See:
Greenhouse Plants, Ornamental-Pythium Seed Rot, Damping-off, and Root Rot
Cause Pythium species are common soilborne pathogens that infect seeds and roots of many greenhouse crops. All Pythium species are strongly favored by abundant moisture and can be especially problematic in poorly drained potting media. Pythium usually attacks seeds or seedlings in the greenhouse. However, cuttings and older plants may also be affected, especially if the potting media is too wet or placed on a poorly-drained surface. Substrate temperatures above 77°F favor the development of some Pythium spp. such as P. aphanidermatum.
Pythium spp. can survive in dust, planting media, or soil particles on greenhouse floors, flats, and pots. Wood benches can also be a reservoir. Spread can also occur via movement of infected plants. Damaged plants are more easily infected. It enters roots damaged by high soluble salts, either from over-fertilization or from letting the rooting medium dry out, even for short periods, during propagation.
Cold irrigation water greatly slows growth for a few hours following each watering and will prolong the maturation period and lengthen the time during which plants are susceptible to damping-off.
Symptoms First symptoms often include poor seed germination and emergence referred to as damping-off. Seeds become soft, shrivel, and disintegrate. If germination occurs, very young, small seedlings appear to suddenly collapse and die. Older seedlings develop brown to gray, water-soaked lesions at or below the soil line. Often the lower part of the stem withers before the upper stem and leaves causing infected seedlings to fall over on top of the soil. Disease progression can be very rapid and spread to adjacent healthy seedlings. In extremely moist conditions, white cottony fungal growth may appear on the surface of the seedlings and the soil.
Older plants can appear stunted or chlorotic, and may wilt even when enough water is available for growth. Diseased roots are discolored brown and/or appear water soaked. Fine roots turn mushy and slough off leaving mostly thicker, older roots behind. The root cortex of the smaller roots can often be easily slipped off, thus leaving a white cylinder of vascular tissue behind.
Cultural control Good greenhouse hygiene will help reduce risk of these diseases. Make cleanliness a priority. Avoid disease-promoting conditions such as poor sanitation, inadequate drainage, and inappropriate irrigation.
- Always use disease-free planting materials.
- Rooted plants acquired from other nurseries should be inspected and placed in a quarantine area where they can be held and monitored for disease development for a few weeks.
- Use new pots, trays, or treat any reused items with a disinfectant, use steam, or solarization techniques (such as with anti-condensation film) to clean them.
- Use soilless growing media that is stored under cover in a concrete bay or in some other clean and sealed container kept dry, cool, and in a low-light environment.
- Growing media could be treated by steam (140°F for 30 minutes), solarization (double-tent at 160°F for 30 minutes or 140°F for 1 hour), or chemically. Storage area surfaces should be easy to clean and disinfest. This could include metal or plastic bins, trailers, trolleys, in bags on a sealed surface, or racks under cover.
- Use ground sources for irrigation and avoid surface water sources for propagation of the crop.
- Good drainage and water management are essential components of a management plan. Ensure growing media and holding areas are well drained.
- Avoid soil moisture conditions of 70% or higher.
- Do not over-fertilize or let plants dry out. Monitor soluble salt concentrations regularly.
- Avoid rough handling of plants, especially when starting and ending the crop.
- Remove diseased plants immediately and carry out rigorous hygiene measures to prevent further spread.
- Remove crop residue and old or low-quality plants that will not be marketed as they can become sources of inoculum.
- Replace wood benches or potting tables for ones made of plastic or wire that will facilitate cleaning and disinfestation.
- Regularly sterilize any tools used in propagation or pruning using 70% alcohol solution or quaternary ammonium-based products.
Chemical control Fungicides are best used as protectants in combination with cultural controls. Once symptoms are observed, the disease is difficult to control. Therefore, prevention is key. Rotate among fungicides with different modes of action to help prevent fungicide resistance from developing. Be sure to check labels for crop safety before application.
Biological control Several studies have shown the effectiveness of some strains of Trichoderma spp. to control or improve suppression of Pythium and other oomycetes in ornamental crops under greenhouse conditions. Use in conjunction with other control tactics such as thorough sanitation.
References Weiland, J.E., Santamaria, L., and Grunwald, N.J. 2014. Sensitivity of Pythium irregulare, P. sylvaticum, and P. ultimum from forest nurseries to mefenoxam and fosetyl-Al, and control of Pythium damping-off. Plant Disease 98: 937-942.