Extension Communications
3614 Administrative Services Building
Ames, Iowa 50011-3614
(515) 294-9915
8/2/99
Contacts:
Leonor Leandro, Department of Plant Pathology, (515)
294-0589, leonorl@iastate.edu
Elaine Edwards, Extension Communication Systems, (515)
294-5168, eedwards@iastate.edu
Yard and Garden Column for the Week Beginning Aug. 6
Doughnuts on your lawn?
By Leonor Leandro
Plant Pathology Research Assistant
Iowa State University Extension
Over the last few weeks, you may have noticed some doughnut-shaped rings of dead grass on your lawn. Is this some kind of prank involving bored teenagers and a bottle of bleach? Evidence of an alien invasion of Earth? Or are more sinister forces at work?
The culprit in this scenario is most likely neither vandals nor aliens, but an inconspicuous fungus named Magnaporthe poae. M. poae plays these tricks on even the most well-tended lawns, causing a disease known as summer patch.
Summer patch is one of the most damaging diseases of Kentucky bluegrass, the main species used in most Iowa lawns. M. poae lives in the soil and makes its living by nibbling on grass roots. Under favorable soil temperatures (65-70 F) and high moisture, the fungus will invade the root tissues and disrupt the flow of water to the shoots. During the spring and fall, Kentucky bluegrass lawns look just fine even though M. poae is busy doing a number on the roots. But in the hot portion of the summer, when your lawn is stressed out by heat and drought, problems appear as the root-deprived grass begins to die of thirst.
Summer patch symptoms are striking. The disease starts with small patches of wilted, gray-green turf that rapidly develop a tan color and form circular or crescent shapes. Rings of straw-colored grass, six to 12 inches in diameter, with a tuft of healthy grass in the center, create the typical characteristic doughnut-shaped patterns known as 'frog-eyes.' In severe cases, the rings may grow together, leaving behind only a few scattered tufts of green grass.
As you might expect with a stress-linked disease, summer patch symptoms tend to appear in areas of your lawn where heat and drying are greatest. This is likely to be a full-sun area that faces south or southwest, or perhaps near sidewalks or driveways. Sometimes, the injured turf recovers a normal look in the fall, and remains healthy-appearing until the first heat wave next summer. In other cases, summer patch damage is so severe that reseeding or resodding is required.
Summer patch can be confused with several other lawn diseases, so it's helpful to get a second opinion. Samples of sick turfgrass can be sent to the Plant Disease Clinic, 351 Bessey Hall, Iowa State University., Ames, IA 50011 (phone: 515-294-0581). An adequate sample should consist of a chunk of turf, including roots, about 6 inches long and 3 inches wide, which traverses the edge of a diseased area. You can bring the sample to a county extension office, or you can send it directly to the Plant Disease Clinic. Place the turf in a sturdy box for mailing, but don't seal it in a plastic bag since it will overheat during shipment. The Plant Disease Clinic charges a $10 fee for sample diagnosis.
After years of experience, diagnosticians like Paula Flynn, head of the ISU Plant Disease Clinic, can pinpoint common risk factors behind many summer patch outbreaks. Among the factors: lawns that have been sodded within the last five to seven years; a tight, poorly drained soil underneath the sod (the legacy of construction crews who smeared the clay excavated from your basement onto the lawn, then covered the evidence with sod); and full-sun exposure.
A key to beating summer patch is to keep in mind is that this disease loves stressed-out turfgrass. Management practices are based on minimizing stresses and encouraging healthy root growth. Here are some recommendations:
1. Aerate your lawn. Compacted, clayey soils make it tough for grass roots to grow and breathe. High-traffic areas can become compacted, too. To moderate this stressful situation, you can core-aerate in the spring and fall. A core aerator opens numerous small holes in the lawn, which will allow the roots to breathe and grow better, improve water penetration, and reduce compaction. You can rent core aerators, or you can have a lawn care professional do the job.
2. Remove thatch. Thatch is a layer of dead grass that accumulates between the soil surface and the grass blades. When this layer is excessive, grass roots tend to grow into the thatch instead of into the soil. Under drought conditions, the thatch dries out, exposing the roots to severe drought stress. You can dethatch your lawn by vertical mowing or a power raking in spring or early fall. Core-aeration also helps to reduce thatch.
3. Balance fertilization. Excessive nitrogen fertilization favors leaf growth at the expense of root growth, making the lawn more susceptible to stress and summer patch. Fall and spring applications of a slow-release form of nitrogen, such as sulfur coated urea, is recommended.
4. Mow the lawn higher in the summer than at other times. Increasing the mowing height to 3 inches will decrease evaporation from the soil and encourage deeper rooting.
5. Maintain adequate soil moisture. Water your lawn when you notice signs of drought stress, applying enough water to soak the entire root zone. Too-frequent watering favors the development of other fungal diseases, so water just once per week. On very hot days, you can reduce heat stress by sprinkling your lawn for 10-15 minutes between noon and 4 p.m. - a practice called syringing.
6. If you plan to reseed a summer-patch-damaged area or seed a new lawn, use resistant cultivars of Kentucky bluegrass, mixed with perennial ryegrass and turf-type fescues. Genetic resistance is the cheapest, and one of the most effective, management tactics.
7. Finally, certain "systemic" fungicides (in other words, they can enter into the grass plant) are effective against summer patch. The catch is that they must be applied preventatively. Keep in mind that although you see no symptoms until summer, the summer patch fungus is busy chomping turfgrass roots. In Iowa, fungicides treatments should be applied at intervals of two to four weeks from early May to July. Some currently labeled products include propiconazole, triadimefon, fenarimol, benomyl, thiophanate-methyl, and azoxystrobin. Effectiveness of these treatments depends on the correct diagnosis of the disease and on proper timing of applications. It's also important to realize that fungicides can't replace the cultural, stress-management practices mentioned above.
Are you in the process of building a new house? If so, perhaps the best summer-patch-prevention tip of all is to watch your construction crews like a hawk, so your lawn can grow into deep, well-drained Iowa topsoil instead of struggling through that bad clay from the basement.
ml: isugarden
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