Pythium Root Rot of Tomatoes

Pythium root rot primarily infects seedlings. It causes seedlings to rot before they even emerge from the soil.

Symptoms of the disease include water-soaked, greasy lesions that sometimes form on the stem part and roots after emergence from the seed. You can prevent the disease using sterilized soil or even artificial soil mix for planting seeds.

How to Identify Pythium root rot

Pythium root rot
Seedling killed by damping-off
Courtesy of University of Florida
Institute of Food and Agricultural Sciences
Pythium root rot
William M. Brown Jr.,
  • Fuzzy white fungus on potting soil.
  • Seedling stems are rotting away at the base.
  • Newly emerged seedlings collapse at the soil-line.
  • Dug-up seeds are rotted with evidence of fungus.


  • Caused by various soilborne fungi.
  • Over-watering.
  • Planting seeds in unsterilized garden soil
  • Leaving covers on planting trays too long.
  • Sowing seeds in warm, humid weather.

Preventative Measures

  • Start seeds in sterilized soil.
  • A lightweight artificial soil mix like Jiffy-Mix (which is made from sphagnum peat moss and perlite) is safe for starting seeds.
  • Water with aerated compost tea and avoid overwatering.
  • Plant only fungicide-treated seeds.


Natural remedies

  • Don't start seeds in garden soil -- use soilless mix instead. (See Growing tomatoes from seed for more information).
  • If you want to use regular soil to start seeds, sterilize it first using a pressure cooker at 5 pounds pressure for 20 minutes.

Chemical treatments

  • A bushel of soil can be disinfected using formaldehyde or formalin. Add 2-1/2 tablespoons to a cup of water and mix it well into the soil. Cover the basket for three days until the formaldehyde smell has dissipated.
  • Disinfect seeds by coating with compounds available at most nurseries.
  • Fungicides can be effective in combination with the preventative methods, but the particular pathogen (fungus) would have to be identified in order to choose the right fungicide.

Got a question about tomato diseases or pests? Ask here. We have the most amazing group of visitors who answer your questions. It's help and be helped!

Credits for technical content

  1. Florida Plant Disease Management Guide


Have your say about what you just read! Leave me a comment in the box below.

Burpee Gardening

Gardener's Supply Company