Protect tomato transplants in the greenhouse from fungal diseases

Overcast, humid weather favors fungal diseases on tomato transplants in the greenhouse. Watch for them and treat them now.

Greenhouse environmental conditions favor the development of gray mold, a disease caused by the fungus Botrytis cinerea. Gray mold causes irregular brown spotting or “blight” of leaves and stem cankers. This is the same Botrytis that infects a wide range of floriculture crops. Under humid conditions, the Botrytis fungus produces gray and powdery spores on the diseased plant parts that can be transported on air currents to cause disease on nearby healthy plants. Gray mold can be prevalent during cloudy periods in the spring, when conditions in the greenhouse are humid and foliage remains wet for an extended period of time.

Establishment and spread of Botrytis can be reduced by increasing air circulation through fans and reducing the relative humidity by venting or heating (depending on outside temperatures). Watering in the morning will help ensure that the plants dry by evening, thereby reducing the disease.

Botrytis gray mold on a seedling

Alternaria is another foliar disease caused by a fungus that can cause leaf spotting and a stem canker on tomatoes and other vegetable transplants in the greenhouse. This disease is not as common as gray mold, but can be destructive. Be especially aware in situations where relative humidity is high and the foliage is dense, resulting in long periods when the leaves are wet. Alternaria typically doesn’t become a problem until the plants are held in the greenhouse for an extended period due to a delay in shipping or planting.

Alternaria spot on tomato foliage

Fungicides are available to control Botrytis and Alternaria on tomato transplants in the greenhouse. Protect DF, Catamaran, and Tanos are among the fungicides that can be used in alternation to limit fungal blights on tomatoes in the greenhouse. Good coverage is critical because the plant canopy is dense at this time of the year. Since Botrytis and Alternaria are often established on the older leaves near the soil, good fungicide coverage can be challenging.

The key to managing Botrytis and Alternaria is to realize that fungicides alone will not solve a disease problem. If these fungicides are coupled with efforts to increase ventilation and reduce relative humidity, then successful disease control is more likely. Once transplants are moved out of the greenhouse and are placed outside to be hardened off, field-based fungicides (chlorothalonil [Bravo, Echo, Equus] Tanos, Quadris, and mancozeb are examples) could be used until the Botrytis or Alternaria appear to be under control.

Dr. Hausbeck’s work is funded in part by MSU‘s AgBioResearch.  

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