BP-61

Preventing Seedling Diseases in the Greenhouse

Daniel S. Egel, Extension Plant Pathologist

Diseases that are likely to affect vegetable transplant production in Indiana fall into two types: damping-off diseases caused by soilborne fungi and transplant diseases usually associated with fungi or bacteria which survive with seed or plant residue. Both types of diseases can cause extensive transplant loss. This bulletin briefly describes both types of diseases and appropriate measures for their control.

DAMPING-OFF DISEASES

Damping-off is a disease that can affect almost all vegetable plants at the earliest stages of growth. It occurs in home gardens as well as commercial fields and greenhouses, and may be caused by several soilborne fungi. Losses due to damping-off can be severe and often appear as reduced stands in seed flats or rapid wilting and death of young seedlings. Prevention is the key to control damping-off and can be achieved by several methods.

Symptoms: Damping-off may occur before or after seedlings emerge from the soil. In the case of pre-emergence damping-off, fungi infect seeds as they germinate. As infection progresses, seeds rot and eventually disintegrate. As a result of pre-emergence damping-off, poor stands become apparent days or weeks later.

Post-emergence damping-off is most often observed in seed flats or among transplants. Fungi infect stems at or near the soil surface. The affected area of the stem takes on a water-soaked appearance and sometimes becomes constricted. Eventually, the stems are unable to maintain structural support of seedlings, which usually collapse and die within 24 to 48 hours (Figure 1).

In some cases, seedlings survive infection in seed flats or as young transplants and are planted in the field. Such plants are likely to exhibit a "wirestem" symptom characterized by an off-color, twisted and constricted stem. This symptom is especially evident in infected crucifer seedlings and results in substantially reduced yields.

Cause: Damping-off is most often caused by any or all of three groups of fungi, namely Pythium, Rhizoctonia, and Fusarium. Species of genus Pythium are especially destructive in commercial vegetable plantings. Any temperature which is unfavorable for plant growth may cause increased damping-off development.

SEEDBORNE/RESIDUEBORNE PATHOGENS

One or more diseases of this type affect most vegetable crops. The pathogens (disease causing microorganisms) which cause these diseases survive in seeds or plant residues, not in soil mixes.

**Table 1:** Vegetable crops frequently grown as transplants and diseases which most often observed on the seedlings. The pathogens that cause these diseases may be borne on the seed
Vegetable crop	Disease
cabbage	black rot Alternaria leaf spot
cucumber	angular leaf spot
muskmelon	anthracnose gummy stem blight
pepper	bacterial spot
tomato	bacterial canker bacterial speck bacterial spot
watermelon	anthracnose gummy stem blight watermelon fruit blotch

Symptoms: Outbreaks of these diseases often show up as clusters of diseased plants. Symptoms on leaves often include brown lesions with yellow halos. In contrast, environmentally induced problems often occur uniformly throughout the seedlings or only in one location (for example, close to an outside wall).

Cause: Several different fungal or bacterial pathogens may be introduced into a transplant facility via contaminated seed (Table 1). Once introduced, these pathogens may continue to cause problems year after year if proper precautions are not taken.

SEEDLING DISEASE CONTROL

Control of damping-off diseases: Losses attributed to damping-off can be reduced by preventing contamination of soil mixes and manipulating the environment so that seedlings escape infection. The fungi associated with damping-off thrive in a wet environment. Therefore, the disease is usually apparent where excess soil moisture exists. Also, poor sunlight and other conditions such as excess nitrogen fertilizer, which inhibits seedling maturation, are favorable for disease development. Cultural control usually involves any treatment which hastens stem maturation and discourages pathogen development. Control recommendations include the following:

Use uncontaminated soil mix. Home gardeners and commercial growers alike are advised to use a commercially prepared soilless growing mix that is sold in 3-4 cu ft. bales or bags. A common mistake is to open a bag of "clean" soil mix and place it on a dirty floor or some other unclean surface prior to planting. Remember, your soil is only as clean as the dirtiest surface that it has come into contact.
Plant seeds shallow and in warm soil.
Use soil mixes that drain well.
Balance nitrogen and potassium fertilizer properly with adequate phosphorus to hasten maturation (NPK 1:2:1). Excess nitrogen promotes seedling growth and succulence rather than maturation.
Chemical seed treatment: Almost all commercially available vegetable seeds are sold already treated with a fungicide. The fungicide should help kill pathogens in the immediate vicinity of the seed for approximately two weeks. Chemically treated seeds, however, are no substitute for the preventative measures discussed here.

Control of seedborne/residueborne pathogens: Several measures should be taken to minimize or prevent the introduction of a seedborne/residueborne pathogen into a transplant facility.

Since many diseases may be seedborne, avoid saving seed unless specifically trained and equipped for seed production.
Frequently inspect seedlings as they are growing. Once established, diseases can rapidly move through a greenhouse. Remove suspicious looking seedlings before a possible disease problem develops.
Seedlots should be separated from one another, for example, by a piece of wood tall enough to prevent the spread of diseases from one seed lot to another. Save all information regarding seed purchases.
Irrigate in the morning to ensure drying of soil and leaf surfaces.
Growers interested in treating transplants with fungicides/bactericides should check the label for specific mention of greenhouse use.
Growers should use good sanitation. The following section outlines sanitation measures that can help reduce the incidence of both damping-off and seedborne/residueborne pathogens.

General control measures: Since the fungi or bacteria which cause plant disease must survive in soil, soilless mix, or plant residue, it makes sense to keep transplant facilities and materials as clean as possible. The following suggestions should help avoid all types of seedling diseases.

Gravel or plastic/cloth floor coverings are better than dirt floors. Dirt from the floor can be splashed onto the foliage of seedlings or the roots may contact the dirty floor, leading to disease. Plastic/cloth coverings have the advantage that they can be cleaned or replaced.
Transplants may be raised off the greenhouse floor onto wooden pallets or cinderblocks. Such a precaution lessens the chance of contamination from the floor, keeps the growing temperature warmer, and facilitates drainage.
Transplant flats should either be new or cleaned before each transplant generation. Trays should be cleaned to remove soil and plant residue and then treated. There are several disinfectants available for use in disinfecting transplant trays (e.g., Greenshield, Physan 20, household bleach). I have found little difference between using Greenshield, Physan 20, or 10% household bleach for 10 minutes to disinfect polystyrene transplant trays. Be certain to follow label instructions including wearing gloves and eye protection.

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