Powdery mildew fungi clog up leaf pores and block light to photosynthetic cells, so the plants are weakened in their ability to use light as an energy source. New growth stops, old leaves fall off, and the plants struggle to stay alive. Fruits produced by infected plants often lack flavour.
Tomato powdery mildew is most likely to develop on stressed plants, so provide ample nutrition, water and mulch to satisfy their needs. If tomato powdery mildew is common in your areas, spray plants preventatively with a mixture of one part milk to five parts water weekly, starting in midsummer.
Pick off individual leaves that show powdery mildew as soon as you see them. Promptly harvest tomatoes from mildewed plants. Compost old tomato plants in an active compost pile so that all infected plant material will be gone by the following summer.