Keep fungal diseases away from vegetable transplants

Know the diseases that infect your crops and how to manage them.

Disease prevention and prompt diagnosis are key components in vegetable seedling production because there are relatively few fungicides registered for controlling diseases on these crops. As long as the greenhouse use is NOT prohibited and the specific vegetable is listed on the label, the fungicide can be used in the greenhouse.

Damping-off (caused by Pythium spp., Phytophthora spp. and Rhizoctonia sp.) affects all vegetable seedlings and is also common among flowering bedding plants. Damping-off results in collapse of the plant at the soil surface. The fungicide Terraclor controls Rhizoctonia and lists broccoli, brussels sprouts, cabbage, cauliflower, peppers, and tomatoes on its label. Previcur Flex is labeled for use in the greenhouse on tomato, leaf lettuce, cucurbits, and peppers for prevention of root rot and damping-off caused by Pythium spp. and Phytophthora spp. To prevent damping-off, avoid over-watering because some fungi that cause damping-off prefer wet conditions.

Good sanitation is the key and ensures that root rot problems from one crop are not carried over to another crop. Root rot pathogens survive in the greenhouse in soil particles or plant parts clinging to containers, benches, walkways, and equipment. If root rot occurs, remove and destroy the diseased plants. Also, remove healthy-appearing plants that are immediately adjacent to the dead plants because the disease may have already spread to them although they are not yet showing symptoms. Plug sheets containing diseased transplants should not be reused.

Botrytis gray mold can infect all vegetable transplants causing an irregular brown spotting or “blight” of leaves and stem cankers. This is the same Botrytis that infects a wide range of floriculture crops producing gray masses of powdery spores. In vegetable transplants, Botrytis is a threat when plants grow and form a canopy of leaves keeping the relative humidity high which favors disease. Since the fungus that causes gray mold depends on water to germinate on the plant surface, increasing air circulation through fans and reducing the relative humidity by venting or heating (depending on outside temperatures) will help prevent condensation of water on plant surfaces and thereby reduce the occurrence of gray mold. Watering early in the day will help ensure that the plants dry by evening, reducing the occurrence of disease. The fungicides Scala, Botran, and Decree can be used on tomatoes in the greenhouse to protect against Botrytis. Decree can also be used on cucumber transplants.

Alternaria blight is caused by a fungus of the same name and causes leaf spotting and a stem canker on tomato and other vegetable transplants in the greenhouse. This disease is not as common as gray mold, but can be destructive when conditions are wet and the foliage thick. Often, Alternaria blight does not become a problem until the plants are held in the greenhouse for an extended period of time due to a delay in planting, shipping or selling.

Fungicides are available to control Alternaria diseases on tomato seedlings and some other vegetable seedlings. The fungicide Dithane is also registered for use on tomato and other vegetable seedlings including onions and cucurbits (melons, pumpkins, etc.). Dithane provides protection against Alternaria and some limited protection against Botrytis. None of the fungicides mentioned are registered for use on pepper seedlings in the greenhouse.

Late blight is caused by a water mold called Phytophthora infestans and is not considered a problem for tomato seedlings in the greenhouse. The late blight pathogen typically overwinters in potato cull piles and is often introduced to production fields via potato seed pieces. Commercial potato growers are vigilant each year for late blight as it is a common problem for them when the weather is cool and wet. The sporangia (seeds) of the late blight pathogen can be easily dislodged from the plant’s surface and carried long distances from one field (or growing region) to another via air currents and storm systems. Weather that is overcast, wet, rainy, and humid allows the late blight sporangia to survive its travels so it can cause disease if it lands on the surface of an unprotected host plant (i.e. tomato, petunia, and weeds such as nightshade). When conditions are bright, sunny, and dry, the late blight sporangium cannot survive long because the sunlight breaks it down and the low relative humidity causes it to shrivel and die.

Control measures for late blight are similar to those recommended for the other tomato diseases and include keeping the foliage dry, providing good air ventilation, spacing plants, and heating when needed to dry out the greenhouse. Fortunately, tomato transplant growers can protect against late blight with the same fungicides they use for Alternaria and Botrytis. In Michigan State University tomato field trials that I’ve run for the last several years (including 2009), the active ingredient in Dithane was excellent in protecting the tomato plants from late blight. The active ingredient in Heritage (azoxystrobin) was also very good. Revus (mandipropamid) is a new product (received a supplemental label that included tomato in August 2009) that has been outstanding against late blight in our outdoor field trials. The use of Revus is not prohibited in the greenhouse on tomato seedlings, but it may not be used on tomatoes for transplant production. Revus could be used in combination with one of the Alternaria/Botrytis products since Revus does not control Botrytis or Alternaria. Revus could be rotated with other helpful late blight fungicides including Curzate, Ranman, and Tanos. All late blight specific fungicides could be used in combination with one of the Alternaria/Botrytis products. Late blight fungicides, combined with one of the Alternaria/Botrytis products, can eliminate the potential occurrence of late blight when used properly and preventively at the greenhouse level.

A, Damping-off of celery, note browning of roots. B, Botrytis blight on a seedling. Late blight on C, leaves and D, stem of tomato in the field.
A: Damping-off of celery, note browning of roots. B: Botrytis blight on a seedling. Late blight on C, leaves and D, stem of tomato in the field.

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