Botrytis blight: Prevention and management tips

Published January 08, 2024

Editor's Note: This article originally appeared in the November 2025 print edition of Greenhouse Management under the headline “Botrytis blight.”

Botrytis blight on marigold
Mary Ann Hansen, Virginia Polytechnic
Institute and State University, Bugwood.org

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 nursery and greenhouse crops. Wounded or weakened plants are particularly susceptible to this disease. The responsible fungi spreads quickly in periods of high humidity or when there is moisture on plant tissue. It can survive year-round in greenhouses due to the relative humidity. Nurseries should be extra vigilant during rainy and drizzly weather conditions.

Source: Missouri Botanical Garden, Michigan State University Extension

The guide to botrytis blight

Symptoms: 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.

Life cycle: Botrytis fungi overwinter as sclerotia on dead plant debris. 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.

Management: 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.

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. Check your state’s extension for a list of registered-in-your-state fungicides, which must be applied in advance of the disease as a protectant. They also must be applied as a foliar spray to be effective in controlling botrytis blight. Rotate fungicides with different FRAC codes to help delay resistance with the pathogen.

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