Do follow advice given here.
A word about powdery mildew (not other diseases):
--- This disease lives on the tops of leaves, so if this is the only reason for spraying there is no need to be sure to get the undersides.
--- Unlike other diseases, powdery mildew spores are prevented from germinating by free water. They germinate in high humidity. Of course this can happen after wetting leaves while watering, so it's still a good idea not to get leaves wet.
--- Most species of plants will have their own particular species of powdery mildew that attack them. For instance, the powder mildew that attack lilacs will never infect roses. Likewise, and especially since roses are unrelated to tomatoes, powdery mildew on roses cannot infect any tomatoes.
