Powdery Mildew

Asked June 16, 2020, 7:28 PM EDT

I noticed powdery mildew on some of the phlox, so I made a mental note to go after it. I researched online and it was suggested that I spray it with baking soda and water. Some suggested adding dish soap. So, I went out to spray, and realized that ALL of the phlox has it!! Some of them are downright covered in it. I started cutting off some of the worst stalks, and then as I looked around, I see that I’m not sure where I draw the line, and I could easily cut half of them. I have always kept relatively smaller groupings so that there is good airflow, because I understand that poor airflow makes it worse. But, the mildew is worse on the exposed side of the plant, so that makes no sense at all. Now, since our gardens are very big, my “smaller” groupings are 1-2 feet in diameter. Is that too much? Is there a way to prevent this stuff? How do you treat something as big as our 5000 sq. ft. garden? It says to spray every 3 days! And, do you spray the whole plant, or just the ones with mildew? Does it cure it and take away the powdery mildew from the infected leaves, or just prevent it from getting worse? Any hints?

Carroll County Maryland

1 Response

We do not recommend homemade sprays as they can be phytotoxic to the foliage and we do not have research on the baking soda.

Powdery mildew is a fairly common fungal disease in some types of phlox. The best way to minimize this problem would be to plant mildew-resistant varieties. (If you are interested, Mt. Cuba Center did a trial of different phloxes and notes the best performing varieties here: https://mtcubacenter.org/trials/phlox-for-sun/).

For your existing plants, there is nothing that will cure the mildew that is already there. Anything you can do to increase air circulation and reduce humidity (thinning out the plants, pruning) can help to minimize the problem. You could also apply a horticultural oil labeled for powdery mildew control to protect new foliage only. Do not spray in high temperatures. Refer to our webpage for additional information. https://extension.umd.edu/hgic/topics/powdery-mildew-annuals-bulbs-groundcover-perennials-and-vines