Why is Powdery Mildew on My Bee Balm?

Written by John Sepulveda, Certified Arborist WI-1454A


Pollinator gardens often feature monarda (bee balm), zinnias and phlox, but these plants are susceptible to powdery mildew. Read this article to learn about this common disease and how to keep plants healthy so they attract butterflies, bees, hummingbirds and other pollinators to your yard.



What is Powdery Mildew?

Powdery mildew is a disease caused by various species-specific fungi. It does not affect conifers but many trees, shrubs, flowers and agricultural plants can be infected. Powdery mildew is rarely lethal but it can be unsightly due to the appearance of powdered sugar on the upper leaf surface and stems of plants. Conditions that favor the spread of this disease include humidity, poor air circulation, and improper watering techniques.


Although many times powdery mildew is merely cosmetic, it can negatively affect monarda (bee balm), zinnia and phlox through severe leaf loss. Fungicide treatments are recommended for plants that suffer multiple years of leaf loss. There are also organic remedies to control the disease. Without proper treatments, plants are at an increased risk of stress due to leaf loss or opportunistic pests.


How to Protect Plants from Powdery Mildew

There are multiple ways to protect susceptible plants, including:

  • proper pruning and spacing of plants, which will increase air flow while helping lower the surface humidity

  • watering plants in the mornings if possible to avoid saturated soils which increase humidity at night

  • avoid late summer, high nitrogen fertilizers which can rapidly increase growth of the mildew


All infected plant material should be destroyed, and not used for compost or mulch since the fungus can survive in temperatures up to 90°F. There are also many plant species that are resistant to the powdery mildew fungi.


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