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When seedlings die soon after germinating, the cause is often one of several pathogens that lead to damping-off disease. Seeds may rot before they germinate, shoots may decay before they appear or stems of seedlings may rot at the soil line. Damping-off is most common when soil is compacted and conditions are wet and cool. To manage damping-off, plant at the right time, germinate seeds indoors in disease-free soils, and follow planting practices that reduce disease risk.
Plant in warm, moist soils with good drainage.
- Don't plant seeds or transplants into cold, wet, compacted soil.
- Wait for the outdoor soil and air temperatures to warm up.
- If starting seeds indoors, keep in a warm area or use heat mats under seedlings.
- Shallow planting will speed up germination outdoors if conditions are marginal.
Improve drainage in compacted, waterlogged soils.
- Use raised planting beds.
- Add soil amendments such as redwood shavings, peat moss, or bark.
- Use potting soil for indoor seeding of transplants.
- Avoid using unfinished or green compost, which holds water and promotes disease development.
- Mist seedlings instead of soaking the soil.
Use good cultural practices for seedling plants.
- Don't overwater.
- Thin plants after they emerge to improve air circulation.
- Don't overfertilize with nitrogen.
- Rotate crops to avoid planting the same crops in the same place year after year.
Employ good sanitation practices.
- Use clean, disease-free pots and soil.
- Remove and discard diseased plants.
- For plants grown in pots or cold frames, use steam-treated soil or pasteurized potting mix.
- Outdoors, consider solarizing soil before planting, a method that uses the sun’s heat and clear plastic to kill soil-borne pests.
- Use the highest quality seed available.
Pest Notes: Introduction
In the garden, planter box, or pot, seeds may fail to come up, or seedlings die soon after they have emerged from the soil. Seeds may rot before they germinate, seedlings may decay before they emerge, or seedlings may wilt and collapse. The problem is caused by plant diseases collectively referred to as damping-off.
Identification and Biology
Damping-off can be caused by several soil-dwelling fungi and fungus-like organisms. Species in the genus Pythium are most often responsible for damping-off, but several other pathogens, including species of Rhizoctonia, Fusarium, and Phytophthora, can also cause decay. The pathogens that cause damping-off are present in virtually all soils. They survive on dead organic matter and produce spores or other structures that may survive for long periods of time and can infect seedlings when conditions favor their development. The young tissue of emerging seedlings is most susceptible to infection. Also, seedlings are more likely to become infected when growing slowly in cold, wet soil. Vigorously growing seedlings pass through the susceptible stage fairly quickly and become established plants. Established plants are more resistant to damping-off pathogens.
Other garden pests such as cutworms, earwigs, flea beetles, snails and slugs, and root maggots may also damage seedlings in the garden. In order to manage the problem, it's important to determine whether the damage was done by one of these pests or by damping-off diseases.
Damage
The first above ground evidence of damping-off is the failure of some seeds to emerge as seedlings. If seeds are attacked before they germinate, they become soft and mushy, turn dark brown, and decay. They may have a layer of soil clinging to them when they are dug up because the soil is interwoven with fine, threadlike growth of the pathogen. Seeds that begin to germinate may shrivel and decay. If seedlings are attacked after they emerge, stem tissue near the soil line is decayed and weakened, usually causing plants to topple and die. If only the roots are decayed, seedlings may continue to grow slowly or may eventually die. As seedlings grow into established plants, they become less susceptible to damping-off pathogens.
Management
Damping-off is prevented by using good sanitation and by planting high-quality seeds in an environment favorable for rapid germination, growth, and establishment. The specific methods to achieve good control of damping-off differ between growing transplants in containers (transplant flats or pots) and directly in garden soil.
Growing transplants
If starting plants in transplant flats or pots indoors, in cold frames, or in greenhouses, good sanitation is the first step to preventing damping-off. If reusing transplant flats or pots, thoroughly wash all potting mix particles off the surfaces, sanitize the flats or pots in a 10% bleach solution for 30 minutes, and then allow the trays to air dry. Use a steam sterilized or pasteurized lightweight and fast-draining seed starting mix. Composted hardwood bark has been reported to reduce damping-off and can be substituted for peat moss in seed starting mixes. Plant when potting mix temperatures are favorable for rapid germination, which is 65-70°F for most seeds. To achieve good temperatures for seed germination, it may be helpful to place the seeding starts in a warm area of the home or use heat mats underneath the flats/pots.
Maximize seedling vigor and rapid emergence by using fresh, high-quality seeds. Avoid using old seeds or seeds that have been stored in poor conditions, such as high temperatures and high humidity. Plant seeds at the right depth so they will germinate rapidly. A good general rule is that planting depth should be twice the width of the seed. Keep the potting mix moist but not saturated with water after planting the seeds. After the initial wetting of the potting mix, it is a good idea to gently mist the surface rather than thoroughly watering. Misting avoids soil saturation with water and prevents dislodging or pushing seeds too deep. Transplant the seedlings outdoors after the soil temperatures have warmed sufficiently to promote good growth. Measure the soil temperature with a soil thermometer since air and soil temperatures are different.
Direct seeding
If seeding directly into garden soil, bed preparation and appropriate environmental conditions are important for managing damping-off disease. In the warmer, sunnier regions of California, soil solarization during fallow periods can reduce soilborne pests, including the organisms that cause damping-off. See references below for specific instructions on soil solarization. Prepare the planting beds so the soil has good drainage. If amending with compost, use only well-decomposed, mature compost. Immature compost can support the growth of some damping-off pathogens and increase disease incidence.
Plant when soil temperatures are favorable for rapid germination, which is 65-70°F for most seeds. Seeds planted in cold soil that is saturated with water are most vulnerable to damping-off. As described above, use only high-quality seeds and plant at the appropriate depth to achieve rapid emergence. Use light sprinkler irrigations to encourage germination and emergence, and do not water to the point of soil saturation. After plants emerge, thin them so that there is good air circulation between the plants. Avoid overfertilizing with nitrogen, which causes tender growth that is susceptible to damping-off infection.
References
Elmore CL, Stapleton JJ, Bell CE, DeVay JE. 1997. Soil Solarization: A Nonpesticidal Method for Controlling Diseases, Nematodes, and Weeds. UC ANR Publication 21377. Oakland, CA.
Flint ML. 1998. Pests of the Garden and Small Farm: A Grower's Guide to Using Less Pesticide. UC ANR Publication 3332. Oakland, CA.
Stapleton JJ, Wilen CA, Molinar RH. 2008. Pest Notes: Soil Solarization for Gardens & Landscapes. UC ANR Publication 74145. Oakland, CA.
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