Pythium Root Rot

While root rot can be caused by several different species of the fungus-like organism Pythium, the three most commonly encountered species are Pythium irregulare, Pythium aphanidermatum, and Pythium ultimum. P. ultimum and P. irregulare are often found in field soil, sand, pond and stream water and their sediments, and dead roots of previous crops. P irregulare has 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 can be 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 and shorefly 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. P. aphanidermatum and P. irregulare pose a threat to crops grown in ebb and flow systems because they form a swimming spore stage that can move in water. This is likely to occur only if irrigation times are long (45 minutes or longer) or if pots sit in puddles of water because the bench or floor does not drain completely. If Pythium infests a cutting bed or if contaminated water is used in propagation, large losses occur. Pythium ultimum is primarily associated with soil and sand. As 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 but can be a problem in ebb and flow systems if the reservoir becomes fouled with potting mix and plant debris particles harboring it. 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 midday and may recover at night.
• Plants yellow and die.
• Brown tissue on the outer portion of the root easily pulls off leaving a bare 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 180 degrees F and held at that temperature for 30 minutes. 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 nontreated 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 suspected of being a source of Pythium, it may be necessary to treat the water before use. First, contact the author (see address above) to arrange to have the water tested for Pythium. Slow sand filtration has been shown to be an effective, simple, and inexpensive method for removing Pythium from water. Heat, ultraviolet light, ozonation, and chlorination can also be effective but are expensive and require some training to be used properly.

Cover ebb and flow 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.

Some populations of Pythium have resistance to metalaxyl, mefenoxam, and/or propamocarb. If these chemicals do not appear to be protecting your plants, switch to another product and contact the author of this fact sheet for assistance in assessing the cause of the problem.

The following biological agents and chemicals are registered for controlling Pythium. Be certain the crop to be treated is listed on the product label before treating the crop.

Chemical Controls

Common name	Trade name
etridiazole	Truban, Terrazole
etridiazole + thiophanate methyl	Banrot
fosetyl-Al	Aliette
mefenoxam	Subdue Maxx
metalaxyl	Subdue
propamocarb	Banol

Genus	Trade name
Gliocladium	SoilGard
Trichoderma	PlantShield, Bio-Trek, RootShield
Bacillus	Companion
Streptomyces	Mycostop