Pythium Root Rot: Identification, Prevention, and Management in Greenhouse Crops

Understanding Pythium Root Rot

Pythium root rot is a pervasive disease in greenhouse production caused by several species of this fungus-like organism. In the northeastern United States, the most commonly encountered species are Pythium irregulare, Pythium aphanidermatum, Pythium cryptoirregulare, and Pythium ultimum. While P. irregulare and P. cryptoirregulare infect a wide range of greenhouse crops, P. aphanidermatum is primarily associated with poinsettia. P. ultimum, historically linked to field soil and sand, has become less prevalent with the shift to soilless potting mixes.

Sources and Spread

Pythium species naturally inhabit field soil, sand, pond and stream water, sediments, and dead roots of previous crops. The pathogen is rarely found in commercial soilless mixes but is easily introduced through contaminated tools, pots, flats, foot traffic, pets, or by placing mixes on unclean surfaces. Fungus gnat larvae and shorefly adults can also vector the pathogen within greenhouses. Over-pasteurization of soil mixes (excessive heat or duration) eliminates beneficial competitors, allowing Pythium to cause severe root rot unchecked.

In ebb-and-flood irrigation systems, Pythium becomes a significant threat only when irrigation cycles exceed 45 minutes or when poor drainage leaves pots sitting in standing water. Hydroponic systems are especially vulnerable. If a cutting bed becomes infested, losses can be substantial.

Symptoms

• Stunted plant growth
• Brown, dead root tips
• Midday wilting with nighttime recovery
• Yellowing and eventual plant death
• Outer root cortex sloughs off, exposing the vascular strand
• Microscopic, thick-walled, round spores (oospores) within root cells

Management Strategies

Prevention First

Control is difficult once rot begins; prevention is paramount. Use heat-pasteurized potting mix (entire pile heated to 180°F for 30 minutes). Longer times or higher temperatures kill beneficial organisms. Store treated soil and commercial soilless mixes in clean, covered areas to prevent recontamination.

Water Management

If using pond or stream water, ensure intake pipes are well above the bottom to avoid drawing in sediment. Slow sand filtration is an effective, inexpensive method to remove Pythium from water. Heat, UV light, ozone, and chlorination can also be effective but are costlier and require proper training.

Cover ebb-and-flood reservoirs to exclude debris. Screen return water to remove potting mix and plant debris before it re-enters the reservoir.

Sanitation

Disinfect all bench surfaces, potting benches, tools, and equipment contacting the potting mix. Periodically clean and disinfect ebb-and-flow reservoirs, benches, and flood floors.

Chemical and Biological Control

In operations with a history of Pythium root rot, apply a fungicide or biological control agent early in the cropping cycle. Biological agents should be applied to the potting mix before, during, or immediately after transplanting, or even to plugs pre-transplant. Do not apply chemical pesticides 10 days before or after biological agent application. Repeat applications may be necessary for sustained protection.

Contact Cooperative Extension for currently registered fungicides and biological agents. Resistance to metalaxyl, mefenoxam, and propamocarb has been documented. High-risk systemic fungicides should be used sparingly and rotated or mixed with chemicals having different modes of action (different FRAC codes) to delay resistance development.