I guess lawn service companies are either bored or desperate for business at this time of the year. I've gotten an average of 5 emails a week for the past three weeks from people asking about lawn services diagnosing them with Gray Leaf Spot (pyricularia grisea). The reason I say the lawn services are bored or desperate, is because Gray Leaf Spot normally rears its head from April through June. It is usually affiliated with our wet spring weather and the fertilizing that takes place from April through June.
Before I get to the basics of the disease and its control, I want to give you a warning: Some unscrupulous lawn maintenance companies may try to scare you into using their services by claiming some very untrue things:
Example #1: You're told that this disease is so "terrible" that you must do something about it "very soon" or you could lose your whole lawn! Well, that's a load of composted cow manure. Granted, if left untreated for years it could look pathetic, but rarely is it life-threatening.
Example #2: A lawn maintenance company claims they have the only "approved" fungicide capable of solving the problem. That's laughable, considering Daconil (chlorothalonil) is the most often-approved fungicide for gray leaf spot, and Daconil can be found nearly anyplace that sells garden supplies. Granted there is not a label for Gray Leaf Spot on a bottle of Daconil, but I'm aware that if used at the rate of 4 ounces per gallon of water, you have the same control a lawn care service has.
So, the truth is that you can take care of this problem yourself. But it does take a little more than Daconil (chlorothalonil) to alleviate the problem. So, here are some great points that research on the subject reminds us of:
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 gray leaf spot 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, Duosan and Heritage are also approved for use on gray leaf spot, although they're harder to find and often more expensive.