Transplant diseases: Identification and control

April 22, 2010 - Author: Mary Hausbeck, Michigan State University Extension, Department of Plant Pathology

Editor’s note: This article is from the archives of the MSU Crop Advisory Team Alerts. Check the label of any pesticide referenced to ensure your use is included.

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. Although Botran 75-W is registered to control Botrytis on tomato seedlings, this fungicide should be used with caution due to concern regarding occasional sensitivity of the plant stem. 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 including onions and cucurbits (melons, pumpkins, etc.). Dithane provides protection againstAlternaria and some limited protection against Botrytis. None of the fungicides mentioned are registered for use on pepper seedlings in the greenhouse.

Bacterial diseases can infect tomatoes and peppers resulting in blighting. Not all spotting on the foliage is caused by fungi. It is important to distinguish between spots caused by fungi and bacteria because disease management differs. On tomato transplants, three bacterial diseases can be problems and include bacterial canker, bacterial speck, and bacterial spot. Peppers are affected by bacterial spot only. Of the bacterial diseases that cause problems on tomatoes, bacterial speck is probably the easiest to identify because of the small, dark-brown spots surrounded by a yellow “halo” that occur on the leaves. Bacterial spot that occurs on tomato and pepper is not as easy to identify as bacterial speck. Bacterial spot disease results in spots or blotches on the leaves and stems. These are larger than those caused by bacterial speck. Symptoms of bacterial canker on tomato transplants include small, tan “blister-like” lesions on the leaves and petioles and progress to form brown streaking and cankering. A diagnosis from an Extension educator or other knowledgeable professional is often warranted to separate symptoms of bacterial diseases from symptoms caused by fungi or other causes.

Tomatoes with bacterial diseases should be immediately removed from the greenhouse and destroyed. In addition, tomato seedlings immediately adjacent to those showing symptoms should also be removed and destroyed. In some situations, all tomatoes within a block or greenhouse will have to be destroyed. Although epidemics may seem to appear overnight, chances are it began in just a few plants and progressed unnoticed for a couple of weeks. Plug sheets containing infected transplants should not be reused. Removing infected transplants from the greenhouse is the most critical component of managing bacterial diseases once they’ve been introduced.

Bacteria move readily in a film of water and can spread through splash droplets. It is important, therefore, to water plants early enough in the day to ensure that the foliage dries completely by evening. Good ventilation, circulation, and low relative humidity are also important in helping to maintain dry foliage. Clipping, pruning or any other type of injury provides a means for the bacteria to enter the plant and should be avoided.

Until recently, growers have had to manage bacterial canker as it occurred in the field. Our research team approached this problem by testing fungicide applications to transplants while in the greenhouse. The greenhouse was targeted because the spread and increase of bacteria is favored by the wet, humid conditions of the greenhouse and the close spacing of tomato transplants. Multiplication and spread of the bacterium is less likely in the field because of the lowered relative humidity and increased plant spacing. Also, it is more economical and efficient to spray transplants while in the greenhouse than to spray plants once placed in the field.

We focused on the health of tomato transplants because it has been our observation that establishing a field with transplants that are infected with the bacterium responsible for bacterial canker results in devastating yield losses. Transplants can be infected while in the greenhouse, yet appear healthy at the time of planting in the field.

Applying a copper hydroxide product alone or in combination with a mancozeb fungicide at five-day intervals to transplants in the greenhouse once true leaves had emerged, even when a bacterial canker epidemic occurred, resulted in transplants that produced yields comparable to that of healthy plants. In our studies, these copper applications were not continued once the transplants were planted in the field. For tomatoes, the efficacy of the copper fungicides may be enhanced by mixing them with a mancozeb-based fungicide. Although mancozeb does not have any action against bacteria, the combination of mancozeb + copper is considered by some to provide a synergistic action against these bacteria. This combination would also provide some control of the foliar diseases caused by fungi (such as Botrytis and Alternaria). While Agri-mycin alone or in combination with copper hydroxide was also effective in our studies, this product does not list greenhouse on its label. However, current interpretations indicate that this product can be used on seedlings in the greenhouse since the label does not prohibit this use. We have not determined whether a seven-day interval of bactericides affords the same protection as the five-day application interval that we tested. Applications of copper hydroxide or Actigard to transplants once in the field may be helpful in reducing bacterial occurrence and fruit spotting.

Impatiens necrotic spot virus (INSV) should be a primary concern of all growers who raise both vegetable transplants and flowering plants. This virus infects a large number of plant species and occurs frequently in bedding impatiens, New Guinea impatiens, dahlia, cineraria, cyclamen, gloxinia and buttercup. This virus moves from infected flowering plants to healthy vegetable seedlings via western flower thrips. A grower may not know that plants are diseased because expression of symptoms may be slow. Meanwhile, if western flower thrips are present, the disease can be spread throughout the greenhouse. It is advisable, therefore, to keep vegetable and ornamental bedding plants separated within 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.

General guidelines

1) Dedicate operations for seedling/transplant production. Greenhouses that grow both tomato transplants and mature plants for fruit production are especially at risk of keeping diseases active in the greenhouse and available to infect new tomato seedlings.
2) Keep the relative humidity as low as possible (less than 85 percent) through heating and venting as appropriate.
3) Space plants to prevent pockets of high humidity from forming.
4) Use fans to move air and vent to exhaust moisture-laden air out of the greenhouse.
5) Scout seedlings two times each week to ensure that problems are detected early when corrective measures can be taken.
6) If disease symptoms are detected, remove affected plants including adjacent healthy-appearing plants.
7) Water at a time of day when plants can dry quickly.
8) Apply fungicide preventively when weather conditions are favorable for disease (i.e. wet, humid).

Tags: msu extension, vegetables

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