Damping off, a generic term for a group of deadly seedling diseases, is fatal for tomato seedlings.
It can be caused by several different fungi (Pythium, Rhizoctonia or Phytophthorathat) which attack tomato seeds, tender stems, and roots. Tomatoes are most affected in humid conditions, especially if the soil is cold and wet. Young seedlings or plants are most susceptible. An epidemic of damping off disease can be a disastrous way to begin a new tomato season.
Although fungi live in the soil and water, spores spread through the air and can move quickly from one seed tray (or garden row), to another. Affected plants appear as if they have been cut off at the base.
Once the process is underway, it’s hard to save even a few of your plants. Prevention is the best cure.
What damping-off looks like
Healthy young tomato plants infected by damping-off disease look pinched or cut off at the base of the stem. They wilt, droop over, wither away, and die. A white mold-like growth may appear on the soil surface and on the dead plants.
Two kinds of damping-off
There are two types of damping off: pre-emergent and post-emergent.
Pre-emergent damping-off: seeds rot in the soil or seedlings decay before they push through the soil
Post-emergent damping-off: seedlings sprout, but then pale, curl, wilt, or collapse at the soil line. The stem is water-soaked and turns gray, brown or black before disintegrating.
How to prevent damping-off
Use sterile containers. If you are re-using last year’s flats, be sure to wash them thoroughly and rinse them in a bleach solution before planting.
Start tomato seeds in a sterile potting medium. Damping-off fungi flourish unhygienic conditions. Soil-less potting mix provides a healthier environment and eliminates potential pathogens from the start. (Read more about sterile growing mediums and find inexpensive seed starting mix to buy.)
Avoid overly damp conditions. Many new gardeners start tomato seeds in damp basements – a perfect breeding ground for damping-off fungi. Choose a seed-starting area with good circulation. You can even run an electric fan in the growing area to keep the air moving.
Maintain a steady temperature. Drafty, cool conditions encourage damping-off.
Sprinkle soil surfaces. Spread a thin layer of sand, perlite, or sphagnum peat moss on the surface of the potting mix or garden soil to discourage fungi and bacteria.
Water from the bottom.
Work for a low pH. As the potting mix or garden soil pH rises, so does a tomato plant’s susceptibility to damping off. Commercially-prepared germination mixes have an average pH around 5.5, while tap water tends to be alkaline. As you water the seed pots and your seedlings with tap water, the pH in your pots gradually increases. Simultaneously, so will the plants’ susceptibility to damping-off diseases. Know the pH of your tap water.
Use a preventative fungicide. Water potting mix, soil, and seeds with soluble copper spray.
Separate infected plants. Damping-off disease spreads quickly from one plant or seed tray to another. Monitor new seedlings carefully. At the first sign of damping-off, move affected plants away from healthy ones.
What to do if your plants are affected
Dispose and destroy plants immediately. Avoid placing debris in the compost pile, since damping off fungi live in soil and will overwinter, ready to attack your new tomato seedlings next year. If tomatoes had not yet been set in the garden, disinfect or discard containers.