Tips and tactics to keep your greenhouse clear

Damping-off is a common problem in greenhouses that grow vegetable and ornamental transplants. It’s a disease that causes rotting, collapse and death of seedlings as they emerge and start to grow. It typically starts on a few plants, but can quickly spread.


Damping-off can occur on germinating seeds still underground or on young seedlings. Even before they come up, the emerging shoots of seeds may have a decayed or water-soaked appearance. Later, the stem of young seedlings along the soil line may become dark and shriveled, with a so-called wire stem appearance, even though the top of the plant may appear healthy. The roots may or may not be decayed.

To avoid damping-off, use new flats or trays or sterilize them prior to filling with a high-quality mix. It may also make sense to add a bio-fungicide labeled for suppression of soilborne diseases to your mix before you plant into it.

In flats of germinating seeds, this disease often occurs in a circular pattern, as the fungal pathogens grow out from the original infection area. In plug trays, where spread is usually by splashing irrigation water, the disease may be more randomly distributed.


The cause of seedling collapse can be one or more common soilborne fungal pathogens, such as Pythium, Phytophthora, or Rhizoctonia. Pythium and Phytophthora are called water molds, and they are usually a problem if the soil is too wet for too long. Most strains of Rhizoctonia infect in drier soil than the water molds. Alternaria is another fungus that can cause damping-off. It is typically seed-borne rather than soilborne, and may cause foliar blights on plants as well.

Prevention is key

The best control of damping-off is to stop it from happening in the first place. Once damping-off has started, it can be difficult to control. The following strategies can help reduce the risk of damping-off in the greenhouse production of vegetable and ornamental seedlings.

Keep it clean.

Damping-off problems can be created by poor sanitation practices. Keep flats off of dirt floors. Hang up the ends of hoses when not in use; never allow the spout end to lie on the floor. If damping-off does occur in a flat, discard the entire flat before it has a chance to spread to other flats.

Thoroughly wash and disinfect hand tools, other greenhouse equipment, benches and especially seedling trays prior to use. Use a bleach solution or another disinfecting product such as hydrogen dioxide before planting. Note that Rhizoctonia produces a crusty resting structure that often looks like dirt, adhering to the surfaces of benches and previously used seedling trays.

Use a good mix.

A high-quality growing medium free of any field soil can minimize the presence of disease inoculum. However, soilborne pathogens that cause damping-off can and do occur naturally in soilless mixes made with peat moss, as well as compost-based mixes. Organic growers should ensure that only well-made, mature compost is used in their growing medium. The medium should have good drainage, aeration and water-holding capacity.

Plant carefully.

Providing seedlings with optimal growing conditions can help prevent damping-off. Low temperatures, excess humidity or improper fertility can encourage disease problems.

Do not plant seeds too deep, because that causes 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. Do not plant seeds too densely because overcrowded, thick plant stands result in poor air movement and low light intensity, which can promote disease development.

The right amount of feed and water.

Take care not to use too much or too little fertilizer, since either will affect the health of the seedlings and thus their ability to defend against disease. Never over-water; always allow seedlings to dry out just a little between each watering. The longer the soil stays wet, the more likely damping-off will be a problem. Of course, allowing soil to dry too much between watering may stress seedlings and encourage damping-off caused by Rhizoctonia.

Optimize growing conditions.

Rapid seedling emergence and growth can help your plants ‘“eat” an infection, since they are most vulnerable while very young. Provide seedlings with the temperature and humidity they need to grow well. Although keeping greenhouse temperatures cool can reduce energy expenses, it not only slows plant growth, but can also lead to excess humidity in the greenhouse. That can cause condensation on cool overhead pipes or other surfaces, which then drips onto seedlings. Damping-off often starts in areas of flats where water is dripping.

Avoid placing hanging baskets directly above flats of seedlings. Damping-off can get started when water drips down from baskets or from condensation on overhead pipes.

Keep greenhouse temperatures warm and the humidity low to reduce the risk of condensation, dripping and thus damping-off. Use horizontal airflow fans to evenly distribute heat if necessary. Rather than run the greenhouse at a cool temperature, it might be better to start your plants a week or two later and use the necessary fuel to maintain optimal growing conditions. Using heat mats under trays is another way to maintain temperatures that stimulate rapid seedling emergence and growth without a lot of energy use.

Organic soil/seed treatments.

These are available to help with the prevention of damping-off; they will not cure diseased plants, and they are no substitute for the practices listed above, but they may enhance their effectiveness. Always check with your organic certification agency prior to using any pesticide.

Mycostop contains Streptomyces griseo-virdis, a beneficial bacterium. It is labeled for suppression of diseases such as Pythium, Rhizoctonia, Botrytis and others on ornamentals, vegetable plants and herbs. It can be mixed into potting media at a rate of 1 to 2 grams per cubic yard, or applied as a spray or drench. Repeat applications every two to six weeks may be necessary. It can also be used as a seed treatment for preventing damping-off in the field or greenhouse. It is OMRI listed.

SoilGard12G contains Gliocladium virens, a beneficial fungus. It is OMRI listed and labeled for suppression of Pythium and Rhizoctonia on ornamentals and food crop plants grown indoors and out. It should be mixed at the rate of 1 to 1.5 pounds per cubic yard of media; granules must be distributed uniformly throughout the medium. It can also be applied as a drench to greenhouse flats, plug trays or pots.

RootShield Granules contains Tricho-derma harzianum, another beneficial fungus labeled for suppression of Pythium, Rhizoctonia, Fusarium and other diseases on a wide range of food crops, ornamentals and trees. Granules are mixed at the rate of 1 to 1.5 pounds per cubic yard of media prior to planting. OMRI listed.

Careful management of seedlings in the greenhouse will pay off as healthy plants ready to be sold to satisfied customers or transplanted into your fields in a couple of months.

Note: Mention of pesticides and brand names is for information purposes only; no endorsement is intended nor is discrimination against products not mentioned.

The author is Vegetable and Berry Specialist with University of Vermont Extension based at the Brattleboro office. He can be reached at