Oval White Spots Under Hydrangea Leaves

Identification

• Hydrangeas are prone to infection from Erysiphe polygoni, which causes powdery mildew. The disorder affects a number of hydrangea species, particularly those growing in greenhouses.

Damage

• Powdery mildew is characterized by the appearance of round to oval, white, cottony spots and patches on foliage undersides. The cottony texture results from the fine threads of fungal growth. The spots have feathery margins and continue to spread with time, covering foliage tops, stems and flowers. The disease stunts plant growth and discolors the flowers.

Control

• Growing plants under optimal conditions is among the best preventative measures. Avoid areas of high humidity, poor ventilation and low light as these favor fungal growth. Water plants during early morning to allow foliage to dry well. In cases of severe infection, use recommended fungicides including horticultural oils, sulfur-based products and potassium bicarbonate.