Understanding Pythium Blight in Lawns: Causes, Prevention, and Treatment

What is Pythium Blight?

Pythium blight, also known as grease spot or cottony blight, is a turfgrass disease caused by several species of Pythium, with P. aphanidermatum being the most common and destructive. The disease manifests as water-soaked, dark, oily patches that can expand rapidly under favorable conditions. In high humidity, these patches may be covered with a fluffy white mass of mycelium, giving it the name “cottony blight.”

Causes of Pythium Blight

Pythium species are natural soil inhabitants, often present without causing harm. However, specific environmental conditions can trigger their proliferation, leading to disease development. Factors that contribute to Pythium blight include:

• High humidity and temperature: The disease thrives in hot, humid weather, particularly when daytime temperatures exceed 82°F (28°C) and nighttime temperatures are above 70°F (21°C).
• Excess moisture: Prolonged leaf wetness, especially when it exceeds 12 hours, creates an ideal environment for Pythium to flourish.
• Poor drainage: Areas with inadequate drainage are more susceptible to disease.
• High nitrogen levels: Over-fertilization, especially with quick-release nitrogen, can promote disease development.

Types of Pythium Diseases

1. Damping-off: Affects seedlings, causing stem cankers at the soil line and leading to seedling collapse.
2. Crown and Root Rot: Results in a general decline of turfgrass, with symptoms including thin, off-color, and stunted turf. This can occur from early spring to late autumn.
3. Snow Blight: Occurs under snow cover in high nitrogen, poorly drained soils. Symptoms include tan to orange spots or uniform blight.
4. Pythium Root Dysfunction (PRD): A specific disease affecting golf greens under certain conditions.

Preventing Pythium Blight

Effective management practices can significantly reduce the risk of Pythium blight:

• Improve drainage: Ensure both surface and subsurface drainage are adequate.
• Water management: Water deeply and infrequently, avoiding overwatering and late-day irrigation.
• Nutrient management: Use slow-release nitrogen fertilizers and avoid over-fertilization. Ensure adequate calcium levels.
• Thatch control: Regularly remove thatch if it exceeds 0.5 inches.
• Mowing practices: Avoid mowing when grass is wet or when mycelium is present. Use lightweight, walk-behind mowers when the grass is dry.
• Air circulation: Prune nearby trees and shrubs to enhance air flow and light penetration.
• Promote root growth: Raise mowing height and reduce mowing frequency to reduce plant stress.

Treating Pythium Blight

If Pythium blight is already present, fungicides can be an effective treatment. It is crucial to use fungicides in tank mixes and rotate among different FRAC groups to prevent resistance. Weather-based applications can increase fungicide efficacy, making treatments more effective. Additionally, using fungicide-treated seed can help prevent damping-off in seedlings. For a comprehensive list of fungicides labeled for Pythium management, refer to the Disease Management chapter of UMass Extension’s Professional Guide for IPM in Turf for Massachusetts.

If you suspect that your lawn may have Pythium blight, it is essential to seek professional help quickly, especially if conditions are favorable for the disease to spread. Rapid intervention can prevent extensive damage and preserve the health of your turf.

For even the most experienced turf care specialist Pythium blight is a formidable challenge for lawn and turfgrass management, but with proper cultural and chemical practices, it can be effectively prevented and controlled. By understanding the conditions that favor its development and implementing proactive measures, you can maintain a healthy, vibrant lawn even in the face of this persistent disease.