Lawns that are heavily shaded can sometimes develop a white color on the grass blades, as if they have been dusted by powder. This is due to powdery mildew, a common fungal disease favored by cool, humid weather and shady conditions. The disease is usually most noticeable in the spring and fall, but can occur throughout the growing season.
Grass blades that are completely covered by mildew usually turn yellow and may die. Affected areas often have thin grass growth. If powdery mildew is a problem, consider overseeding the area with a more shade-tolerant grass. As feasible, try to improve sun and air penetration to these areas by careful pruning of nearby trees. Or, replace turfgrass under trees with an organic mulch, such as wood chips. Chemicals are typically not necessary for powdery mildew control in home lawns.