The Botrytis battle

Botrytis blight is one of the most common diseases in the greenhouse affecting many different annuals and herbaceous perennials, vegetables, and herb bedding plants. Botrytis can cause leaf and flower spot and blights, stem cankers, damping off, and cutting root rot.

Plants may be attacked at any stage, but the new tender growth, and freshly injured tissues are the most susceptible. Botrytis blight produces its characteristic gray fuzzy appearing spores on the surface of infected tissues. Tan stem cankers can develop from blighted leaves on basil, fuchsia, calibrachoa and poinsettia. This can be especially problematic on hanging baskets with dense plant canopies, where it is difficult for sprays to reach the blighted leaves that are in contact with the growing media.

Air currents and splashing water can easily disseminate the Botrytis spores. Germination of spores and infection is dependent on a film of moisture for approximately 8 to 12 hours, relative humidity of 93 percent or greater and temperatures between 55°F and 65°F. After infection, colonization of plant tissues can occur at temperatures up to 70°F.
 

What can a grower do?

Because Botrytis can survive as a saprophyte, rapidly invade host plants, and produce abundant spores that are easily moved within the greenhouse on air currents, this disease is a formidable opponent. Growers need to utilize all of these control tactics: manipulation of environmental conditions (reducing humidity and duration of leaf wetness), sound cultural practices and the proper use of fungicides. Fungicides alone cannot control Botrytis, and this pathogen has a long history of fungicide resistance, making it important for growers to do the following:

• Follow proper sanitation and sound cultural practices
• Control weeds and remove plant debris before and during production.
• Dispose of diseased plants and debris in a plastic trash bag. Keep the bag closed to help prevent spreading spores to uninfected plants as the bag is removed from the greenhouse.
• Cover trash cans to prevent the airborne spread of spores from diseased plant tissue.
• Avoid growing ornamental hanging baskets above especially susceptible crops. The spent flowers dropping on plants below serve as an energy source for the fungus helping to encourage the development of Botrytis blight.
• Always water in the morning to reduce the length of time the leaves stay wet after irrigating. Rising temperatures during the day will evaporate water from the foliage, so the leaves stay dry.
• Adequate spacing of plants and use of wire mesh benches helps improve air circulation.
• Heat and vent two or three times per hour in the evening after the sun goes down and early in the morning at sunrise. Heating and venting can be effective even if it is cool and raining outside. Air movement, even in a closed greenhouse, helps reduce moisture on the plant surfaces and surrounding the plants. Using horizontal airflow (HAF) also reduces condensation.
• Proper plant nutrition (avoiding low calcium or excess nitrogen) and height management techniques help to prevent lush, overgrown plants thereby reducing humidity within the canopy.

Resistance has been reported to thiophanate-methyl (FRAC Group 1), iprodione (2) and fenhexamid (17). One rotation option, suggested by Ann Chase, is to alternate between pyraclostrobin and boscalid (7 and 11) and cyprodinil and fludioxonil (9 and 12). Chlorothalonil (M5), fenhexamid (17), polyoxin D zinc salts (19), trifloxystrobin (11), mancozeb (M3), azoxystrobin (11), fludioxonil (12), pyraclostrobin (11) and Bacillus subtiwlis can also be applied in rotation. At the end of the crop cycle, when crops are in flower, extreme care must be exercised in selecting fungicides to avoid leaving unsightly residues or inadvertently damage the sensitive flowers or bracts.

Pundt is an extension educator at the University of Connecticut and a frequent contributor to Greenhouse Management. Have a question? She can be reached at leanne.pundt@uconn.edu.