Allen D. Owings, Koske, Thomas J. | 4/20/2005 12:28:16 AM
Localized dry spot (LDS) is one of the most serious summer problems that we encounter during dry periods. When attempting to determine the cause of dying patches of grass during the summer, always check the soil first. If the top couple of inches of soil is dry, trying to correct the problem with pesticides may well be futile unless chinch bugs are present.
Localized dry spot first appears as small diameter patches of dead or dying turfgrass. The spots will continue to enlarge in a circular pattern, as any number of diseases appear to do. Soil beneath the spots will be powder dry to a depth of 1 to 2 inches. This is especially true of sandy or thatchy turf.
After a good summer rain, the turf appearance will usually improve, but only for a few days. This improvement is short-lived because little moisture actually penetrates the soil. If the soil can absorb moisture only at the rate of a tenth inch per hour, rainfall at 1 inch per hour will be of little value, since much of it will just run off. In addition, when a thatchy lawn becomes dry, the thatch becomes hydrophobic (water repellent). It could take several hours of light rainfall just to penetrate the thatch.
Dry spot most often occurs from one or more causal agents. Fungi or humic acids coat sand particles and make them repel water, a compacted 'plow pan' layer or buried debris stops water percolation or tree roots dry areas of feeder root extensions past the drip line of the tree.
Also, there is a type of non-toadstool fairy ring that can leave arcs or rings of thin, dying grass because of its water repellent fungal body. Pull a soil core or wedge and place a few drops of water on the first few inches beneath the turf. See if it beads up or penetrates. If it beads, you have a hydrophobic localized dry spot.
To correct localized dry spot problems, you must irrigate. Application of a special soil-wetting agent is very helpful in enhancing water penetration. Several lighter applications are usually less yellowing than one heavy application. Most sprinklers apply water very slowly, at rates of 1/4 to 1/2 inch of water per hour. However, if the water begins to run off before you have thoroughly wet the top 2 to 3 inches of soil, irrigate in cycles. Wait an hour or so before continuing to irrigate. To determine how deeply the soil has been wetted, probe down into the soil using a knife, screwdriver, soil probe or spade. Once the soil has been properly irrigated, do not irrigate until the surface begins to dry again. If the weather is extremely hot and dry and the soil type sandy, you may need to irrigate again within 4 or 5 days. If, on the other hand, the weather is fairly cool at night, irrigation may not be necessary again for another one to two weeks.
Forking or core aerifying those affected areas will greatly help get water into the soil. If you have a fairy ring problem, Insignia and Heritage(strobilurins) or Prostar fungicide may be applied monthly as needed to infested areas to remove the fungus problem.