Botrytis blight on bigleaf hydrangea (Hydrangea macrophylla) blooms
Botrytis blight on Chinese dogwood (Cornus kousa)
These leaf spots on New Guinea impatiens (Impatiens hawkeri 'Sunpatiens') may look colorful, but they are probably caused by Botrytis blight, a fungal disease
Botrytis blight on rose (Rosa) blossom
Fungal spores of botrytis blight on New Guinea impatiens flower (Impatiens hawkeri group)
Botrytis blight, also know as gray mold, is a fungal disease caused by several species in the genus Botrytis. It affects the buds, flowers, leaves, and bulbs of many plants including African violet, begonia, chrysanthemum, cyclamen, dahlia, geranium, lily, peony, rose, and tulip. The extent and severity depend on weather conditions and cultural practices. This disease is the primary cause of decay in cut flowers.
Symptoms and Diagnosis
Botrytis blight causes buds and flowers to develop abnormally and turn brown. Flowers may have irregular flecks and brown spots; older flowers tend to rot quickly. Soft, brown spots appear on leaves, stems, and flowers following a cool damp period. Affected parts may be covered with a gray mold following damp, cool weather.
Botrytis fungi overwinter as sclerotia on dead plant debris in the garden. In the spring, spores form and spread by wind or splashing water to infect dying, wounded, or extremely soft plant tissues. Fungal mycelial strands (web blight) from previously infected plant parts can grow onto healthy plant parts and infect them. The fungus is capable of invading tissue during all periods of the growing season and multiplies rapidly in declining foliage, hence, the need for good sanitation.
Integrated Pest Management Strategies
1. Practice good sanitation. Remove and destroy all infected plant parts as soon as they are observed.
2. Avoid overcrowding. Give adequate space between plants to allow for good air circulation. The fungus thrives in areas that are cool and moist and where plants are overcrowded.
3. Do not overfeed. Avoid fertilizing with excessive amounts of nitrogen. This can cause tender growth that is very susceptible to the fungus. Get a soil test to guide fertilizer practices.
4. Avoid overhead watering. Water on foliage and flowers from overhead irrigation, especially on cool, cloudy days, promotes the disease. Try to keep buds and flowers dry. Water early in the day so the plants have enough time to dry off completely.
5. Use fungicides. Depending upon the susceptibility of the plant to this disease, spray every 10 days with a fungicide. Pesticides registered for use include copper, captan, chlorothalonil (Daconil), mancozeb, maneb, sulfur, and thiophanate methyl (Cleary 3336). Fungicides must be applied in advance of the disease as a protectant.
Strategies 1, 2, 3, and 4 (if organic fertilizers are used) are organic approaches. Of the fungicides listed in Strategy 5, consult the Organic Materials Review Institute (OMRI™) for appropriate organic copper or sulfur products.