Damping Off

Rhizoctonia spp., Fusarium spp., Pythium spp., Alternaria spp.

Damping off is a disease caused by multiple fungal pathogens, commonly Rhizoctonia solani, Alternaria, and Fusarium spp., as well as oomycete species in the genus Pythium. This disease primarily affects seedlings and usually causes plant death. Damping off can affect plants direct seeded in the field, as well as seeds sown for transplants in the greenhouse.

Identification

Damping off is classified as pre- or post-emergence. Pre-emergence damping off occurs when a pathogen infects seeds before germination, resulting in poor stand. Post-emergence damping off develops after seeds have germinated. When post-emergence damping off occurs, the pathogens infect the roots of the plant, as well as the stem at the soil line. The stems of infected plants will collapse, cotyledons will wilt, young leaves will become gray or discolored, and the plant will subsequently die. The outer layer of infected roots can slough off, or roots can fully collapse. Mycelial growth may occur at the soil level.

Life Cycle

Damping off is caused by soilborne fungal and oomycete pathogens. These pathogens are ubiquitous in most soils and can be transferred to seedlings through transplanting into contaminated field areas, potting media, pots, or tools. These pathogens generally thrive in wet, cool soil, although Rhizoctonia can also cause disease under dry soil conditions.
Damping off pathogens are usually weak pathogens that cannot infect healthy, full-grown plants. They primarily infect seedlings and young plants.

Cultural Controls & Prevention

In greenhouse transplant production

Damping off is directly related to water. Take care not to overwater, paying special attention to smaller, slower-growing plants like onions, leeks, and peppers. For these crops that require less water, a light media and/or a misting wand may be beneficial. A lighter media with a lower percentage of peat or compost will also help reduce growth of algae on the soil surface and will support fewer fungus gnats—as they are attracted to moist soil where fungi thrive—whose larvae can cause wounding of roots and lead to damping off. One could also use the same soilless mix but add in some extra perlite or coarse sand to lighten it up. Also, choose the right size cell for the crop—if the crop is slow growing, a very small cell may be best so that there is not too much extra soil in the cell retaining excess moisture. Reduce watering on overcast days to account for lower transpiration and evaporation rates. Water in the morning and, if a second watering is needed, make sure it is made early enough in the day that the soil has a chance to dry out before morning. Avoid overcrowding plants and use fans to improve airflow and dry out soil more quickly. 

In field settings

Damping off can also occur in the field where it may cause reduced stand or stunted growth in direct-seeded crops. This is especially true when plant germination and growth is slow due to cool spring soils. Though it is tempting to try to push things and plant early, it is best to wait until the soil is adequately warm for the crop you are planting. Planting seeds too densely can also result in increased damping off and patchy or poor stands.
Once plants are established in the field, they are not usually susceptible to damping off, though one of the damping off pathogens, Rhizoctonia solani, can cause disease throughout a plants life. These diseases include wirestem in brassicas—where the hypocotyl becomes skinny and turns brown, sometimes causing the plant to die—and bottom rot in cabbage and head lettuce—where the lower leaves that are in contact with the soil turn brown and rot away.

General management recommendations:

• Practice good greenhouse sanitation: Use new transplant trays or if reusing transplant trays, clean with soapy water and sanitize using a 10% bleach soak for 30 minutes. Do not reuse potting mix or use field soil for transplant production.
• Wait to plant until soil warms to an appropriate temperature. Don’t plant into cool soil.
• Avoid overwatering.
• Use vigorous seed.
• Do not seed or plant too densely.

Chemical Controls

• Use seeds treated with a fungicide, whether synthetic or biological.
• Several OMRI-approved biopesticides, e.g. Rootshield Plus and Mycostop Mix, can be mixed into soilless media before seeding. These biopesticides are living bacteria or fungi that colonize roots and protect roots from infection by damping off pathogens.
• Several fungicides both synthetic and biological are labeled for use in the greenhouse. Fungicides are not considered effective in a field setting, where the focus should be on planting vigorous seed into warm soil at the proper spacing. Consult the New England Vegetable Management Guide Vegetable Transplants section for materials and more information.