How to prevent seedling damping off
Damping off is a disease of seedlings. Infected seedlings rarely survive to produce a vigorous plant. Quite often a large section or an entire tray of seedlings is killed by damping off. Once plants have mature leaves and a well developed root system, they are better able to naturally resist the damping off pathogens. There is a critical period of growth between planting and maturity, where special care needs to be taken to protect sensitive seedlings.
Host and pathogen
A wide variety of vegetables and flowers can be affected by damping off. Young leaves, roots and stems of newly emerged seedlings are highly susceptible to infection. Under proper environmental conditions, damping off pathogens can cause root rot or crown rot in mature plants.
The fungi, Rhizoctonia spp. and Fusarium spp., along with the water mold Pythium spp. are the most common pathogens responsible for damping off.
- Seedlings fail to emerge from the soil.
- Cotyledons (the first leaves produced by a seedling) and seedling stems are water soaked, soft, mush and may be discolored gray to brown.
- Seedling stems become water soaked and thin, almost thread like, where infected.
- Young leaves wilt and turn green gray to brown.
- Roots are absent, stunted or have grayish brown sunken spots.
- Fluffy white cobweb like growth on infected plant parts under high humidity.
Mushy tan spots indicate infection by the damping off fungi on this overwatered seed tray.
All of the pathogens responsible for damping off survive quite well in soil and plant debris. The pathogens can be introduced into the seedling tray in several ways. Pots, tools, and potting media that have been used in previous seasons and are not properly cleaned can harbor the pathogens. Spores of Fusarium spp. can be blown into the production area, carried by insects like fungus gnats, or move in splashing irrigation water. Pythium spp. is often introduced on dirty hands, contaminated tools or by hose ends that have been in contact with dirt and debris. Once introduced to a seedling tray, the damping off pathogens easily move from plant to plant by growing through the potting media or in shared irrigation water.
Garden soil often contains low populations of the damping off pathogens. If garden soil is used to fill seedling trays, the damping off pathogens are likely to be present and initiate disease in the warm wet conditions favorable for seed growth. Seeds that are directly seeded into the garden can also suffer from damping off. Disease is particularly severe in garden seedlings when seeds are planted in soils that are too cool for optimal germination or when weather turns cool and wet after planting resulting in slow germination and growth.
The damping off pathogens thrive in cool wet conditions. In addition, any condition that slows plant growth will increase damping off. Low light, overwatering, high salts from over fertilizing and cool soil temperatures are all associated with increased damping off.
Infection of seedling stems by the damping off fungi results in thin wiry rotted stems that cannot support the seedling.
- Sterilize all used pots and trays in a solution of 10% household bleach by soaking for 30 minutes.
- Use new potting media to fill trays. Do no reuse potting mix or use garden soil or compost.
- Clean all tools that will be used in planting and maintenance of the seedlings. Store them in a clean location when not in use.
- Use a heating pad under trays to warm soil to 70-75°F for indoor plant production.
- Wait until garden soil has reached optimal temperature for germination before planting outdoors. This temperature varies depending on the plant (Table 1).
- Use a potting media with good drainage. Water to keep potting media moist but not soggy. Use pots with drainage holes to insure good drainage of excess water.
- Keep hoses and water heads off the floor.
- Use clean (like tap water) warm (68 – 77F) water to water young seedlings. Cool water (50F) slows plant growth and increases the opportunity for infection.
- Do not apply fertilizer to seedlings until several true leaves have developed. Then apply 1/4 strength standard soluble fertilizer. Many potting mixes contain slow release fertilizer and do not require any fertilizer application.
- Provide 12-16 hrs of light from a soft white fluorescent or grow light to seedlings. Light from a window is rarely sufficient.
Table 1. Soil temperatures necessary for proper seed germination of various vegetable crops.
|Minimal soil temp||Optimal soil temp|
|Cucumbers, melons, squash||60F||95F|