Powdery Mildew on Bee Balm
The leaves of my bee balm plant are covered with a white substance. The leaves eventually brown and die. New shoots and flowers keep appearing. Should I be concerned?
Powdery mildew is the culprit. This is a common fungal disease on lilac, garden phlox, zinnia, squash and other vine crops and lilacs. The good news is the plants will survive. Unfortunately, they look pretty ugly by the end of the season. Planting something a bit shorter in front of the bee balm will help mask the damage while allowing the colorful flowers to show through. Or replace this plant with a more mildew resistant variety like Raspberry Wine, Blue Stocking, Marshall’s Delight, or Violet Queen. Grow the plants in full sun with proper spacing to increase the health and vigor of the plants and reduce the risk of disease. Further reduce the risk of powdery mildew on susceptible plants by removing ¼ of the stems in spring to increase light penetration and air flow. You can use an eco-friendly fungicide as soon as the disease is spotted and throughout the remainder of the season. Cornell University discovered a weekly application of one Tablespoon baking soda mixed with 1 teaspoon insecticidal soap or light weight horticulture oil (these act as spreader stickers) in a gallon of water will suppress powdery mildew. The organic fungicide Neem and other commercial products can also be used. Check the label and make sure the product is labeled for controlling mildew on the plants you are treating.