Pythium Root Rot

Pythium attacks juvenile tissues such as the root tip and newly germinated seedlings. After gaining entrance to the root the fungus may cause a rapid, brown to black rot of the entire primary root and may even move up into the stem tissue. As the soil dries, new roots may be produced and the plant may recover or never show symptoms of disease. Under wet conditions brought about by poor soil drainage or excess irrigation, more and more roots are killed and the plant may wilt, stop growing, or even collapse and die. Bulbs of susceptible plants turn brown to black, gradually desiccate, and form a hard mummy.

The pathogens that are responsible for Pythium root rot, also known as water molds, are present in practically all cultivated soils and attack plant roots under wet conditions. These fungi can be spread by fungus gnats and shore flies and end up contaminating potting mixes in containers. There are many species of Pythium; a few of these species are beneficial in that they compete with or parasitize the pathogenic species. Of the many pathogenic species, some have limited host ranges while others, such as Pythium ultimum, have very wide host ranges.

Some Pythium species, such as P. aphanidermatum, are pathogens only at high temperatures (above 77°F), and some are active only at low soil temperatures. Soil moisture conditions of 70% or higher of available water capacity are conducive to infection by Pythium. It is likely that soil from a field contains several pathogenic Pythium species.

Pythium species form several types of spores, but not all species form all types. Zoospores, which are produced in sporangia, are motile in water. Oospores, which result from a sexual process, usually undergo a period of dormancy and can withstand long periods of drying. Some species also form chlamydospores, which are asexual and have thick cell walls. These structures can serve as survival or overwintering structures. Sporangia and zoospores in general do not survive in air or dry soil for long periods of time.

In the control of Pythium diseases, emphasis is placed on providing good drainage and water management.

• Steam (at 140°F for 30 minutes), solarize (double-tent at 160°F for 30 minutes or 140°F at 1 hour), or chemically treat the growing medium.
• Sanitize well because Pythium spp. can survive in dust, planting medium, or soil particles on greenhouse floors and in flats and pots.
• Remove and discard diseased plants.
• Use of properly composted pine bark as 20% of a potting mixture is reported to provide some control of Pythium and Phytophthora root rots. Additionally, the mycoparasite Gliocladium virens is used as a Pythium biocontrol agent.

For flower production in outside fields, solarization has been successful for control of damping–off in many crops that are grown in warmer climates. There are reports of inadequate control of some high temperature species (e.g. P. aphanidermatum). Solarization and steaming are acceptable for organic production.