Mildew on lilacs and black spot on roses make me think of poison ivy and chicken pox on children. The biggest difference is that the ivy and pox will heal and disappear (if they don't scratch and get infected), but the mildew and black spotted leaves won't. Plants don't "heal" like people do. Their way is to produce new leaves to replace the damaged ones. So we must prevent fungal diseases unless we want to look at the aftermath for the rest of the season. Some, like damp off and Pythiumor Phytophthora, are so damaging to plants that we must practice prevention or lose them.
The best prevention, if we have the choice, is to plant a resistant cultivar in the first place. There have been recent introductions of mildew resistant phloxes and monardas and many shrub roses with Rugosa heritage are less likely to be attacked.
Carefully choose where you put susceptible plants. An open, sunny spot with good air circulation is best. If fungal root diseases are the problem as can happen with Rhododendrons, don't plant them in poorly drained clay soil. If that is the only spot you have, choose a different plant better suited to the conditions. It takes all the fun out of gardening to be constantly battling diseases and always looking at sickly plants. One of the problems I had dealt with was an alpine currant hedge in a shady area that was experiencing its annual bout with powdery mildew. In this situation (shade, close spacing of the shrubs) another plant should have been used for the hedge - one both mildew resistant and shade tolerant. Where the hedge is already in place, thinning the canes of the shrubs to allow greater air circulation will help. Sometimes you just have to acknowledge that you don't have a place where your favorite plant will thrive and move on to a new favorite that likes the spot you have to offer. Believe me, the thriving second choice will make you much happier than your sickly first love.
