Summer patch has been showing up on Iowa lawns for the last 2-3 weeks. This disease is one of the most damaging problems of turfgrasses and sometimes one of the most frustrating to control. The main host plants are Kentucky bluegrass - the main component of almost all Iowa lawns - and fine-leaved fescues, which are often found in "shade-adapted" grass seed mixes.
Summer patch is caused by a fungus, Magnaporthe poae, which lives in the soil and attacks the roots. It produces dark-colored hyphae (fungal strands) that can damage and kill roots. The symptoms of summer patch are striking: circles or semicircles of grass, 6 to 12 inches in diameter, that are light tan in color and matted in appearance. Tan circles with green grass in the center are often referred to as "frog-eye" symptoms because of the dark center and light border (don't ask me where the "frog" part comes from!). Symptoms can proliferate in a lawn to the point where some areas have more damage than healthy lawn. A similar disease, called necrotic ring spot, produces identical symptoms in fall and spring, but summer patch typically appears in late June and July. Experience indicates that summer patch is by far the more common problem in Iowa.
Scenarios for outbreaks of severe summer patch often have certain elements in common. One of these shared elements is water stress; that is, the root system is deprived of water during the hottest parts of the summer. Ironically, water stress that leads to summer patch is often worst on lawns that are maintained very intensively; in other words, the very lawns whose owners work the hardest, and pay the most, for picture-perfect turfgrass. A common soil scenario for severe summer patch in Iowa goes like this: a site that has been sodded within the last 3-5 years, on either a heavy-textured (clay) subsoil or a light, sandy or gravelly subsoil. In both types of soil, the midsummer situation for the roots can be similar: they can't get enough water when they need it most. The roots of the sod on the clay soil are often very slow to penetrate the tough, poorly oxygenated clay, so they don't extend deeply. In the sands and gravel, the entire soil profile dries out rapidly due to its high porosity. The result is the same: root systems that are weakened by drought and heat stress. These weakened roots are easy prey for the summer patch fungus. Additional factors that make a lawn more vulnerable to summer patch also contribute to drought stress; these include excessive thatch (which often comes with the sod when you buy it), compaction (typical on lawns around newly constructed homes), and mowing too low.
Management of summer patch starts with good cultural practices. This means raising the mowing height to at least 3 inches in summer. If thatch is a problem, it will be helpful to vertical-mulch or power-rake the lawn in spring and fall (appropriate equipment can be rented, or you can contract to have this done twice a year). Core-aerating helps relieve compaction and helps roots obtain oxygen in heavy soils (again, you can rent equipemnt or hire a specialist). One researcher on summer patch has had success in preventing summer patch by syringing (applying water for brief periods, say 10-15 minutes) every 1-2 hr during the hottest part of midday in midsummer.
A complementary approach is application of fungicides. Systemic fungicides labeled for summer patch control, such as Banner, Rubigan, Lynx, and others, can be applied every 3-4 weeks beginning when the grass starts to grow (early May) and continuing through the end of June.
When summer patch is severe, overseeding is probably the only option. It's important to overseed with blends and mixtures of grasses with tolerance or resistance to summer patch. Check with you seed supplier for appropriate blends that work in your area. A mixture of Kentucky bleugrass with perennial ryegrass and turf-type tall fescue varieties will lessen the risk of severe summer patch and also reduce the risk of other diseases. This is a long-term strategy that can help the lawn avoid severe summer patch die-off in the future.
This article originally appeared in the August 7, 1998 issue, pp. 105-106.