IPM Identification and Mangement Of Turfgrass Diseases

Author(s)

Summer Patch

Pathogen: Magnaporthe poae
Hosts: Kentucky bluegrass, annual bluegrass and fine fescues

Symptoms and signs

Aboveground symptoms of summer patch in Kentucky bluegrass initially develop in June through August as small patches of turf, 2 to 6 inches in diameter. Grass blades in the patch change to a dull reddish brown, then tan, and finally a light straw color. In the final stages of the disease, circular patches of dead grass form throughout the turf area. Healthy grass often occurs at the center of the patch, resulting in a characteristic frogeye pattern (Figure 1). Circular patch symptoms develop because the fungus grows radially from a central infection point.

When weather conditions are ideal for disease development, affected areas may overlap, resulting in large areas of affected turf (Figures 2, 3). In mixed stands of grasses, the pattern of symptoms is much more irregular. Summer patch can also cause extensive damage to annual bluegrass on golf course putting greens. Infected annual bluegrass surrounding unaffected bentgrass clones turns yellow to bronze (Figure 4). This preferential damage of annual bluegrass is a useful diagnostic feature of the disease.
Roots and crowns of infected plants are discolored (Figure 5). With a compound microscope, darkly pigmented runner hyphae are visible in and around infected roots (Figure 6).
Symptoms of summer patch can be confused with leaf and sheath spot, insect damage (grubs, billbug), or drought stress. These possible causes should be eliminated before a diagnosis of summer patch is made. The best means of confirming this disease is to submit a turfgrass sample along with images of the symptoms to a plant disease diagnostic lab for confirmation. The turfgrass sample should be cut from the margin of the patch that contains both healthy and diseased turf. Ideally, the sample should be 6 inches square and deep enough to include much of the root system.

Conditions

Although aboveground symptoms of summer patch don’t appear until summer, Magnaporthe poae colonizes grass roots and crowns in the spring. It first grows on the root surface (ectotrophically), but later invades the inner root tissue. Conditions in spring are optimal for turfgrass root growth, and even though fungal colonization is occurring, the turfgrass is able to absorb water and maintain growth.

Infected roots either die or become dysfunctional during the hot, dry summer months. Disease symptoms can be particularly severe on south-facing slopes. Infected roots are no longer able to supply adequate water to the foliage and plants exhibit aboveground symptoms of moisture stress. Severely damaged plants may die. In others, recovery is slow because generation of new roots is inhibited by high soil temperatures, and plants cannot rapidly replace diseased roots.

The information herein is supplied with the understanding that no discrimination is intended and that listing of commercial products, necessary to this guide, implies no endorsement by the authors or the Extension Services of Nebraska, Colorado, Wyoming or Montana. Criticism of products or equipment not listed is neither implied nor intended. Due to constantly changing labels, laws and regulations, the Extension Services can assume no liability for the suggested use of chemicals contained herein. Pesticides must be applied legally complying with all label directions and precautions on the pesticide container and any supplemental labeling and rules of state and federal pesticide regulatory agencies. State rules and regulations and special pesticide use allowances may vary from state to state: contact your State Department of Agriculture for the rules, regulations and allowances applicable in your state and locality.