Gray Leaf Spot 2020


It’s been a couple of years since I covered gray leaf spot in detail, so here’s an update.

Seen from afar, gray leaf spot can make grass appear to have a blotchy yellow hue. But a close examination of the blades will reveal very distinct lesions of brown and yellow.

Before going further, here’s a warning: Some unscrupulous lawn maintenance companies may mislead homeowners, trying to scare them into using their services. You may be told your lawn has a disease so terrible, rapid action is required or you'll lose your whole lawn! That's a load of compost. Gray leaf spot untreated for a couple of years can make a lawn look pathetic, but it’s rarely life-threatening.

Other companies may claim they have the only "approved" fungicide capable of solving the problem. That's laughable. Daconil (chlorothalonil) is the most often-used fungicide for GLS, and it can be found at nearly any place that sells garden supplies.

The truth is that you can take care of this problem yourself. Here is some material pulled from various university research papers:

Gray leaf spot may be showing up due to prolonged leaf wetness, brought about by nighttime watering, frequent rainfall, high humidity or heavy dew plus rapid, lush growth courtesy of recent fertilizations. Lawns with severe gray leaf spot have areas that seem to just fade or melt away. The decline often starts in shaded locations and low spots with poor drainage. Individual leaf spots on grass blades are typically elongated with dark margins.

The good news is that gray leaf spot usually subsides in late July and August, once things heat up, rain frequency decreases and grass growth slows. But the damage can be reduced sooner with some standard cultural practices.

  • Do not over-fertilize
  • Do not water at night
  • Mow frequently
  • Catch clippings in problem areas for the time being

Fungicides can be used to control it, but that may be difficult if the disease has already done significant damage.

Lesions begin as tiny round or oval gray, brown or black spots on the blades. They then enlarge into oval or elongated areas on blades, sheaths, and stems, with the size depending on the grass species and variety. The spots may be surrounded by a yellow halo or general chlorosis with purple to brown borders. The blades may be blighted gray, usually from the tip downward. During moist periods, the lesions are covered with a gray, velvety fungal mycelium. Diseased blades may wither and turn brown, giving them a scorched appearance.

Besides chlorothalonil-based fungicides like Daconil, others such as Banner, Banner-Maxx, and Heritage are approved for use on gray leaf spot, but they're often harder to find and more expensive.

By the way, chlorothalonil-based fungicides are not labeled for use with gray leaf spot. The reason is a long story, but I think it’s a CYA kinda thing. Without indicating the product is for gray leaf spot, the manufacturer can’t be blamed or sued if your lawn dies. (Thanks, lawyers.) But I’ll recommend four ounces of a chlorothalonil-based product per gallon of water, and one treatment will usually conquer the problem in under two months. If, after mowing three or four times, things have not improved, do one more treatment.

Researchers at the Extension and Experiment Station at Texas A&M University approved the fungicides below over 30 years ago. I can only confirm that the first three are still available at retail stores, but if you can find one of the others and it seems cost-effective, give it a try. If you buy something other than chlorothalonil and it isn’t labeled for GLS, call me and during the GardenLine show I’ll talk you through the right dosage.

  • Chlorothalonil
  • Mancozeb
  • Propiconazole (Banner-based as in Banner Maxx)
  • Azoxystrobin
  • Pyraclostrobin
  • Fenarimol

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