by Alison
Stoven
Colorado State University Cooperative Extension
Master Gardener
Larimer County
Q: What is the white, flour-like stuff on my perennial leaves?
A: It sounds like powdery mildew, a fungal disease. This fungus affects a wide variety of perennials, and usually develops mid-summer through fall when warm days are followed by cool nights. During the day, wind carries fungal spores to the host plant, and at night when the humidity increases, the spores germinate.
As the disease worsens, plant leaves may become distorted and drop prematurely. If a plant continues to get powdery mildew year-after-year, it may permanently weaken plants and increase susceptibility to other pest and disease problems. Powdery mildew occurs in landscapes with poor air circulation and abundant shade. Dense plantings increase humidity, and can increase powdery mildew spore germination.
Plants like garden phlox, aster, black-eyed Susan, bee balm and delphinium are more prone to the disease. There are powdery mildew-resistant varieties of many plants. Try ‘Blue Stocking’ bee balm and ‘David’ garden phlox.
To avoid powdery mildew, improve air circulation in your landscape and thin plantings. Water at the base of plants instead of the leaves and avoid excessive fertilization. Monitor plants for disease development, and if necessary, use protective fungicides. Fungicides will not kill existing mildew, but it can protect uninfected foliage.
In the fall, rake up fallen foliage to reduce spore development next year. The powdery mildew spores overwinter in leaf and stem debris, and when the conditions are right, will germinate next season.
Q: I hear a lot about Cooperative Extension and
know it’s affiliated with Colorado State University. But what exactly
is Cooperative Extension?
A: Cooperative Extension has a long history. In 1862 the Morrill Act (also known
as the “Land Grant College Act”) stated that one institution in
each state was to be designed a land grant college or university. In turn, the
institution must educate individuals in agriculture, home economics, mechanical
arts and other professions. The Morrill Act was initiated by Vermont congressman,
Justin Smith Morrill. His vision was to allow for higher education be available
to all social classes.
In Colorado, Cooperative Extension is a join effort between the United States Department of Agriculture, the land grant university, Colorado State and the 64 counties in Colorado. Cooperative Extension offers unbiased, research-based information for the citizens of Colorado.
Information is available from Cooperative Extension on topics like horticulture, agriculture, natural resources, finance, food safety and preservation, nutrition and many others. For more information about Cooperative Extension, please contact your local County Extension office.
Gardening and Insect Fact Sheets are available on-line by clicking HERE.
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