Click for larger image Damping-off of snapdragon seedlings (Antirrhinum) caused by Rhizoctonia sp. Note constriction of stems at the soil line. R. K. Jones, NCSU,

When apparently healthy seedlings die suddenly soon after germination the likely cause is “damping-off.

Symptoms and Diagnosis

Damping-off is caused by a number of soil fungi in several genera including: Rhizoctonia, Pythium, Botrytis, Phytophthora, and Fusarium and is often accompanied by low rates of germination.   Affected stem tissue on the seedlings may have black water-soaked lesions at the soil line which constrict the stem causing the seedlings to topple.  Occasionally, white/pink/gray cottony fungus may appear at the base of the stem.

Life Cycle

The fungi that cause damping-off are ubiquitous and can multiply and readily infect seedling when conditions favor their development. There is no recommended cure for damping off.  Survivors are generally weak and non-productive.  However, there are actions which can reduce the likelihood of the condition reoccurring.  These include elimination of pathogens and control of environmental conditions.

Integrated Pest Management Strategies

1. Sanitation. Dispose of all plants and soil in the affected containers and washing all containers and tools with soapy water and soak/rinse with a 1:10 solution of bleach to water.

2. Start clean. When starting seeds always use a sterile soil-less growing mix.  Do not use garden soil or reuse contaminated soil.

3. Use seeds treated with a fungicide, if available, or apply a layer of milled sphagnum moss on the soil surface. Milled sphagnum moss retards the growth of damping-off fungi. Placing a 1/8 inch layer on the soil after the seeds are planted can provide some protection to the young developing seedlings.

4. Provide an environment that is not conducive to the development of the disease. After seeds germinate place plants in an area with good air circulation. Remove plastic or glass coverings after half of the seeds have germinated. Bottom water seedlings and do not overwater. Thin out and fertilize seedlings after the first true leaves have developed.

Organic Strategies

Strategies 1, 2, and 4 are strictly organic approaches.

More images:

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Damping-off of cotton seedlings (Gossypium) caused by Rhizoctonia solani. Clemson U, USDA CES Slide Series,
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Damping-off of chrysanthemum seedlings caused by Rhizoctonia sp. R. K.Jones, NCSU,
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Damping off in seedllings caused by Pythium. Note black water-soaked lesions constricting the stem at the soil line. Clemson U - USDA CES Slide Series,
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Flats of healthy lettuce seedlings