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When I started working at Gardener's Supply in the 1990s, my Vermont backyard was pretty green—with grass. Today, there's just a tiny bit of the original lawn left. Most of the available space has given way to trees, shrubs, perennials, annuals and stonework. Watch a slideshow of my garden in Burlington, VT.
In addition to my work at Gardener's Supply, I work in the gardening division at Church Hill Landscapes. In that role, I maintain dozens of gardens and learn a lot in the process. I believe that all gardening is good gardening.
Mid- to late summer is when most plant diseases start becoming noticeable. Powdery mildew, black spot, and gray mold (Botrytis) are three of the most common flower diseases. Learn how to recognize them and what you can do to avoid them in your own garden.
Powdery mildew, as the name suggests, resembles a white, powdery coating on leaf surfaces. The term is used for several closely related fungal species, each of which affects one or more specific plants. So, the powdery mildew on your cucumbers may not be the same species as the one affecting your phlox.
If severe, in addition to leaf surfaces it also might appear on stems and the flowers themselves. Affected leaves eventually turn yellow, then brown. Dead foliage typically falls off the stem, though it will sometimes remain in place.
Although not fatal to plants, powdery mildew makes the foliage unattractive and repeated bouts of the disease will gradually weaken the plant. Annual flowers that are particularly susceptible to powdery mildew include zinnias, snapdragons, and verbena. Perennials that are commonly infected include delphiniums, lungwort, bee balm, and garden phlox.
Unlike most fungal diseases, powdery mildew doesn't need water to spread; it can spread under conditions of high humidity.
Black spot is another common fungal disease. It is a big problem with roses. (See photo at top.) The disease typically begins as black spots on the foliage. These spots are most prevalent on upper leaf surfaces, and may be up to 1/2" across. Leaves eventually begin to yellow around the spots, then become all yellow and fall off. The spots may also appear on rose canes, first being purple and then turning black.
Gray mold is perhaps the most common disease of flowers. It is especially problematic during periods of high rainfall and cool temperatures. Like powdery mildew, gray mold is well named. It appears as a gray mold, primarily on old and dying leaves and flowers. It begins as water-soaked spots and eventually develops into the characteristic gray, fuzzy coating.
Last updated: 5/14/19
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