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ISSUE 14   August 2, 2001



High heat, rain, high humidity; frequent rain, heat, high humidity! The cycle repeats itself over and over, with the result being a plethora disease problems showing up on turfgrass. Here is the list of what may be showing up on your turfgrass at this time of year, under the weather conditions we have been experiencing pretty much state-wide:

Cool-Season Turfgrasses:

Dollar Spot - circular patches from 1-6" in size of bleached turf; leaf lesions that are light tan with reddish-brown borders, that may also girdle the grass blade; white fungal (looks like cobweb) growth may be visible with morning dew present. Shows up on Kentucky bluegrass, bentgrass, and fine fescue lawns.

Rhizoctonia Blight, aka Brown Patch - Circular patches up to 2 feet in diameter of light brown turf; close-cut turf may have a dark border or smoke ring; higher-cut turf may have patch borders that are yellowish-green in color; light brown lesions with dark borders occur on leaf blades, with the areas beyond the leaf lesions being possibly dark in color - a water-soaked appearance; dark fungal growth at the base of the plant. Affects all cool-season grasses, especially in areas of poor drainage, the shade, or that stay continuously wet.

Summer Patch - Begins as dark circular patches that appear drought-stressed, later, turning a light straw color. The shapes of the patches may be streaks, crescents, or circles, with the circular patches forming "frog-eyes" of green turf surrounded by brown circles. The crowns usually die. This, and necrotic ring spot are very similar in visual appearances, with near identical conditions that favor disease development. They, along with Brown Patch, are quite common occurrences with peat backed sod that has been laid on heavy clay soil. Humid conditions, drought stress, and heavy thatch are also major contributors to the development of these diseases.

Pythium Blight - common on seedling emergence under high temperatures, poor air circulation, and high humidity. Irregular spots that are brown, usually merging, and likely with black edges. The disease is known as damping-off when it effects emerging seedlings.

Powdery Mildew and Rust - Both of these pathogens are manifesting themselves earlier than normal (usually late August). Powdery mildew covers the leaves of the turfgrass with a white to gray powdery growth, starting usually in the shade. Rust shows up as a yellowish-orange color that gets kicked into the air and on the shoes during the mowing process. Both pathogens are common on grass that has been planted within the last year.

Controlling the pathogens: Maintain a good fertility level, where nitrogen is not excessively applied. Also, core aeration followed by power raking will help to improve drainage from the surface, get air into the root zone, and improve nutrient availability. Antagonistic microbes from the coring and power-raking activity will help to decrease thatch thickness and sequester many of the disease organisms. Fungicides for treatment abound on the market, but use is advised only on a limited, selected basis because of resistance problems.

Ron Smith
NDSU Extension Horticulturist and Turfgrass Specialist


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