How to Prevent Damping Off Disease When Starting Seeds Indoors

Updated on January 13, 2020
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Caren White is a Master Gardener and instructor at Home Gardeners School. She has been associated with Rutgers Gardens for over a decade.


Starting seeds indoors has many advantages. You can grow varieties that are only available through seed catalogs. You can get a jump on the growing season by having plants rather than seeds to plant in the spring. And the cost is much less than purchasing plants at a nursery. But all of that convenience comes with a price: disease. Here is how to prevent the most common disease, damping off, that kills seedlings.

What is Damping Off Disease?

Damping off disease is a fungal disease that kills seedlings. There is not a single type of fungus that causes the disease. Rather, it is a number of different fungi and molds that cause a range of symptoms but all result in the death of the seedlings. Damping off is specific to seedlings because once plants have developed leaves and a root system, they are able to resist attacks by the fungi and mold. Cool temperatures and high humidity are ideal conditions for these organisms to flourish.

There is no treatment for damping off disease. If you see it develop in a tray of plants, discard the entire tray and start over.

A stem that has been killed by damping off.
A stem that has been killed by damping off. | Source

What are the Symptoms of Damping Off Disease?


The fungi can attack the roots so look for the absence of roots or roots that are stunted or discolored with grayish brown spots.


The cotyledons (the first set of leaves) are soft and mushy. They may also be discolored, gray or brown.


The stems get very thin and are unable to stand up.


The true leaves, which develop after the cotyledons, wilt. They may be discolored, gray or brown.

The entire tray of seedlings or just some of the seedlings may develop symptoms and die . Usually the die-off is in a circular pattern as the fungi moves from seedling to seedling. Discard the entire tray of plants to prevent further spread of the disease.

Can Damping Off Kill Seedlings That are Growing Outdoors?

The fungi and molds that cause damping off live in the soil outdoors in your garden. They can infect outdoor seedlings just as readily as indoor seedlings. Since cold damp conditions encourage the growth of the organisms, wait until the soil has warmed in the spring to sow your seeds.

Roots that have been infected with damping off.
Roots that have been infected with damping off. | Source

How to Prevent Damping Off Disease

Grow in a Sterile Environment

Keeping as sterile an environment as possible is key to preventing damping off disease. Always use sterile potting soil and new containers and trays. If you must re-use containers and trays, sterilize them first with a 10% bleach solution. Sterilize all of your tools before use with the same bleach solution and then store them somewhere clean. Don’t leave them laying around the growing area where they can become infected with fungi or mold.

Use Sterile Potting Soil

Don’t use soil from your garden or compost. Both contain the fungi and molds that cause damping off disease. Disregard the so-called instructions you see online on how to sterilize garden soil. These methods do not work. Make sure that the potting mix that you are using drains well and the containers that you use have drainage holes. Saturated soils encourage the growth of mold and fungi.

Use a Heat Mat

Place a heating mat under your trays. Cool soil encourages fungi and mold to grow. Keep your soil warm, 70⁰ - 75⁰F. This will also hasten the growth of your seedlings so that they will quickly get large enough to be able to resist disease.

Use Warm Water

Use warm water (68⁰F - 77⁰F) to irrigate your seedlings. Cold water encourages the growth of organisms that cause disease. If possible, water from the bottom of the tray. When you water from above, the water hits the soil with force causing the soil to splash on to the leaves or cotyledons of the plants, infecting them with diseases that are in the soil.

Do Not Overwater

Be careful not to overwater your plants. Don’t allow them to sit in water. Soggy soil is a perfect environment for a host of organisms that cause disease in plants. Empty any excess water from the trays.

How to Make Your Own Fungicide

Many gardeners make their own fungicides to prevent the growth of fungi and molds that cause damping off. The ingredients are found in most homes.

  • Use a tea made with chamomile or cinnamon to water your seedlings or mist them.
  • Lightly dust the soil your plants are growing in with powdered cinnamon.
  • Mist your seedlings with a 3% solution of hydrogen peroxide.

All three ingredients, chamomile, cinnamon and hydrogen peroxide, are natural fungicides. Gardeners often use these remedies if damping off has killed a few but not all of their seedlings. They discard the diseased plants and then begin treatment with a homemade fungicide in hopes of saving the rest of their plants.

© 2019 Caren White


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