By Brynne Eisele, Master Gardener
First and foremost, know that, although powdery mildew on your plants may not be esthetically pleasing, your plants are most likely not permanently damaged. This fungal disease shows up as white to grey powdery spots (think baby powder or cobwebs) on leaves, buds, or stems. Infected leaves may twist or cup, looking distorted. Powdery mildew thrives in humid conditions and can show up anytime, spring through fall, in Minnesota. Due to its preference for moist conditions, powdery mildew is commonly found in shaded or dense areas of plants. It can also be more persistent if leaves are consistently wet from rain or irrigation without the chance to dry in between.
If you have identified powdery mildew on your shrub or perennial there are steps you can take to reduce the chance of it recurring the following year or spreading during the current year. Remember that this fungal disease is likely not permanently damaging to your plant and does not require any management. Since powdery mildew thrives in moist and dense environments remember to follow proper spacing on the plant tags when planting. This will prevent overcrowding of plants and promote good airflow around the plants. Make sure to clean up leaf debris and other material that gathers at the base of your plant or inside the shrub during the winter. This will open up the plant for good light penetration and airflow and discourage an environment where powdery mildew can thrive.
Many trees, shrubs, and perennials are now offered in powdery mildew resistant varieties. Selecting disease resistant varieties along with good plant management practices can help you combat this fungal disease and keep your plants looking healthy and green.
For further information on powdery mildew and a more complete listing of plants that this fungal disease might affect, read this article from the University of Minnesota Extension.