Temperatures have been high both day ( 85F) and night ( 60F). If high humidity and 10 hours or longer of blade wetness is added to the scenario, your lawn may come down with a case of brown patch, also known as Rhizoctonia blight. The common fungus Rhizoctonia solani causes brown patch. Most types of grasses and several non-grass plants can be infected by the fungus.
Typical symptoms of brown patch are light brown circular patches that may be up to two 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 appear 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. These spots may mimic other diseases. In order to diagnose brown patch, the overall pattern in the lawn in combination with blade symptoms need to be considered.
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 include the use of slow-release nitrogen fertilizer, removal of clippings from diseased areas, improving air circulation where possible by pruning trees and shrubs, watering deeply and infrequently early in the morning, manually remove 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). Dull mower blades create entryways for the fungus. So, keeping blades sharp can help prevent spreading the fungus. Fungicides can be incorporated in to the management program, but will not effectively control brown patch if used alone.
This article originally appeared in the August 10, 2001 issue, p. 102.