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Identifying, Fighting and Preventing Lawn Diseases

Protecting Your Lawn From Fungal Disease

When fungal disease hits lawn grasses, control can be difficult if you don't understand the cause behind the disease. Fortunately, proper lawn maintenance and corrections in care prevent and resolve most fungal problems. To keep your lawn healthy and fungus-free, learn to recognize and eliminate the conditions that leave lawns at risk.

Recognizing Disease

In order for fungal diseases to strike, three factors must be in play: disease pathogens, weather conditions that favor their development, and susceptible lawn grasses. Without all three, grasses keep the upper hand. While you can't control the weather, you can recognize fungal threats and act.

Active fungal disease usually occurs in times of stress. For example, warm-season grasses are hit when abnormally cool temperatures slow their growth; cool-season grasses are most vulnerable under warmer than normal conditions. Excessive rain, drought and overcast skies all impact your lawn's ability to fight back.

Some common fungal lawn diseases for warm- and cool-season grasses can be identified as follows:

Brown patch

  • Circles of dead, sunken grass that can vary in size up to 3 feet wide.
  • Grass blades darken, wilt and die, leaving the circle's perimeter smoke-gray in color.
  • In the spring and fall, warm to hot weather conditions set the stage for disease.
  • Wet leaves, high humidity, overfertilizing and excess water facilitate the spread.

Dollar spot

  • Silver-dollar-size circles spread up to 6 inches wide and eventually merge into large, irregular blocks.
  • Grass develops straw-yellow spots that look water-soaked. Morning light reveals cobweb-like growth.
  • Late spring through fall, mild-to-warm temperatures support disease development.
  • Underfertilizing, excessive moisture, drought stress and thatch leave grasses susceptible.

Gray leaf spot

  • Small, irregular patches grow and join together to form large areas of damage.
  • Grass blades develop small, bleached-out spots with dark brown edges. The spots turn fuzzy gray when wet.
  • Moderately high temperatures with high precipitation from spring to fall favor the disease.
  • Poor air circulation, overwatering, overfertilizing and too much shade contribute to the spread of disease.

Leaf spot

  • Grasses display elongated, oval spots with dark purple-brown margins and brown centers.
  • Crowns and roots of grass plants develop dark brown rot, and grasses thin and die out.
  • Unseasonably warm temperatures threaten cool-season grasses, while unseasonably cool temperatures put warm-season grasses at risk, particularly in overly dry, poorly aerated soil.
  • High humidity, extended moisture, mowing too short and too much high-nitrogen fertilizer support the disease.


  • Circles of withering, blackened blades expand in patterns that follow the flow of water through the lawn.
  • Leaves look greasy and stick together, roots turn brown and rot, and grasses die quickly.
  • Warm temperatures, consistently wet conditions and high humidity from early summer to fall encourage the disease.
  • Compacted soil, overfertilizing, overwatering and too much shade hasten the spread.

Red thread

  • Red or bleached-looking patches appear as grass blades wither from the tips.
  • Reddish-pink threads bind blades together; red spores show up on shoes, mowers and tools.
  • Mild, early spring temperatures and cool, wet conditions favor the disease.
  • Underfertilizing, high humidity, poor air circulation, shade, thatch, leaves wet for an extended period of time, and compacted soil contribute.


  • Irregular patches of weak, thinning grass develop yellow specks that become rust-colored.
  • Orange-red pustules cover grass blades and hitch a ride on tools, shoes and mowers.
  • Mild to moderately warm temperatures from early summer through fall are primary disease periods.
  • Drought, high humidity, extended wetness, shade and underfertilizing leave stressed grass susceptible.

Snow mold

  • Small, circular, pinkish spots spread up to 1 foot or more.
  • Grass blades become water soaked, turn red-brown, and then tan. White or pinkish fungal threads can be seen in mornings.
  • Overly cool fall temperatures combined with wet conditions set the stage for disease to strike in winter, especially under snow cover.
  • Overfertilizing with too much nitrogen in fall, poor drainage and too much shade leave grass vulnerable.

Summer patch

  • Circles of dying and dead tan-colored grass show green, healthy-looking grasses in their centers.
  • Dark brown fungal threads may cover the crowns, rhizomes and stolons of grass plants.
  • Unusually high temperatures in late spring through summer support disease development.
  • Excessive soil moisture, compacted soil, poor drainage and mowing too low give the disease an advantage.

Preventing and Resolving Problems

Once the disease is identified, maintenance that prevents it also corrects it. Prevention and prompt corrections in care at the first signs of trouble restore the advantage to your lawn. Keep your lawn at its competitive peak with these fungal-fighting practices:

1. Keep soil pH at a level lawn grasses need. Most lawn grasses prefer slightly acidic soil near 6.0 to 7.0 pH, where essential nutrients stay readily available. Regular soil testing

and appropriate soil amendments help keep your grass healthy and strong.

2. Fertilize properly. Underfertilizing leaves grass undernourished, and overfertilizing promotes weak, rushed growth; both mistakes set up lawns for disease. Fertilize with premium products, such as those in the Pennington UltraGreen line of lawn fertilizers, that release essential nutrients at the right levels. Follow label instructions for proper amounts.

3. Water for lawn health, strength and resilience. Irrigate early in morning when evaporation rates are low and the emerging sun speeds drying, reducing the amount of time grass spends wet. Don't water frequently; instead, water deeply, one to two times weekly, to encourage strong, deep, drought-tolerant roots. The equivalent of 1 inch of precipitation soaks soil to a 4- to 6-inch depth, which is perfect for healthy, established grass. Don't leave irrigation on automatic; act according to rainfall or the lack of it.

4. Choose premium, water-wise grass seed products. Use grasses recommended for your region and suited for your yard's sun exposure. The right grass is key to avoiding disease. Pennington Smart Seed grass seed saves up to 30% more water versus ordinary grass seed, reducing watering-related problems. Pennington New and Improved One Step Complete grass seed combines premium Smart Seed grass seed, professional-grade fertilizer and improved wood mulch in an easy-to-use, premixed product to establish resilient grass and correct problem areas.

5. Follow good mowing practices. Mow at heights recommended for your grass type. Keep your mower blades sharp, and mow according to grass growth, not the calendar. Remove only one-third of the blade height in a single mowing. Collect and dispose of lawn clippings if you suspect fungal disease. Don't compost; some diseases spread through clippings, infecting compost.

6. Dethatch lawns and aerate soil regularly. Thatch up to 1/2 inch thick acts like beneficial mulch, but amounts beyond that restrict water and air and encourage disease. Compacted soil does the same. Reduce compaction with regular aeration and dethatch so air, sun and moisture can reach their goal and keep disease at bay.

7. Use pesticides for severe infestations. When corrections alone aren't sufficient, choose pesticides specifically approved for the disease at hand. Lilly Miller Moss Out! 5 in 1 Broadleaf Weed & Moss Killer, for example, controls diseases such as dollar spot, snow mold and rust. Most otherwise healthy lawns recover quickly with proper corrections. Many other lawn fungicides are only available to licensed applicators, so preventing problems is key.

By recognizing conditions that promote fungal lawn diseases, and by circumventing the threat, you can solve problems promptly. With the help of Pennington grass seed and Pennington lawn health products, you can keep your lawn grasses beyond the reach of fungal disease.

Pennington, Smart Seed, and One Step Complete are trademarks of Pennington Seed, Inc. Ultragreen, Lilly Miller, and Moss Out! are registered trademarks of Central Garden & Pet Company.
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