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Damping-off of Vegetable Transplant Seedlings

One of the most common problems encountered in vegetable seedling production this time of year is "damping-off". Damping off is a disease that results in the rotting, collapse and finally death of seedlings just before or soon after they emerge. It often starts out in a few plants but can spread quickly throughout the entire flat and into neighbouring flats. One or more soil-borne pathogens may be responsible for this disease, depending on the environmental conditions in the greenhouse.

Soil-borne pathogens such as Pythium and Phytophthora, often called water molds, can be particularly destructive if the soil is kept too wet for a long period of time. Although temperature is important, different species of these so-called water molds can infect at different temperatures. Pythium is the most common water mold pathogen found on diseased greenhouse vegetable seedlings and is often associated with excessive nutrition or ammonium toxicity.

Damping-off symptoms in vegetable seedling flat (left) causing seedlings to collapse (right)

Rhizoctonia solani is another common soil-borne pathogen that causes seed rot, damping-off and root rot during the production of vegetable seedlings in the greenhouse. Most strains of Rhizoctonia infect in somewhat dryer soil than the water mold pathogens. Often, Rhizoctonia will girdle the stems above and below the soil line of susceptible seedlings resulting in the seedling toppling over. On some plants such as cole crop seedlings, the lower stems are completely girdled, and the stiff vascular tissue is all that is left holding up the seedling, and causing the symptom known as "wire stem".

The best control of damping-off is to avoid it altogether. Once damping-off has started in a seedling flat, it may be difficult to control. The following are a few tips that will help avoid or reduce the risk of damping off becoming established in the greenhouse production of vegetable seedlings.

Sanitation: Many outbreaks of damping-off can be attributed to poor sanitation practices. Disinfect equipment, benches, and particularly seedling trays with a 1-part bleach to 4-parts water solution or with another disinfecting products such as Virkon before planting.

Growing medium: Use sterilized or heat-treated growing mediums. Soil-borne pathogens that cause damping off can naturally occur in peat moss and soil mixes. Ensure that growing medium is properly prepared and managed to provide good drainage, structure, aeration, and water-holding capacity.

Seed Treatment: Plant seeds that have been treated with a registered fungicide seed treatment to protect seeds and seedlings from infection by "damping-off" pathogens. Be sure to choose the proper seed treatment. Seed treatments such as Apron XL registered for protection against the water mold damping off pathogens will not necessarily control damping off caused by Rhizoctonia.

Seeding Depth: Do not plant seeds too deep and ensure that the right temperature is maintained for rapid seedling emergence and growth. Deep seeding cause stress on the seedlings as they grow through the soil and results in more tissue exposure below the soil line to the damping off pathogens.

Plant Density: Try not too over crowd or plant seedling too densely. Proper seeding rates to avoid thick plant stands, poor air movement and low light intensity, will help prevent disease from developing.

Fertilizer: Provide seedlings with the proper nutrients. Applying too much or too little fertilizer will affect the health of the seedlings and their ability to defend against disease.

Watering: Never over-water and always allow the seedling plugs to dry out just a little between watering. The longer the soil stays wet or saturated, the more likely water molds will infect and cause disease problems. Allowing soils to dry too much between watering may stress the seedlings and encourage damping-off caused by Rhizoctonia.

Many soil-borne diseases that start in the greenhouse can be easily introduced to the field on infected transplants. Once a soil-borne pathogen is introduced to a field it can quickly become established and may require more expensive management techniques in that field for years to come. By following a few simple tips in the greenhouse will result in healthy transplants ready to grow in a couple of months and avoid transferring diseases to the field.