Pythium ultimum

Black leg of seedlings


Pythium is considered a ‘weak pathogen’, meaning that it can usually only affect plants that are in a sub-optimal condition or suffer from stress, caused for example by too much water, too high or low temperatures or sudden fluctuations in temperature.

Pythium ultimum mostly occurs at low soil or water temperatures and causes among others cereal diseases.

Life cycle and appearance of Black leg of seedlings

Pythium belongs to the Oomycetes, a separate group of microorganisms and is therefore not a real fungus. It survives as oospores, resting spores which are resistant to unfavourable conditions, especially dehydration. Germination of the oospores is stimulated by exudates from roots of a susceptible host nearby. The oospores germinate and form so-called sporangia from which subsequently zoospores are produced. Zoospores are spores with flagellae, which enable the spores to move through water. The zoospores are chemically drawn to the growing roots of young plants and move towards them through the soil water. They infect the roots through small wounds, where the side roots are formed or through other similarly damaged spots.

On the infected plant, new sporangia with zoospores are formed, that allow the disease to spread to neighbouring plants.

The zoospores disperse via water. Therefore, this disease spreads most rapidly in wet conditions with a high (>70%) water content of the soil or substrate.

Oospores are dispersed when soil particles are moved around by humans or machinery. Pythium can survive very well in the soil or substrate in the absence of plants. Many species are entirely saprophytic, so these do not harm plants.

How to prevent Black leg of seedlings

  • Use resistant cultivars or graft on tolerant cultivars
  • Use clean planting material
  • Ensure regular growth
  • Avoid over-watering
  • Do not provide excess nitrogen since this promotes the disease.
  • In greenhouses:
    • Prevent high temperatures immediately after planting. Avoid cold and excessive warm irrigation water. Keep the water content of the substrate below 70% and ensure good drainage
    • Disinfect recirculating nutrient solutions
    • Increase the distance to the main substrate at the start by using higher substrate blocks for the seedlings
  • In field crops:
    • Avoid the use of fields with inadequate drainage or improve the drainage before planting/sowing

Prevent plant diseases by optimizing plant potential and crop resilience.