Answer: Brown patch is indeed a fungal disease, and it loves hot, humid weather. It attacks all turfgrasses, but likes some more than others. Sometimes the grass in the middle of the patch will recover, and you'll end up with a brown ring surrounding a green island. If the weather dries out or cools a bit, your lawn should recover fully within a few weeks. If it continues to be hot and damp, the disease could kill the affected grass completely. At this point, you may be able to stop the spread of the fungus with a professionally applied fungicide. If you are willing to let your lawn weather this summer through without chemical treatment, and follow the guidelines below, your lawn should recover, with a bit of reseeding at most. That's cheaper and safer than the chemical approach.
Nutrient levels play a role, so fertilize your lawn judiciously, based on a soil test. Excess nitrogen tempts the fungus, especially in the spring, and lack of calcium, phosphorus and potassium make things even much worse. If your lawn is on poorly drained soil, this problem will likely crop up every wet summer unless you improve the drainage. Hope this helps!
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