When I was doing my research in grad school, I did almost all of my work in greenhouses. Every night we would burn sulfur to decrease the outbreak of powdery mildew. (Powdery mildew is a fungus that looks like that fuzzy, grayish-colored stuff growing on leaves of plants. Watermelons are especially susceptible to it). This practice worked fairly well, but it was done in an indoor setting. I don't believe that it would have the same effect in an outdoor setting. Not to mention, it may be difficult to find sulfur to burn anyway.