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Mid- to late summer is when most plant diseases start becoming noticeable. Powdery mildew, gray mold (Botrytis), and black spot 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. If severe, 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.
As with most fungal diseases, gray mold is spread by wind and splashing water. To prevent problems, avoid getting irrigation water from splashing onto the leaves and flowers.
Most fungal diseases are spread by microscopic structures called "spores" that are transfered on wet foliage. However, powdery mildew thrives in high humidity. Keeping plants well-spaced and removing weeds will help ensure good air circulation and reduce the humidity around plants.
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.
Black spot is another common fungal disease. It is a big problem with roses. The disease typically begins as black spots on the foliage. These spots are most prevalent on upper leaf surfaces, and may be up to ½″ 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.
Last updated: 11/1/15
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