Hello,

I apologize for the delay in responding to your question. Ask Extension Responders do not endorse sure-fire solution to gardening problems. Powdery mildew is a fungal disease that affects the leaves, stems, and flowers of the plant. The fungus spores germinate best in hot dry weather with cool nights which occur often in the spring. If it comes in on developing leaves, it can cause distorted leaves and stems. Although the fungus spores live in the soil, they can survive cold winters by colonizing plant buds and under the right conditions in the spring start up new infections when the plant buds out. Read more about it here: https://www.gertens.com/learn/plant-diseases/powdery-mildew.htm

Powdery mildew can be controlled by certain cultural practices. Because the fungus survives in the soil, it is best to rake up and dispose of any leaves or branches that have fallen to prevent any future infection. Increase air circulation around the shrub by pruning out the oldest, biggest shoots right to the ground. Avoid fertilizer especially high nitrogen fertilizers because they promote new bud growth which the fungus preys upon. Mulch under the plant to provide soil moisture. At the end of the growing season, prune out any infected stems and leaves and clean up and dispose of the infected material.

And here: https://extension.umn.edu/plant-diseases/powdery-mildew-trees-and-shrubs#trees-and-shrubs-commonly-affected-by-powdery-mildew-1584111

And here: https://hgic.clemson.edu/factsheet/powdery-mildew/

Powdery mildew is impossible to control once it is evident. Fungicides can be applied early in the spring before the disease appears. However, when the disease has progressed, a fungicide whether home-made or a commercial chemical will not help it.

When using chemicals, read the label to ensure that it can be used against that disease on that specific plant. Follow the label directions on how to use the chemical.

Thank you for contacting Ask Extension.

Hello Barb,

Moving the phlox to a sunnier location with good air ventilation and carrying out good cultural practices may prevent it from getting powdery mildew. However, some phlox species are more susceptible than other phlox species and you may have better luck with a different plant. The choice is yours.

Good luck with your plant.

Thank you for contacting Ask Extension.