While root rot can be caused by several different species of the fungus-like organism Pythium, the most commonly encountered species in the northeastern U. S. are Pythium irregulare, Pythium aphanidermatum, Pythium cryptoirregulare and Pythium ultimum.

Some species of Pythium are found in field soil, sand, pond and stream water and their sediments, and dead roots of previous crops. P. irregulare and P. cryptoirregulare have been isolated from almost every type of greenhouse crop grown but P. aphanidermatum seems to be associated primarily with poinsettia and very few other crops. Pythium is very rarely found in commercially available soilless potting mixes. It is easily introduced into pasteurized soil or soilless mixes by using dirty tools, dirty pots or flats, walking on or allowing pets to walk on the mixes and by dumping the mixes on benches or potting shed floors that have not been thoroughly cleaned.

Fungus gnat larvae and shorefly adult activity may also be involved in moving Pythium from place to place in greenhouses. When introduced into a soil mix that has been heat-treated for too long or at too high a temperature, Pythium can cause severe root rot because it has few competitors to check its activity.

Although Pythium species that form zoospores have long been considered a threat to crops grown in ebb and flood irrigation systems, this has not generally been true unless irrigation times are long (45 min. or longer) or if pots sit in puddles of water because the bench or floor does not drain completely. Pythium in hydroponic systems is definitely a threat to the crops. If Pythium infests a cutting bed, large losses occur. Pythium ultimum is primarily associated with soil and sand. When commercial growers switched to soilless mixes, this species became less important than when growers used field soil in the potting mix. P. ultimum does not form the swimming spore stage. Almost all plants are susceptible to Pythium root rot. Root tips, very important in taking up nutrients and water, are attacked and killed first. Pythium also can rot the base of cuttings.

Symptoms

• Plants are stunted.
• Root tips are brown and dead.
• Plants wilt at mid-day and may recover at night.
• Plants yellow and die.
• Brown tissue on the outer portion of the root easily pulls off leaving a strand of vascular tissue exposed.
• The cells of roots contain round, microscopic, thick-walled spores.

Management

Pythium root rot is difficult to control once rot has begun. Every effort should be directed toward preventing the disease before it begins by using heat-pasteurized potting mix (entire pile heated to 180F and held at that temperature for 30 min. Longer times and higher temperatures will kill beneficial organism in the soil.). Cover the treated soil and store it or commercial soilless mixes in an area that will not be contaminated through the introduction of non-treated soil.

If pond or stream water is used for irrigation, be certain the intake pipe is well above the bottom so that sediment is not drawn in. If the water supply is found to be a source of Pythium, it may be necessary to treat the water before use. Slow sand filtration has been shown to be an effective, simple, and inexpensive method for removing Pythium from water. Heat, ultraviolet light, ozone, and chlorination can be effective but are expensive and require some training to be used properly.

Cover ebb and flood irrigation system reservoirs to prevent contaminated debris from entering the system. Pass return water over a coarse screen to remove potting soil and plant debris in order to help keep Pythium out of the reservoir.

Disinfect all bench surfaces, potting benches, tools, and equipment that will contact the potting mix. Periodically, thoroughly clean and disinfect ebb and flow reservoirs, benches, and flood and drain floors.

In a greenhouse operation with a history of Pythium root rot, apply a fungicide or a biological control agent as early in the cropping cycle as possible. Biological agents should be applied to the potting mix before, during or immediately after transplant. They can even be applied to plants in plug trays before transplanting. Do not apply ANY chemical pesticides to the potting mix 10 days before or for 10 days after applying the biological control agent. Biological control agents and fungicides may have to be applied more than once in order to maintain adequate protection.

Contact Cooperative Extension for information on the fungicides and biological control agents registered for use against Pythium. Some populations of Pythium have resistance to metalaxyl, mefenoxam and/or propamocarb. Certain fungicides, usually systemic fungicides, are said to be 'at risk' to the development of resistance if they are used repeatedly. It is recommended that chemicals at high risk be used sparingly and in rotation or mixed with chemicals with different modes of actions (different FRAC number).