves normally stay attached long after they are infected. Preventing infection of the new growth in the spring and planting in a proper site are keys to controlling powdery mildew. Cultural controls include: (1) planting in a sunny area; (2) not crowding the plants; (3) avoiding overhead watering; (4) pruning out and destroying heavily disease branches; and (5) raking and destroying fallen leaves. Chemical applications should begin before new foliage is infected. Fungicides will protect foliage from infection but will not remove the dense, white fungal growth from infected leaves. Adding a wetting agent to the spray solution will greatly improve coverage. Liquid household detergents make good wetting agents when one teaspoon is added for each gallon of spray solution.
Crown Gall (bacterium – Agrobacterium tumefaciens): Large, rounded galls with an irregular rough surface, appear on roots or stems. Plants lack vigor and may die back. Avoid wounding stems or roots. Prune out and destroy affected plant parts. Sterilize pruning shears in 90% alcohol or 10% bleach after each cut. Dig up and destroy severely affected plants.
Anthracnose (fungus – Colletotrichum spp.): Symptoms consist of small, brownish spots on the leaves with light colored centers. Tiny cracks in the leaf spots indicate fruiting structures of the fungus. Overhead watering makes the disease worse and considerable defoliation can result. Fungicide applications will help control this disease.
Leaf Spots (fungi – Cercospora destructiva, Phyllosticta euonymi, Septoria euonymi): Various sizes and colored spots on leaves. Collect and burn fallen leaves. Prune to thin out shrubs. Protect with fungicide sprays when needed.
Cotton Root Rot (fungus – Phymatotrichum omnivorum): Plants suddenly wilt and die. when plants are removed from the soil, the bark on the roots is decayed. (See the section on Cotton Root Rot)
Oedema (physiological): Small, raised masses of tissue on the underside of leaves. the raised mass of tissue later becomes rough and corky. May sometimes occur on stems. Control by reducing moisture level. In greenhouses, reduce humidity, improve air circulation and avoid overwatering during overcast periods.