Botrytis (Gray Mold)
Thursday, 05 April 2007 13:53
Overcast cloudy weather this week has encouraged Botrytis Blight in greenhouses. Plants are filling out and tender flowers, leaves and stems are often infected. Look carefully under canopies and around the crowns of plants such as petunias for familiar fuzzy grayish-brown spores which are easily spread on air currents and by splashing water. Common symptoms of Botrytis are spotting and blight on flower petals, leaves and stems that eventually develop into soft fuzzy tissue. (Photos) Sometimes, less obvious symptoms show as tan colored cankers on stems that can cause entire branches of plants to wilt, while the rest of the plant appears healthy. This sometimes occurs on Fuchsia hangers when one branch wilts and drops its leaves while the rest of the plant looks fine - for a while. If left alone, more branches wilt one by one. The roots may look fine making the cause puzzling until fuzzy spores form on stems deep in the canopy.
To manage Botrytis, promptly clean plants, water early in the day, so foliage can dry rapidly and space plants to reduce humidity. Dispose of debris in plastic trash bags and avoid spreading spores by keeping the bag closed while moving it through the greenhouse. Reduce humidity by heating and venting several times in the evening and in the morning. See the factsheet, Reducing Humidity in the Greenhouse . Keep garbage cans covered so spores are not released into the greenhouse via air currents. Some growers place netting below hanging baskets to catch any flowers that drop.
Growers often rely on fenhexamid (Decree) which is a non-systemic fungicide with both protective and curative activity, chlorothalonil (Daconil) or iprodione (Sextant, 26 GT). Botrytis strains have reportedly shown resistance to Sextant and 26 GT, so rotate these with other products to delay resistance. There are also reports of widespread resistance to the benzimidazole fungicides (Cleary's 3336 and Fungo Flo). Cultural and environmental management is especially crucial for Botrytis blight on herbs or vegetable bedding plants in the greenhouse, because many of the fungicides labeled for ornamentals are not labeled for herbs and vegetable bedding plants.
More Information:
2007 New England Recommendation Guide - Botrytis section (PDF)
Photos of Botrytis on tomato plants and fruit
Photos:
Dropping flowers with botrytis
Botrytis on Lilies
Tina Smith
