Q:
My phlox gets a powdery coating that kills off the plant. I’ve tried cutting back when I first see it, but I usually lose the battle.
– M. Shannon, La Grange
A:
Powdery mildew is a common, easily recognizable disease that attacks phlox as well as other plants. It looks like powdery splotches of white or gray on leaves and stems and can affect flowers, buds and fruit. Powdery mildew does stress plants; severe or repeated infections will weaken them. When the infection is severe, leaves fall off. Periods of high humidity will increase the severity and occurrence of this disease on phlox.
My personal preference: Choose a different phlox to reduce maintenance. Cultivars exist that are more resistant to powdery mildew. Go to the Chicago Botanic Garden’s website (chicagobotanic.org) and click on the “Your Garden” tab; go to “Plant Evaluations” for results of a study on phlox performance (newsletter No. 13).
If you have a cultivar that is particularly susceptible to powdery mildew, it is best to replace it with another phlox that is resistant. Phlox “David” is a resistant cultivar that is readily available.
The following cultural practices can reduce the incidence of powdery mildew on phlox, as well as other plants: Place plants in full sun. Thin stems to improve air movement. Reduce or eliminate overhead watering. Remove any infected plant parts from the garden.
There are fungicides labeled for powdery mildew (follow label instructions when using). However, you could try treating the plants weekly with a baking soda-and-water solution. Like most synthetic fungicides, it is most effective when applied before powdery mildew appears.
To mix your own solution, combine 1/2 teaspoon of liquid soap, 1 tablespoon ultralight horticultural oil and 1 tablespoon baking soda; add to 1 gallon of water. Test the mixture on a couple of the plants to make sure there is no damage before applying it to the whole clump.
MilStop is an example of a commercial potassium bicarbonate solution that is labeled for use on plants as a fungicide.
Tim Johnson is director of horticulture for the Chicago Botanic Garden in Glencoe. Send questions to: Gardening Q&A, Sunday, Chicago Tribune, 435 N. Michigan Ave., Chicago, IL 60611-4041; e-mail to
sunday@tribune.com
.
