Thatch, fungi may be cause for ugly lawns

It's getting downright ugly out there. I am talking about the brown spots and large areas of lawn turning brown. It is unsightly, but not surprising. There are possible causes for this blighting, but watering, weather and other lawn care practices are at the root of the problem.

Despite watering deeply and less frequently, most area lawn owners have not followed this sage advice that I offer year after year ... after year. I talked earlier this season about watering when plants need it, not by relying on a timer set at the beginning of the season and never adjusting.

This year, we experienced extended cool spring weather. That did not keep the irrigation timers from being set as soon as water was available. Because of the cooler weather, grass remained wet for long lengths of time, setting up the perfect conditions for lawn fungi.

Pythium may be one of the fungi-causing problems. Pythium fungi kill the roots and crown. During cooler weather, the disease may start as small, yellowish patches that coalesce into larger areas. When it turns warm, the disease shows up as large areas of wilted and dying turf.

This disease can be avoided with deep and infrequent watering and irrigating early in the morning instead of at night. To reduce spreading the disease, collect and remove grass clippings when mowing. Remove excessive thatch, and do not fertilize heavily during warm weather.

Thatch, another topic I have covered numerous times, may also be one of the problems contributing to lawn ugliness. Thatch is an intermingled layer of organic matter that comes from the grass plant itself. It consists of undecomposed grass stem, crown and root debris. Thatch is not caused by an accumulation of grass clippings as once thought. It happens when grass produces this material faster than it decomposes.

Lawns in our area are predisposed to develop thatch because our soils, especially sandy soils, are generally low in microbial populations responsible for breaking the organic matter down. Plus, most of our lawns are composed of Kentucky bluegrass and fine fescue grasses that form more thatch than bunch-type grasses, like turf-type perennial ryegrass.

Many poor lawn care practices encourage thatch buildup and discourage microbe populations. Frequent, shallow irrigation promotes thatch. Excessive nitrogen fertilization makes grass grow faster and develop thatch at a quicker rate. Infrequent mowing encourages the development of stem tissue and more thatch. Excessive irrigation and compacted soil discourage microbe activity.

The best defense against thatch is good preparation of the soil before seeding or sodding a lawn, followed by sound lawn care practices. This includes deep, infrequent watering, mowing regularly at the recommended height, fertilizing at recommended rates, aerating to relieve soil compaction and removing thatch when the layer exceeds one-half inch.

Finally, another common cause of large brown spots during this hot weather is sprinkler coverage. Check how much water is being applied to the brown areas when the sprinklers are on.

-- Marianne C. Ophardt is a horticulturist for Washington State University Benton County Extension.

This story was originally published July 17, 2014 at 12:00 AM with the headline "Thatch, fungi may be cause for ugly lawns ."