A fast-producing weed known as crab grass and a fungal grass disease cause some areas in the lawn to turn brown, writes Doug Caldwell, a landscape entomologist. Crab grass weed tends to climb on top of St. Augustine grass, and can only be contained by keeping the turf healthy and using pre-emergent substances. The brown patch disease spreads on moist lawns, so it can be prevented by tapering down irrigation and fertilizer use, and through an early application of fungicides, Caldwell notes.

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