Extended periods of cool, wet weather this spring may have you noticing symptoms of a common pathogen infecting plants: Pythium. This water mold pathogen particularly attacks seeds, seedlings, and young plants (Figures 1 and 2). Pythium is present at low levels in nearly all soils around the world, but tends to become a problem under cool rainy conditions or persistent water saturation of roots.
Symptoms of Pythium include narrowing of stems at the soil line, symptoms of nutrient deficiency, stunting, and a darkened root ball with outer tissue layers that slough off of roots when smoothed between your fingers.
Recommendations for Pythium Management
- Do not start seed for transplants in natural soil, or if planting outside, buy seed dusted with a fungicide like thiram or captan.
- Keep transplant facilities, potting areas, cell packs, and transplant trays very clean. If reusing trays (particularly for tobacco), ensure these have been steamed, glazed, or cleaned with bleach and rinsed thoroughly.
- Avoid introduction of natural soil into floatbeds, bags of media, or greenhouse facilities. Do not store bags of media on natural soil and do not allow them to get wet prior to sowing seed.
- Recommended fungicides for transplant production (see labels for rates, application instructions, and safety information): Terramaster for tobacco, Terramaster or Ranman for greenhouse tomato, or Previcur Flex for several veggies.
Before applying a commercial fungicide, be sure to have symptomatic plants diagnosed correctly. Pythium-induced diseases may be indistinguishable from root rots caused by other pathogens, such as the true fungus Rhizoctonia. However, fungicides labeled for Pythium would not manage such a fungus, due to vast differences in these pathogens’ biology.
For more information on Pythium, please see the following publications:
- Pythium Root Rot in Tobacco Float Systems (PPFS-AG-T-01)
- Damping-off of Vegetables and Herbaceous Ornamentals (PPFS-GEN-03)
By Emily Pfeufer, Extension Plant Pathologist