Gray leaf spot plagues St. Augustine lawns

Gray leaf spot plagues St. Augustine lawns

Lawns look like they’ve been blow-torched

COLLEGE STATION — One day you have a lush, green St. Augustine lawn and then you notice gray spots on the leaves. Soon, large parts of your lawn look like they’ve been blow-torched.

The culprit is most likely a fungus called gray leaf spot, said Dr. Young-Ki Jo, a Texas AgriLife Extension Service plant pathologist who works with turfgrass diseases statewide.

The bad news is that gray leaf spot fungus thrives under hot, humid conditions and that it may be particularly prevalent this time of year, Jo said.

The good news is that with or without fungicide treatment, St. Augustine lawns will eventually recover.

But the question is how long can the homeowner put up with an unsightly lawn, Jo said.

“Once you get it under control, the lawn recovers relatively quickly,” said Dr. Karl Steddom, plant pathologist based at Overton. “You should start to see results in a week or two after fungicide treatments.”

Jo said recovery is matter of time, and depends on environmental conditions and homeowner actions. The general recommendation is not to over-water or over-fertilize infected turfgrass.

“But it’s more complicated than that,” Steddom said. “What you don’t want to do either is to water early in the morning or at night.”

Steddom said watering at these times is not recommended because cool and damp conditions are conducive to fungal growth.

“Watering at 6 a.m is okay. At 8 a.m., it is not,” Steddom said. “But watering at 10 a.m. is okay. But definitely do not water at night.”

This is because at 6 a.m., the lawn is likely already damp from dew, so a reasonable amount of irrigation won’t contribute further to the conditions the fungus thrives upon, Steddom said. But watering a couple of hours later will mean the leaves stay wet for hours. But by mid morning, however, the heat of the day will quickly dry the grass out.

“You don’t want the leaves to stay wet any longer than you can help it,” Steddom said.

Jo said that stressed turf will show more severe symptoms.

“Raise the mowing height to reduce the stress,” he said.

Jo said the most effective fungicides recommended for gray leaf spot available to homeowners are those containing active ingredients such as thiophanate methyl or azoxystrobin.

Several product brands containing one of these ingredients and labeled for turf should be available at home improvement stores.

Jo emphasized that home owners should follow label instructions closely.

“Water volume for an application should be at 2 gallons per 1,000 square feet,” Jo said. “If you see no improvement in three to four weeks after fungicide application, please consult with AgriLife Extension agents or turfgrass specialists before planning another fungicide application. It’s always is good idea to have a proper diagnosis.”

Jo said that products containing thiophante methyl are of relatively low toxicity and the lawns can be re-entered within 24 hours after treating.

“Carefully read the label of each product and make sure to check the safe re-entry interval,” Jo said.

Many products used by commercial turf growers should not be used by residential homeowners, he noted.

For example, Daconil, another good fungicide used for gray leaf spot control on commercial turf, is not registered for home lawns, Jo said.

“Trizole fungicides containing active ingredients such as propiconazole, myclobutanil or fernarimol will also control gray leaf spot but have a potential of phytotoxicity causing discoloration of turf,” Jo said.

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