Kentucky bluegrass (Poa pratensis), annual bluegrass (Poa annua), and fescues (Festuca sp.) can be affected during the summer by an interaction of environmental factors and a root and/or crown rot caused by the fungus Magnaporthe poae. This diseases is known as Summer Patch. Bentgrasses (Agrostis sp.) may also become infected but show few symptoms and may continue to perform where other grasses decline.
Summer Patch occurs between June and September. It can be difficult to diagnose this disease by symptoms alone in the early stages. The disease begins as scattered small round patches of thin, wilted or slow growing turf. Initially, affected patches may be only 3-8 cm in diameter, but thay may enlarge to about 30 cm in diameter (about 12 inches) and range from gray-green to light tan or straw-colored. In rarer circumstances, they may get to be twice that size. As patches enlarge, they may coalesce, and form crescents of yellow or tan turf. Where turf within the center of a patch begins to recover, necrotic rings may become evident.
Because Necrotic Ring Spot disease may exhibit similar symptoms, microscopic examination is often necessary to determine the cause of the problem. The roots, crowns and stolons of infected plants may appear to be dark brown as the dark mycelium of the funugus invades the tissue. As the disease progresses, the cortex may begin to rot, and plants may die.
Summer Patch blighted areas often occur on lawn sites that receive direct sun and are on south-facing slopes, or near sidewalks, driveways, buildings, or other "hot spots" or otherwise stresed areas in the yard or on golf courses. In the cool weather of autumn, the grass may begin to grow into these dead areas again. The disease, however, is likely to reappear in previously affected areas the following summer, and to increase in intensity. Summer patch usually occurs during the hotter periods (June, July, August) of the year. Summer patch is less of a problem during cool summers with adequate rainfall.
Disease Cycle
The fungus, Magnaporthe poae, survives unfavorable conditions as mycelium in infected plant tissue or plant debris. The optimum temperature for fungal growth is 28°C. Infection takes place in late spring when soil temperatures reach 19-20°C. Spread during the growing season occurs as the fungus grows between roots. Symptoms may not be evident until the temperature increases very drastically during wet weather. The pathogen may also be spread by movement of infected plant material and on mechanical equipment.
Management Strategies
The primary stresses that influence disease development include excesses of thatch, fertilizer, and turf canopy temperature, as well as incorrect timing of fertilizer applications, low mowing height, and pH extremes. Each of these stresses can be reduced through appropriate cultural practices described below. Disease severity may worsen at a higher pH, so try to maintain the pH of the soil and Rhizosphere at 5.5 to 6.0. Use an acidifying fertilizer where the pH is above 6.0, and try to avoid the use of products that may raise the pH.
For most bluegrass lawns, two to five lbs of nitrogen/1000 sq.ft. is sufficient. Apply this in a fertilizer balanced by phosphorus and potassium. Do not apply even small amounts of fertilizer during the June-August stress period because this will tend to stimulate the disease. Therefore, fertilize only in autumn (September through November) and in late spring (May). Deep watering is essential for proper root growth. Water the soil under disease-prone areas to a depth of 15 to 20 cm every 7-10 days during the dry periods of the summer. Soaker hoses are very useful for supplementary watering on steeper slopes where other sprinklers are inefficient. The harmful effects of excessive temperature can be reduced by a light sprinkling of the surface at mid-day. Proneness to disease in turf is increased as the cutting height is decreased. Cut lawns at 5 to 10 cm height, and often enough so that less than 1/3 of the leaf blade is removed during each mowing. Thatch (the layer of organic matter between the mineral soil and the green grass) should be no more than 1.5 cm (1/2 inches) in thickness. Thatch can be removed by vertical slicing machines and/or aeration during the spring and early fall. Over a longer period thatch will be reduced by using the cultural practices discussed above.
Kentucky bluegrass cultivars such as Adelphi, America, Aspen, Columbia, Eclipse, Glade, Midnight, Nassau, Parade, Ram I, Sydsport, Touchdown, Vantage, Windsor, and Victa are less susceptible to summer patch than others. Blend seed of resistant cultivars with that of one or more otherwise desirable cultivars.
Chemical treatment is efficient only when the previously mentioned cultural practices are first used. Furthermore, applications must be made before the crown rot develops sufficiently to cause visual symptoms of the disease. Fungicides containing the active ingredients azoxystrobin, cyproconazole, fenarimol, propiconazole, iprodione, thiophanate-methyl, trifloxystrobin or triadimefon are available for control of summer patch in commercial applications. Other products containing azoxystrobin, myclobutanil, thiophanate-methyl, or triadimefon may be available for use on home lawns by homeowners to manage this disease. Thoroughly water (applying 2 to 3 cm of water) areas with a history of this disease several days before applying the fungicide. To determine the best time to treat, monitor soil temperature to a depth of 2 inches, and make the first application when the soil temperature reaches 55°F. or about 13°C. Additional applications may be required for certain fungicides. Always adhere to the rates and procedures recommended on the fungicide package label.
Reprinted in part from "SUMMER PATCH ON TURFGRASS" a Cornell University Plant Disease Diagnostic Clinic factsheet http://plantclinic.cornell.edu/factsheets/summerpatch/summer_patch.htm
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