If you've grown zinnias, lilac, bee balm or phlox you've seen this, a white powdery substance on the leaves. This fungal disease is known as powdery mildew. It grows on leaf surfaces, causing infested leaves to turn yellow, brown and eventually wither. The disease won't kill the plant - it just looks bad.
So what can you do? Start with resistant varieties of susceptible plants like Garden View Scarlet bee balm. Grow plants in full sun and properly spaced for good air circulation.
If you decide to treat this disease, try one of the more eco-friendly products. Cornell University found mixing one tablespoon of baking soda with one teaspoon insecticidal soap or lightweight horticulture oil in a gallon of water works well.
Spray the Cornell mix or Neem, a plant derived fungicide, weekly. Start at the first sign of the infection and repeat as needed until the end of the season.
A bit more information: Reduce the risk of powdery mildew with a bit of spring thinning. Remove ¼ of the stems of garden phlox, beebalm and other susceptible perennials back to ground level in early spring. This increases air flow and light penetration throughout the plant, resulting in stiffer stems and often less powdery mildew. Or keep it simple and just cover up the unsightly leaves by planting slightly shorter mildew resistant plants around them. The surrounding plants mask the unsightly leaves so they do not detract from the beautiful flowers.