The leaves on my bee balms are covered with a grayish white "powder." What is the problem?
The symptoms on your bee balms (Monarda spp.) are most likely due to powdery mildew. Powdery mildew is a common fungal disease on bee balms. Symptoms are most severe on overcrowded plants and those growing in partial shade.
Cultural practices can reduce the severity of powdery mildew. When planting bee balms, select a site that receives at least 6 hours of direct sun each day and space plants 2 to 2½ feet apart. Move plants growing in partial shade to a sunny location. Divide bee balms every 2 to 3 years to prevent overcrowding. Remove and destroy disease-infested plant debris in fall. If cultural practices fail, fungicides can also be used to control powdery mildew.
Probably the best way for home gardeners to avoid the annoying problem of powdery mildew is to select and plant mildew resistant varieties. Varieties that possess good to excellent resistance to powdery mildew include ‘Marshall’s Delight’ (bright pink flowers), ‘Jacob Cline’ (deep red flowers), Grand Marshall™ (fuschia-purple flowers), and ‘Raspberry Wine’ (wine red flowers).