Controlling Gray Leaf Spot

posted by Randy Lemmon - 

Recently on Facebook, I shared a bit of old advice about handling gray leaf spot. The pictures here show great examples of what it looks like.  From afar, gray leaf spot can give grass a blotchy yellow hue, but a close examination will reveal very distinct lesions on the blades.

Before I get to controls for this disease, I want to give you a warning: Some unscrupulous lawn maintenance companies may try using some untruths to scare you into using their services. You may be told that your lawn has a disease so "terrible" that you must act "very soon" or lose your whole lawn! That's a load of compost!  Granted, if gray leaf spot is left untreated for a couple of years, the lawn could look pathetic. But it’s rarely life-threatening. 

Or a lawn maintenance company 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. There is a catch to using chlorothalonil-based fungicides, but I will get to that later.

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 because of nighttime watering, frequent rainfall, high humidity, heavy dew (i.e. prolonged leaf wetness), 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.

Management practices are very important for gray leaf spot control.

  1. Do not over-fertilize
  2. Do not water at night
  3. Mow frequently
  4. Catch clippings in problem areas.

As grass growth slows in late July and August, and rain frequency decreases, gray leaf spot usually subsides. Fungicides can be used to control it, but control may be difficult if the disease has already done significant damage.  The "spots" associated with GLS first appear as tiny brown- to ash-colored spots with purple to brown margins that enlarge and become diamond-shaped. Lesions begin as tiny, round or oval gray to brown or black spots on leaves. Spots enlarge into oval to elongated areas on leaves, sheaths, and stems, with the size dependent on the species and variety. Spots may be surrounded by a yellow halo or general chlorosis with purple to brown borders. Leaves may be blighted gray, usually from the tip downward. During moist periods, lesions become covered with a gray, velvety fungal mycelium. Diseased blades may wither and turn brown giving them a scorched appearance.

Besides the commonly found Daconil, the fungicides Banner, Banner-Maxx, and Heritage are also approved for use on gray leaf spot, although they're harder to find and often more expensive.  

And here’s the catch with chlorothalonil-based fungicides:  None of them are labeled for GLS. The reason is a long story, but in my opinion it’s a CYA kinda thing. Without indicating the product is for gray leaf spot, the manufacturer can’t be blamed if your lawn dies, and you can’t sue them or something like that. But I’ll recommend four ounces of chlorothalonil-based product per gallon of water, and one treatment will usually conquer the problem in under two months.  If another fungicide you buy for this control isn’t labeled for GLS, don’t give up. Just call me, and I’ll talk you through the right dosage based on what you’ve got.

GardenLine with Randy Lemmon

GardenLine with Randy Lemmon

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