Q. I have a problem with my heirloom tomatoes  the leaves on top have mildew on them what is causing this? I have never had this problem before. The plant is producing tomatoes and the leaves on top are still green.  I am baffled! This is not tomato but it gives you an idea what powdery mildew looks like. It looks like someone sprinkled the plants with wheat flour A. I would be very interested if what you are seeing is actually mildew and if you mean the disease powdery mildew. Powdery mildew is a disease that is very specific to certain plants. For instance the powdery mildew on roses cannot attack tomatoes. Powdery mildews can attack numerous plants but each type of powdery mildew is specific to that plant. Powdery mildew is a common disease in desert environments because it can be very active under very low humidities. The environment for this disease to occur is usually the same; somewhat shady areas frequently irrigated with overhead sprinkler type irrigations. This causes water to splash when it hits surfaces. This splashing water can carry the disease from one plant of the same type to a neighboring plant. Nonchemical control of powdery mildew is the same for all types; reduce or eliminate the shade problem and the splashing water problem. Use drip irrigation. Improve air circulation among the plants by not planting too close together and don’t let them shade each other. Give them some space and dont over fertilize them so they get real bushy and have alot of internal shading. Thin out the canopies of the plants to improve air circulation. Go to a different type of growing. Dont let them sprawl but stake them and tie them to stakes to get them more vertical and improve air circulation. Usually if you can do all this you will not need a fungicide. Otherwise there are fungicides you can apply to keep it from spreading.