Many species of Pythium cause diseases of roots, crowns, and/or foliage that result in a general decline of turfgrass stands. Disease may occur in small patches or involve large areas, especially on highly maintained golf course greens. Depending on environmental conditions and the species present, symptoms may appear any time from early spring to late autumn. Microscopic examination of infected tissue typically reveals oospores (survival structures) and/or sporangia (spore-bearing structures) and zoospores (motile spores).
Pythium species are natural inhabitants of the soil. It is not unusual for the organism to be present at low levels without causing disease. Conditions that suppress turf growth and encourage the proliferation of Pythium can disrupt this natural balance and lead to disease development. Pythium may cause disease in several different ways.
Damping-off: This disease occurs only on seedlings. When Pythium is growing rapidly and seedling growth is slow, the pathogen forms stem cankers at the soil line, causing seedlings to lodge. If moisture, temperature, and light conditions are not optimal for seedlings, seed decay and damping-off become more likely.
Crown and Root Rot: Several species of Pythium cause diseases of roots and crowns that result in a general decline of turfgrass stands. The disease may occur in small patches or involve large areas, especially on highly maintained golf course greens. Symptoms are nonspecific with the affected turf appearing thin, off-color, and stunted. Symptoms may appear from early spring to late autumn and typically occur after a period of wet weather. Large areas of turf may wilt, turn brown, and die. The crowns of individual plants may appear water soaked and discolored. The root system may be greatly reduced and discolored, and roots may have a "rat-tail" appearance due to the separation of the cortex from the vascular cylinder. Crown and root rot can occur under both low and high soil temperatures depending upon the species of Pythium present.
Pythium Root Dysfunction (PRD): This disease occurs only on golf greens under very specific conditions. See our PRD fact sheet for information.
Snow Blight: This uncommon foliar blight is favored by high nitrogen fertility, poor drainage, extended periods of cold weather, and saturated soil underlying snow cover. At least six Pythium species can cause snow blight. Symptoms may appear as small tan to orange spots or as a more uniform blight. Foliar mycelium may or may not be present and roots are mostly unaffected, but crowns can be extensively rotted, resulting in plant death. Snow blight is most serious on turf growing in unfrozen, saturated soil underneath snow cover.
Pythium Blight (Grease Spot, Cottony Blight): Although several Pythium species are capable of causing blight on turf, P. aphanidermatum is the most common and typically the most destructive. Pythium blight can occur in cooler weather if another species is present, but with P. aphanidermatum the disease usually appears suddenly in hot, humid weather. Symptoms may appear as water-soaked, dark, oily looking patches ("grease spot"). Spots may also be bronze or tan and are generally less than 6 inches in diameter, but these can expand rapidly and large areas may be affected. Blighting may also appear in streaks that follow the movement of water or machinery. When humidity is high, especially at night, the collapsed leaves may be covered with a fluffy, white mass of mycelium ("cottony blight"). Spots may coalesce to form large, irregularly shaped areas of dead turf, especially in areas of poor drainage. High nitrogen fertility promotes disease development. Foliar blight is most severe in periods of high humidity when day temperatures are above 82°F (28°C) and night temperatures are above 70°F (21°C). Disease is favored when the leaf wetness period exceeds 12 hours. Crown and root rot often accompany foliar blight.
Cultural Management
- Improve surface and subsurface drainage.
- Water thoroughly and infrequently. Reduce automatic overhead irrigation and syringe on an as needed basis.
- Avoid over watering and watering late in the day.
- Ensure adequate nutrition. Avoid overfertilizating and use slow-release N. Calcium deficiency can also increase susceptibility to Pythium blight.
- Mechanically remove thatch if it exceeds 0.5".
- Do not mow when the grass is wet or when foliar mycelium is present. Affected areas should only be mowed when dry, preferably with a lightweight walk-behind mower. Never mow on rainy days or when soil is excessively wet.
- Prune nearby trees and shrubs to improve air circulation and light penetration.
- Promote vigorous root growth by raising mowing height, reducing mowing frequency, and other management practices to reduce plant stress.
- Avoid aeration, vertical cutting, or topdressing of diseased putting greens.
- Plant less susceptible turfgrass species and cultivars. For more information, see The National Turfgrass Evaluation Program website (www.ntep.org)
Chemical Management
Fungicides targeting Pythium should be applied in recommended tank mixes and rotated among FRAC groups. Rotation is critical for the prevention of resistance development. Resistance to strobilurins (11), mefenoxam (4), and propamocarb (28) has been documented in Pythium populations. Repeated applications of fungicides with the same mode of action favor the development of fungicide resistance. Fungicide efficacy can be increased when applications are based upon weather forecasting models. Use fungicide treated seed to avoid damping-off.
For a listing of fungicides currently labeled to manage this disease, refer to the Disease Management chapter of UMass Extension's Professional Guide for IPM in Turf for Massachusetts.