Powdery mildew on grass is a common fungal disease caused by the Blumeria graminis fungus. This obligate parasite requires a living host and manifests as a superficial, dusty, white or grayish coating primarily on the lawn’s blades. While the disease rarely kills turfgrass outright, it significantly weakens the plant by interfering with photosynthesis. This leads to a thinned, unhealthy-looking lawn. It is particularly common on cool-season grasses like Kentucky bluegrass, fine fescues, and perennial ryegrass.
Identifying Powdery Mildew on Your Lawn
The defining characteristic of this disease is the appearance of fine, white or gray patches resembling a dusting of flour or talcum powder across the grass blades. This powdery substance is a mass of fungal growth (mycelia) and millions of asexual spores (conidia). The disease usually begins on the upper surfaces of the lower leaves and spreads upward, often appearing in localized patches.
If you lightly rub the affected blades, the white material will easily wipe off onto your fingers, confirming the fungus. As the infection progresses, the grass tissue underneath the mildew coating is starved of sunlight. The leaves often turn yellowish, then tan or brown as they begin to die. Heavily infected turf stands take on a dull, pale white appearance, indicating a serious loss of plant health.
Factors Causing Powdery Mildew Development
Powdery mildew thrives when environmental conditions align, primarily favoring reduced light intensity and poor air movement. The fungus is most severe in shaded areas, such as under dense trees, along fences, or next to buildings, where sunlight is blocked. Lack of direct sunlight prevents grass blades from drying out and weakens the turf, making it more susceptible to infection.
The disease prefers air temperatures between 60 and 70 degrees Fahrenheit combined with high relative humidity. Unlike many other turf diseases, powdery mildew does not require free water or standing moisture on the leaf surface to germinate. High humidity provides the necessary moisture for the fungus to flourish, especially when air circulation is stagnant. Furthermore, excessive nitrogen fertilizer can contribute to the problem by promoting dense, tender new growth that is highly susceptible to infection.
Curative Treatments for Removing Mildew
Immediate treatment of existing powdery mildew focuses on killing active fungal growth using contact-based solutions. One effective non-chemical treatment is a baking soda solution, which temporarily increases the leaf surface pH, creating an alkaline environment toxic to the fungus. A common mixture involves dissolving one tablespoon of baking soda (sodium bicarbonate) and one-half teaspoon of liquid soap in one gallon of water to help the solution adhere.
Another natural option is horticultural oil, such as neem oil, which functions as both a fungicide and a preventative measure. These oils work by smothering the fungal spores and mycelia. When using any oil-based spray, apply it in the late afternoon or evening to prevent leaf burn, especially when temperatures are high. Repeat applications are often necessary, typically once a week for three to four weeks, to eliminate the recurring infection.
If the infection is severe and environmental modifications are insufficient, commercial fungicides may be required. These products contain active ingredients such as propiconazole, azoxystrobin, or myclobutanil, which are labeled for turfgrass powdery mildew control. Fungicides are classified as either contact or systemic.
Contact fungicides remain on the plant surface, forming a protective barrier that prevents spores from germinating, offering immediate control but requiring frequent reapplication. Systemic fungicides are absorbed by the grass and move within its tissues, providing longer-lasting protection. Always follow the product label’s instructions precisely, and consider rotating between different chemical classes to minimize the chance of resistance development.
Modifying the Environment to Prevent Recurrence
The most sustainable strategy involves altering the microenvironment to make it unfavorable for fungal growth. Since shade and poor airflow are the primary triggers, pruning overhanging tree limbs or thinning dense shrubbery will significantly increase both sunlight penetration and air circulation. This practice allows the grass blades to dry more quickly, which is detrimental to the fungus.
Adjusting irrigation practices is a crucial cultural change. Watering the lawn only in the early morning allows the grass to dry completely before nightfall, preventing moisture from lingering on the blades. Avoid watering in the late afternoon or evening, as this keeps the turf wet overnight and promotes fungal development. Furthermore, ensuring the lawn is not over-fertilized, especially with high-nitrogen products, reduces the amount of susceptible new growth.
Other maintenance practices that increase air movement and plant health include maintaining the highest recommended mowing height and ensuring proper dethatching or aeration. If the affected area remains persistently shaded, overseeding with a more shade-tolerant grass species, such as fine fescue, or choosing a powdery mildew-resistant cultivar may be the only permanent solution.