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: 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. 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.

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