Bright yellow spots on tomato plant leaves can be a symptom of a powdery mildew infection. The disease rarely affects home gardens, but losses can exceed 50 percent in commercial tomato crops. The extent of the damage varies depending on environmental conditions.
The Leveillula taurica fungus that causes powdery mildew infections mainly affects crops in the western United States. The fungus, which is most active when the weather is hot and dry but punctuated by an occasional shower, produces specialized structures that invade the tomato plant's cells and extract nutrients from plant tissue.
The Leveillula taurica fungus creates irregularly shaped yellow patches on tomato leaves. The patches eventually turn brown and are surrounded by yellow halos. The infected leaves die, but they rarely fall from the plant. The disease does not affect the stems or fruit, but the exposed fruit is easily sunburned without protective foliage. Although powdery mildew does not kill plants, it significantly decreases their vigor and fruit productivity.
Removing old or diseased leaves helps control powdery mildew infections. The Utah State University Extension recommends applying a fungicide such as sulfur at the first sign of disease, followed by repeated applications every two weeks during the growing season. Sulfur can burn plants in conditions over 90 degrees Fahrenheit, however, and should only be applied during cooler weather.