how to treat plants that had powdery mildew last fall - Ask Extension
Hi!
Most of my beebalm had powdery mildew last fall. I did not treat it last fall unfortunately. I've read that powdery mildew fungi can survive t...
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how to treat plants that had powdery mildew last fall #895800
Asked March 29, 2025, 2:13 PM EDT
Hi!
Most of my beebalm had powdery mildew last fall. I did not treat it last fall unfortunately. I've read that powdery mildew fungi can survive the winter. Is there something I can do to preventively treat the beebalm that I knew had powdery mildew last fall? If the fungi has survived the winter, how would you recommend that I treat it when I see signs of it?
Thanks in advance for guidance! Happy spring!
Denver County Colorado
Expert Response
Dear Anna,
Powdery mildews are very common. It has undoubtedly survived the winter.
Several practices will reduce or prevent powdery mildews. Many plants have been developed to be resistant or tolerant to powdery mildew. Inquire about resistant varieties before a purchase. If resistant varieties are unavailable, do not plant in low, shady locations.
Once the mildew becomes a problem, avoid late-summer applications of nitrogen fertilizer to limit the production of succulent tissue, which is more susceptible to infection.
Avoid overhead watering to help reduce the relative humidity.
Remove and destroy all infected plant parts (leaves, etc.).
Selectively prune overcrowded plant material to help increase air circulation. This helps reduce relative humidity and infection.
For infected vegetables and other annuals, remove as much of the plant and its debris in the fall as possible. This decreases the ability of the fungus to survive the winter. Do not compost infected plant debris in your home compost. Dispose of infected plant material in the trash.
If cultural controls fail to prevent disease buildup or if the disease pressure is too great, an application of a fungicide may be necessary, such as sulfur, neem oil (Rose Defense, Shield-All, Triact), triforine (Ortho Funginex), or ornamental use only
potassium bicarbonate (Kaligreen, First Step). Chemicals are most effective when combined with cultural controls. Apply fungicides at seven to 14-day intervals to provide continuous protection throughout the growing season. Follow the instructions on the fungicide label for use on specific plant species, varieties, rates to be used, timing of applications, and waiting periods before harvest.
An alternative nontoxic control for mildew is baking soda (similar to the potassium bicarbonate listed above) combined with a lightweight horticultural oil (Sunspray). Applications of one tablespoon baking soda plus 2.5 tablespoons of Sunspray oil in 1 gallon of water are still experimental.
Here is a link to extensive information about powdery mildew.
https://extension.colostate.edu/topic-areas/yard-garden/powdery-mildews-2-902/
Here is a link to some information specifically about powdery mildew on beebalm.
https://yardandgarden.extension.iastate.edu/faq/leaves-my-bee-balms-are-covered-grayish-white-powder-what-problem
Thank you for your inquiry and best of luck with your beebalm.
Powdery mildews are very common. It has undoubtedly survived the winter.
Several practices will reduce or prevent powdery mildews. Many plants have been developed to be resistant or tolerant to powdery mildew. Inquire about resistant varieties before a purchase. If resistant varieties are unavailable, do not plant in low, shady locations.
Once the mildew becomes a problem, avoid late-summer applications of nitrogen fertilizer to limit the production of succulent tissue, which is more susceptible to infection.
Avoid overhead watering to help reduce the relative humidity.
Remove and destroy all infected plant parts (leaves, etc.).
Selectively prune overcrowded plant material to help increase air circulation. This helps reduce relative humidity and infection.
For infected vegetables and other annuals, remove as much of the plant and its debris in the fall as possible. This decreases the ability of the fungus to survive the winter. Do not compost infected plant debris in your home compost. Dispose of infected plant material in the trash.
If cultural controls fail to prevent disease buildup or if the disease pressure is too great, an application of a fungicide may be necessary, such as sulfur, neem oil (Rose Defense, Shield-All, Triact), triforine (Ortho Funginex), or ornamental use only
potassium bicarbonate (Kaligreen, First Step). Chemicals are most effective when combined with cultural controls. Apply fungicides at seven to 14-day intervals to provide continuous protection throughout the growing season. Follow the instructions on the fungicide label for use on specific plant species, varieties, rates to be used, timing of applications, and waiting periods before harvest.
An alternative nontoxic control for mildew is baking soda (similar to the potassium bicarbonate listed above) combined with a lightweight horticultural oil (Sunspray). Applications of one tablespoon baking soda plus 2.5 tablespoons of Sunspray oil in 1 gallon of water are still experimental.
Here is a link to extensive information about powdery mildew.
https://extension.colostate.edu/topic-areas/yard-garden/powdery-mildews-2-902/
Here is a link to some information specifically about powdery mildew on beebalm.
https://yardandgarden.extension.iastate.edu/faq/leaves-my-bee-balms-are-covered-grayish-white-powder-what-problem
Thank you for your inquiry and best of luck with your beebalm.
