The monardas include bee balm (Monarda didyma) and wild bergamot (Monarda fistulosa), both native plants that thrive in herb, butterfly and wildflower gardens in U.S. Department of Agriculture plant hardiness zones 3 through 9. Although monarda plants suffer from few pest and health problems, they are susceptible to powdery mildew.
About Powdery Mildew
Powdery mildew is a caused by fungal pathogens that often attack bee balm and wild bergamot plants grown in shady, overcrowded conditions. The fungi overwinter on fallen plant tissue or in infected flower buds and release spores that spread via wind, insect activity or water droplets in spring. Powdery mildew thrives in warm temperatures ranging from 60 to 80 degrees Fahrenheit. Unlike other fungal pathogens, the mildew fungi don't need free moisture to germinate and infect plant tissue. The fungus does require high relative humidity levels to promote spore germination. The powdery mildew disease cycle can continue throughout the entire growing season if your area experiences warm days and cool, damp nights.
Powdery Mildew Symptoms
Powdery mildew infections typically start with a few small patches of white, powdery growth that appear on leaves, stems and, occasionally, the flowers. The white spots often grow until entire plant surfaces become covered with the growth, which looks somewhat like baby powder. Over time, the growth might turn mealy and brown or gray in color. Because the layer of mildew interferes with photosynthesis, infected leaves often experience stunted growth, yellowing, curling and drop from the plant. Infected buds often look misshapen or fail to open. Although mildew infections rarely kill bee balm or bergamot, infected plants may be stunted or seem overall sickly.
Planting wild bergamot and bee balm in full sun helps reduce the risk of mildew infections because direct sunlight often kills the shade-loving fungal pathogens. Increasing the air circulation around plants also helps prevent mildew. Thin out crowded stems to allow more air and sunlight to penetrate entire plants. Raking up and destroying debris around the plants in fall prevents the fungi from overwintering near your bee balm or bergamot plants. Water-stressed plants are particularly attractive to powdery mildew pathogens. Although established wild bergamot plants are somewhat drought-tolerant, they grow best in medium moist soils with good drainage. Bee balms are not drought-tolerant and need consistent moisture throughout the growing season to remain in good health. Water your plants every seven to 10 days, allowing the water to soak the top 6 to 8 inches of soil each time. Watering from above the plant reduces the risk of mildew infections because the water washes the fungal spores from the leaves. Applying a 2-inch layer of organic mulch around plants helps retain soil moisture.
Treating Powdery Mildew
Using a simple home remedy can help treat powdery mildew and keep the pathogens from spreading, but you must start spraying your plant as soon as you notice the symptoms for this treatment to work. Established mildew infections are hard to control and eliminate. Mix 3 tablespoons of horticultural oil and 1 1/2 tablespoons of baking soda into 1 gallon of water. Put the mixture into a sprayer and thoroughly coat your bee balm or bergamot. Horticultural oil solutions only work on contact, so completely spray the foliage until it glistens with moisture. Repeat applications every seven to 10 days for as long as the environmental conditions favor the growth of powdery mildew.