Late this summer and into early fall, I noticed a light white coating on the grass blades in my lawn. It is not all over the lawn, just in certain spots. It looks like a light coating of white spray paint. What is this, and should I be doing anything about it this fall?

Your symptoms bear the trademark of powdery mildew, a common problem on bluegrass lawns in shady areas. The mildew fungus (Erysiphe graminis) attacks the surface of the grass leaves, developing a fine, fungal growth that resembles a white powdery substance on the leaves. In dense shade, powdery mildew causes the affected areas of leaves to turn yellow, eventually resulting in the yellowing and dying of lower leaves. Even when on non-turf plants, it can often lead to plant death.

Powdery mildew develops in areas of dense shrubbery or trees where there is poor air circulation, considerable shade, and high humidity. It thrives in temperatures of 60 to 72 degrees. In many cases, selective pruning of shrubs and trees to allow better air circulation and greater penetration of sunlight can control powdery mildew. Our weather conditions this season have provided ample opportunity for mildew to occur.

Since the mildew isn’t all over your yard, take a look at the places where it is occurring. Is there limited air circulation? Are there pockets of dampness and poor drainage? If you have an irrigation system, has the timing been adjusted for natural rainfall? How often is the lawn being fertilized? Often cultural practices can be modified and corrected to minimize optimum growing conditions for mildew. If powdery mildew continues to be a problem, consider planting or overseeding with a less-susceptible grass variety or plant alternative groundcovers in those areas. Turf is not always the best green surface in areas with constant humidity and little air circulation.