Let in Light and Air
By now on the East Coast, lots of roses have mildew or black spot on the leaves. Prevention through good cultural conditions -- selecting disease-resistant cultivars and planting where there are 8 hours or more of sunlight and good air circulation -- is the best approach. As this is not the time to move roses, consider removing nearby tree and shrub branches to increase light and air flow.
Use Organic Control for Mildew and Other Diseases
Try an organic control against mildew on roses and perennials, even food crops. Serenade's Bacillus subtilis is a biofungicide for powdery mildew, downy mildew, bacterial spot, rust, and more. It stops plant pathogen spores from germinating and disrupts pathogen growth.
Spray Sooner Rather Than Later
Despite initial cynicism, I've seen the light -- rather the roses and phlox with AND without white mildew. Two seasons using EDEN Bioscience's Messenger convinced me that early-season spraying nearly stops mildew and black spot; belated spraying on mildew-covered foliage doesn't kill fungi but subsequent new growth is healthy. Messenger uses harpin proteins (derived from fireblight) to stimulate a plant's natural reactions of disease resistance, growth, and flowering.
Prune Off Spent Lilac Flowers
Remove dead lilac flower clusters by pruning just above a node (where leaf joins stem). Then remove older or crowded branches with a pruning cut at the base of the shrub. That's right, squat or bend over to reach inside the shrub just above the soil, and clip the branch low. Also prune off crossing or damaged branches. Leave the strong, healthy branch; remove the broken, spindly, or weak one.
Pinch for Bushiness
Pinching off the stem tip on basil, mums, and sweet potato vines will make the plants grow more bushy. In northern climes, pinching or pruning off the summer phlox's stem tip right now also will promote bushy growth without flower loss. Doing it much later, though, will remove flower buds.