Informational table showing disease name, symptoms, pathogen/cause, and management of Hydrangea diseases. Disease Symptoms Pathogen/Cause Management Botrytis Blight Petals turn brown and fall. Leaf spots form, especially where faded petals have fallen. Flower buds are killed before opening. Botrytis cinerea Space plants to insure good air circulation. Maintain low humidity. Avoid watering late in the day. Remove crop debris. Apply a fungicide to protect plants.
Cercospora Leaf Spot Tan spots with reddish-brown halos develop on leaves. Cercospora Water in a manner that keeps moisture off the leaves. Apply a fungicide to protect plants. Powdery Mildew Yellow areas form on leaves. These may become purplish. White, cottony fungal growth forms on the lower surface of the leaf. Erysiphe polygoni Apply a fungicide as soon as any mildew is observed.
Virescence Flowers are green and may be stunted. Leafy shoots grow from the flower parts. Plants decline and die. Phytoplasma Discard infected plants. Maintain good insect control. Viruses Leaves may be mottled, have yellow spotting, dead flecks, line or ringspot patterns. Plants may be distorted or have flower color breaking, or few flowers. Hydrangea ringspot, tomato ringspot, impatiens necrotic spot Plant virus-indexed cuttings. Maintain good insect and mite control. Destroy infected plants.

Cercospora leaf spot.