Extension Communications
3614 Administrative Services Building
Ames, Iowa 50011-3614
(515) 294-9915
3/19/01
Contacts:
Stephen Wegulo, Extension Plant Pathology, (515) 294-0589,
wegulo@iastate.edu
Elaine Edwards, Continuing Education and Communication
Services, (515) 294-5168, eedwards@iastate.edu
Yard and Garden Column for the Week Beginning March 23
What's Wrong With My Seedlings?
By Stephen Wegulo
Assistant Scientist, Plant Pathology
Iowa State University Extension
Spring is approaching and soon many gardeners will be getting ready to seed or transplant various plants. If you have grown green beans, peas, peppers, tomatoes, cucumbers, muskmelons, watermelons and other garden plants, you may have noticed rapid collapse and death of seedlings or failure of seedlings to emerge from the soil. This is often referred to as damping-off, a disease that affects seeds and seedlings of most plants.
Seeds and seedlings are most susceptible to damping-off between planting and about four weeks after they begin to grow. As the name suggests, damping-off develops primarily when conditions are damp or moist. The disease is caused by several fungi that live in the soil and feed on organic matter (decomposed plant and animal material). The most common one is Pythium, but other fungi such as Phytophthora, Fusarium and Rhizoctonia also cause damping-off. Pythium thrives best in cool, wet weather.
If the current weather trend continues, we are likely to have a cool, wet spring and damping-off may be a problem. Although damping-off is most common in the spring within four to six weeks after planting, the disease can occur any time of the year whenever seeds are planted or seedlings are transplanted during favorable weather. Seedlings raised indoors can be attacked if damping-off fungi are present in the soil.
When damping-off fungi attack plant seeds, they fail to germinate, rot and disintegrate. Young seedlings can be attacked before they emerge from the soil. This is called pre-emergence damping-off. In post-emergence damping off, seedlings already emerged are attacked at the roots or in the stems at or below the soil line. Water-soaked and discolored areas appear on the stems. The lower part of the stem softens and becomes thinner than the parts of the seedling above it. The seedlings then fall over on the soil and die. The most obvious symptoms of damping-off are failure of seedlings to emerge and collapse and falling over of young seedlings.
During extended periods of wet weather, damping-off fungi can invade the fleshy and often most valuable parts of older plants, such as the actual cucumber, melon or tomato, especially when these parts are in contact with the soil. A cottony fungus growth results, and the infected parts become soft, watery and rotten.
Several factors influence the extent of seedling damping-off. The disease, when caused by Pythium or Phytophthora, is most prevalent when soil moisture is high. Planting when temperatures are warm enough (above 61o F) to encourage fast seedling growth can help reduce damping-off. As seedlings mature, they become more resistant to attack by damping-off fungi. At low soil temperatures, damping-off increases because seeds or seedlings are exposed to the fungi for longer periods of time. Ensure adequate lighting as it results in sturdy seedlings that are more resistant to damping-off.
High plant density favors damping-off because of reduced light penetration, greater competition for nutrients and greater retention of moisture. Competition for nutrients weakens seedlings, making them more susceptible to attack by damping-off fungi. Greater amounts of moisture are retained in soil planted with high density stands due to inadequate air circulation. Therefore, allowing adequate spacing between plants can reduce damping-off.
Damping-off can be more severe in certain soil types. Soils with a high organic matter content favor growth of these fungi. Seedlings planted in such soils are more likely to be heavily attacked than those planted in sandy soils. Damping-off fungi grow best in slightly acidic soils, but growth is inhibited in more acidic soils. Maintaining soil pH between 5 and 6 can help reduce damping-off.
The best approach to managing damping-off is to avoid it altogether. It is difficult to contain the disease once it starts. Because seedlings in vigorous health are more resistant to damping-off, practices that promote seedling health can help prevent or reduce the disease.
* Use sterile soil to raise seedlings for transplanting.
* Plant or transplant when soil temperatures are above 61 degrees F.
* Choose a well drained and lighted site to plant your seeds or transplant your seedlings and avoid over-watering.
* Allow adequate spacing between plants and mulch at a minimum during the first few weeks after planting or transplanting.
* Apply enough fertilizer to keep plants in good health, but do not over-fertilize because high-nitrogen fertilization results in fast-growing, spindly seedlings more susceptible to damping-off.
The key to preventing damping-off is to use cultural practices that maintain vigorous seedling health and reduce excessive moisture in the soil.
ml: isugarden
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