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Horticulture and Home Pest News
Horticulture & Home Pest News is filled with articles on current horticulture, plant care, pest management, and common household pests written by Iowa State University Extension specialists in the Departments of Entomology, Horticulture and Plant Pathology.

Air Conditioner Diseases (Pythium Blight and Brown Patch)

This article was published originally on 7/11/2003

The continual use of an air conditioner is a sign that it s time to look for brown patch (Rhizoctonia blight) and Pythium blight. These two diseases commonly affect lawns and golf courses in Iowa when the weather is warm and humid. The fungi that cause these diseases can infect most types of grasses.

Pythium blight starts as small (about 6 inches), circular, reddish brown spots. The disease pattern may occur in streaks following the path that water drains and the path of mowing equipment or foot traffic. Blighted patches can grow to become large, irregular shapes. White cottony fungal threads (mycelium) form on infected blades early in the morning. Infected plants collapse and the diseased areas may become matted and greasy in appearance.

If the temperature drops or the relative humidity lowers, Pythium blight symptoms halt. Management strategies:

  1. Fill in areas where water puddles.
  2. Mow when the grass is dry. Avoid removingmore than 1/3 of the blade at one time.
  3. Improve air circulation by pruning shrubs and trees.
  4. Follow a balanced fertilizer program.
  5. Water deeply and infrequently in the earlymorning hours.
  6. Core aerate in the spring or fall to avoid thatch buildup.
  7. Preventive fungicide may be used on highlyvaluable turfgrass. The treatment should be sprayed before the onset of hot, humid weather.Subsequent applications may be required.

    Brown patch symptoms are light brown circular patches that may be up to 2 feet in diameter. The patches may join together and form a very large diseased area. Sometimes the grass in the center of the patch may look unaffected, causing confusion with other turfgrass diseases such as summer patch. At close inspection of the blades, elongated, irregular shaped bleached spots surrounded by a dark margin can be observed. On warm, humid mornings a smoke ring (grayish mycelium) may be seen on the edge of the patch.

Brown patch is usually most severe on heavily fertilized, irrigated lawns. Poorly drained soils, thatch, and nighttime irrigation that extend the duration of blade wetness all contribute to disease severity. Management strategies:

  1. Using slow-release nitrogen fertilizer.
  2. Removing grass clippings from diseased areas.
  3. Improving air circulation where possible by pruning trees and shrubs.
  4. Watering deeply and infrequently early in the morning.
  5. Manually removing morning dew by dragging a hose or rinsing off the grass with water(droplets on the blades contain sugars that fungi like to feed on).
  6. Mow with sharp blades and keep the grass about 3 inches. Avoid removing more than 1/3 of the blade at one cutting.
  7. Core aerate in the spring or fall to avoidthatch buildup.
  8. Fungicides can be incorporated into themanagement program, but do not effectively control brown patch if used without good cultural practices such as those mentioned above.



This article originally appeared in the 7/11/2003 issue.

Year of Publication: 
2003
Issue: 
IC-489(17) -- July 11, 2003