How You Get Fungus on Your Grass

Fungal outbreaks make normally green lawn appear white, red or even black.
Fungal outbreaks make normally green lawn appear white, red or even black. (Image: Jupiterimages/BananaStock/Getty Images)

Unusual, colored patches across your lawn often indicate a fungal outbreak, especially if you notice grass die-back in the same area. In most lawns, small fungi populations exist because the fungi feed on organic material at the soil level. Improper lawn care, however, may encourage fungi replication. Combating the issue with fungicides stops the spread only temporarily until cultural practices such as mowing height and heavy shading are altered.


Most grass species need more than four hours of sunlight each day for maximum photosynthesis. If your grass is heavily shaded, it weakens from reduced energy production, making it susceptible to fungal outbreaks. Reduced sunlight affects air circulation and humidity. For example, dense tree canopies block air from moving across a lawn. As a result, the air warms near the grass as humidity levels rise accordingly. Powdery mildew (Erysiphe graminis) thrives in shaded areas, especially during wet conditions. Trim your surrounding landscape to allow sunlight penetration across your lawn, reducing fungal spread.

Improper Irrigation

Slime molds replicate on grass blades that remain wet, especially if the grass is watered at night. Reduce fungus on wet grass by watering your lawn in morning. Sunlight striking the grass has all day to evaporate the water droplets, preventing fungal spread. If you want a strong turf, consider watering your grass deeply to a 6-inch soil depth. Grass roots stretch downward in search of moisture, providing solid establishment and resilience to fungal invasions. Watering lightly only encourages shallow roots and weak grass that may allow fungus to proliferate when conditions are wet.


Diligent lawn fertilizing adds critical nutrients to soil for healthy turf growth, but applying too much or too little fertilizer causes fungal outbreaks. In general, fertilize two or three times each year, based on your grass species. Choose a nitrogen, phosphorus and potassium fertilizer ratio of 3-1-2 for most grass species. Following the fertilizer manufacturer's instructions correctly should prevent fungal spread rather than encourage it.


Trimming a lawn too low causes significant stress to the turf; the grass cannot photosynthesize well, and its height exposes soil to excessive evaporation. As a result, fungus may set in because the grass is weakened. Cut your lawn to an average height of 2 1/2 inches tall. Also, removing only one-third of the turf's height at a time prevents widespread stress. In general, mowing once each week during the growing season should help to maintain the ideal turf height, 2 1/2 inches. That grass blade length provides visual appeal along with ample surface area for energy production. A healthy turf fights off fungal invasions easily.

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