Warm, gray and fuzzy?

Don’t allow Botrytis to flourish in your greenhouse.

Botrytis cinerea is a fungus that causes disease on greenhouse ornamentals including leaf spots, blighting, stem cankers, and damping-off of young seedlings (Photos A-C). Blighting is the most common symptom and affects leaves, petioles, blossoms and stems. Botrytis produces large masses of gray conidia or spores (Photo D) – hence the name “gray mold” – that are carried on air currents to healthy plants where blight can become established. On bedding and stock plants, Botrytis typically becomes established and produces conidia on older lower leaves that are near the moist soil surface and under the plant canopy. Botrytis also readily infects the broken or cut stem surface of stock plants and progresses downward, causing a dieback of the entire stem.

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Photo A. Blighting; Photo B. Stem cankers on geranium; Photo C. Damping-off of petunia; Photo D. Microphotograph of Botrytis conidia on a leaf surface.
Photo A. Blighting; Photo B. Stem cankers on geranium; Photo C. Damping-off of petunia; Photo D. Microphotograph of Botrytis conidia on a leaf surface.

Botrytis has a wide host range including many ornamentals, vegetables and some herbs. It continues to cause significant losses at all stages of floriculture production. Botrytis spores can survive at least three weeks before germination, and can be shipped between greenhouses. Failure to control Botrytis at one production stage in any crop can have negative impact on subsequent stages and other susceptible crops in the same facility.

When the weather is moist and humid, susceptible plants may need to be protected from Botrytis infection. Monitoring whether lower leaves are showing the beginnings of brown or gray fuzziness can signal the need for implementing control measures. While fungicides (Table 1) alone cannot control Botrytis blight, they are often necessary partners in a successful disease management program that includes environmental control and sanitation. Even the best fungicides will not provide the needed control if the relative humidity is high and exceeds 85 percent and the leaves stay wet for six hours or more in a 24-hour period. Plants may also be susceptible if they become wet from water dripping from overhead, dew, or condensation. This moisture allows the Botrytis conidia to germinate and penetrate the plant.

Table 1.

Watering in the morning so that the foliage can dry rapidly is one way to minimize Botrytis. Practices that reduce the relative humidity are also helpful and include spacing plants further apart and providing good air circulation. Reduce the relative humidity for a minimum of 24 hours immediately following the harvesting of cuttings to help “dry” the wounded stems and thereby limit stem blight.

This research was funded in part by Floriculture Nursery and Research Initiative of the Agricultural Research Service under Agreement #59-1907-5-553 and by the American Floral Endowment.

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