Q: I have powdery mildew on my new-ish bee balm plants (Monarda x 'Raspberry Wine', purportedly a mildew-resistant variety - part of the reason I bought it!)
There are tiny white dusty-looking patches on the leaves AND the stems. A few leaves are also afflicted with holes (a couple look like Swiss cheese, a couple of others look like they have more conventional insect damage, a bite out of an edge here and there).
The plant is making new leaves, which aren't damaged, and each stem has a nice-looking bloom. But the leaves toward the bottom of the stems are very sick-looking (white/yellow/brown/black). The rest of my garden (knock wood) is fine, including the plant adjacent to the bee balm.
These plants were planted this spring in a border on the north side of our garage, in an area that gets sky light all day and direct sunlight for four hours in mid- to late afternoon. They're planted 12-14 inches apart.
Thanks for whatever help you can offer. - Judi, Racine.
A: This is the time of year for powdery mildew to flourish and wreak havoc on many of our beloved garden plants. And yes, Raspberry Wine is a Monarda cultivar labeled as powdery mildew resistant, but some species are so delectable that even "resistance" doesn't guarantee disease free.
So, what to do? First, you need to understand that "powdery mildew" is a descriptive term for many different fungal diseases. Powdery mildew on bee balm is not the same as powdery mildew on squash and pumpkin, or on lilac. But they are closely related diseases with a lot in common, mainly, the environmental conditions that are present when they start.
Typically, powdery mildew develops when plants are in the shade, are planted too closely together, are overwatered and/or the weather is cool and damp. Given our odd summer, it is no surprise that even "resistant" plants are covered in the white powdery fungus.
To control this disease, you had the right idea in buying a resistant cultivar. That is a good start. Next, plant in full sun and increase the spacing for more air flow. Then, consider using an organic product like neem oil or a copper sulfate spray (which may color the foliage bluish) as soon as you see the first sign of powdery mildew. For more pesticide options, please read our garden fact sheet on the Wisconsin Horticulture Web site at:
This year, remove all affected foliage and get it out of the garden. Do not compost. If the plants are not flowering, cut them back to the ground. If they are still flowering and producing healthy leaves, you could treat with neem oil to protect those new leaves. Fungicides do not really kill the fungal disease, they protect new leaves from being infected.
The same is true if you are seeing powdery mildew on phlox, squash, melons and pumpkins. Just be sure that the product you select to spray is labeled for the plant you are spraying, and follow all safety precautions on the label. One of the pluses of using neem oil to protect your new leaves from powdery mildew is that it can act as an insect repellant on your plants as well.
The holes in the leaves may be from a grasshopper, although there are many insects out now chewing on plants. Japanese beetle is feeding voraciously, but they leave a lacy looking leaf. Slugs and earwigs chew at night, so people often do not see them. A product containing iron sulfate works wonders on slugs.
UW-Extension Master Gardener volunteers serving as plant health advisors can help answer your questions at email@example.com or (262) 886-8451 at the Racine Horticulture Helpline. They are in the office Tuesdays, Wednesdays and Thursdays from 9 a.m. until noon.
Dr. Patti Nagai is the horticulture educator for Racine County UW-Extension. Submit your questions for The Journal Times Q&A column to Dr. Nagai at Patti.Nagai@goRacine.org and put "Question for RJT" in the subject line.