Greenhouse - Botrytis

Botrytis is a common problem in crowded greenhouses, especially during cloudy weather when plants are in bloom. The following is more information.

The extended period of cloudy weather has resulted in Botrytis Blight on closely spaced, tender spring annuals and herbs. Growers are all too familiar with the fuzzy grayish-brown spores (Gray Mold) that are easily spread on air currents and by water splash. Sometimes, less obvious symptoms show as tan colored cankers on stems that can cause entire branches of plants to wilt, such as Fuchsia hanging baskets, while the rest of the plant appears healthy. If left alone, more branches wilt one by one. Fuzzy spores will eventually form on stems deep inside the canopy.

Botrytis blight is best managed by combining environmental and cultural controls with chemical controls.

Environmental controls If you see active fungal sporulation, begin by first reducing the humidity in the greenhouse. Heat and vent in the evening (3x) and early in the morning to exhaust the moist, humid air and replace it with cooler, drier air.

Cultural controls Promptly removing severely infected plants, such as tender herbs, helps to reduce the disease pressure. Place severely infected plants that are covered with the grayish spores, in a plastic bag before removing from the greenhouse. This will help reduce potential spread of the easily airborne spores. Keep garbage cans covered so spores are not released into the greenhouse via air currents. Water early in the day, so foliage can dry rapidly. As plants are sold, provide more space to your existing crops to reduce humidity levels within your crops.

When Botrytis blight develops on herbs, cultural and environmental management is especially crucial because many of the fungicides labeled for ornamentals are not labeled for herbs.

Chemical Controls
Apply preventative fungicides before cutting back plants so the fungal spores are not released onto open wounds as workers handle plants. On ornamental crops, a number of fungicides are labeled for use against Botrytis Blight. The following are some of the fungicides listed under Botrytis management in The New England Recommendation Guide:

Azoxystrobin (Heritage), Bacillus subtillis (Rhapsody, Cease), chlorothalonil(Daconil, Echo and others), copper salts (Camelot), copper sulfate pentahydrate (Phyton 27), fenhexamid (Decree), fludioxonil (Medallion), iprodione (Chipco, 26GT and others), mancozeb (Dithane, Protect), polyoxin D zinc salt (Endorse), trifloxystrobin (Compass), and triflumizole (Terraguard). Combination products are also available containing some of these active ingredients.

Growers often rely on fenhexamid (Decree) which is a non-systemic fungicide with both protective and curative activity, chlorothalonil (Daconil) or iprodione (Chipco, 26 GT). Rotate among mode of action groups to delay the buildup of resistant strains. There are reports of widespread resistance to the benzimidazole fungicides (Cleary’s 3336 and Fungo Flo) as well as resistance to iprodione.

From the current issue of the New England Greenhouse Uptdate http://www.negreenhouseupdate.info/greenhouse_update/index.php