Growing Trends:

Quick Tip 1
Botrytis blight caused by Botrytis cinerea is one of the most prevalent and destructive diseases of greenhouse crops. Winter is a time when many growers battle Botrytis blight. But greenhouse conditions are usually conducive for disease development throughout the year, so constant vigil is required. Optimum temperatures for disease development are 70°F-77°F, but fungal growth occurs under a broad temperature range. Moisture often is more of a limiting factor than temperature. Eliminating moisture is a key to controlling Botrytis.

Botrytis causes a variety of diseases in addition to blights such as leaf spots, damping off and fruit and bulb rots. Senescent and dead plant tissues are readily colonized and may serve as a “base” from which the fungus spreads to healthy tissue. Affected tissue is soft and brown and sometimes has a water-soaked appearance.

Control is challenging and a preventive management program with scouting to detect disease early is imperative. An integrated strategy combining environmental management, cultural practices and fungicides will most effectively manage this disease.

 

Quick Tip 2
Clean growing areas before each new crop as well as during crop production. Remove old flowers and old or dead plants that could serve as a reservoir for sporulating Botrytis. Collect in plastic bags and secure so spores do not escape. Dispose of sealed bags outside of production areas. Protect plant wounds with a fungicide or remove wounded plants from the greenhouse. Use a protectant fungicide prior to cleaning the growing area; reduce humidity to less than 65 percent for three days after cleaning to minimize spore dispersal and spread. Remove old flowers and plant debris in refrigerators before storing harvested crops.

Start production with healthy vigorous plant material that can resist infection. Crops such as exacum, geranium and impatiens are especially susceptible, so look for cultivars with some disease tolerance. Locate susceptible plants in the driest, best ventilated areas.
Increase plant spacing to facilitate air circulation and thorough fungicide coverage and minimize leaf wetness. Closely monitor older plants with dense canopies, which are more prone to disease.

Keep plants well ventilated so humidity stays below 85 percent. Vent and heat in the evening and early morning to exhaust moist, humid air and replace it with cooler, drier air. Use horizontal airflow systems and open-mesh benches to improve air circulation. Reduce relative humidity for a minimum of 24 hours following the harvesting of cuttings to “dry” wounded stems and limit stem blight.

Avoid overhead irrigation especially during flowering. If this is the only method of irrigation available, irrigate early in the day and maximize the interval between irrigations so that plants dry as rapidly as possible.

 

Quick Tip 3
In some crops such as chrysanthemum, it is important to apply fungicides at an early stage when the lower foliage can be adequately covered by the chemical. Otherwise lower foliage may serve as a reservoir of spores that infects the flowers when the plants are larger and thorough coverage is more difficult.

There are reports of widespread resistance to the benzimidazole fungicides (Cleary’s 3336 and Fungo Flo) as well as resistance to iprodione. Alternate fungicide applications between materials with different modes of action to prevent resistance development.
Another strategy is to tank mix fungicides with different modes of actions or add protectant (contact) fungicides such as mancozeb, fenhexamid, copper or chlorothalonil to systemic fungicides such as trifloxystrobin or azoxystrobin.
 

Have a question? You can write Julie at jpnewman@ucdavis.edu.