Wet conditions promote rust fungus on hollyhock (Alcea rosea). Infected leaves eventually turn brown, wilt and die. Symptoms typically start on lower leaves and the disease progresses to stems and upper leaves over the summer. The undersides of infected leaves show raised brown or brownish-red spots about the size of a pinhead and the upper leaf surface has small yellow spots. Hollyhock rust is caused by the fungus Puccinia malvacearum. Fungicides available for the home garden to treat powdery mildew include horticultural oils, neem oil and products that contain chlorothalonil or sulfur. These fungicides are mainly protective, not curative. To be effective, fungicides should be applied according to label directions at the first sign of the disease and repeated on a regular basis.
Planting resistant varieties also minimizes problems with this disease in species that are prone to becoming infected.įungicides are rarely warranted for home gardens, but if cultural practices do not provide enough control, they can be used.
In overcrowded plantings, improve air circulation by digging and dividing perennials. Plants growing in shady locations should be moved to a sunny site. Cutting off and removing diseased plant debris in fall can reduce the amount of pathogen present in the garden next year. The fungal pathogens that cause powdery mildew can survive the winter. Optimal conditions for powdery mildew are cool nights followed by warm days.Ĭultural practices can reduce the severity of powdery mildew. Powdery mildew is most commonly found on plants growing in shady areas and in crowded plantings with poor air circulation. Infected leaves eventually turn yellow and then brown. The fungal disease produces a grayish-white coating or “dusting” on the leaves starting on lower leaves and progressing upward. Powdery mildew is caused by a number of different fungus species and is a common disease of garden phlox (Phlox paniculata) and other perennials such as bee balm, asters, peony, mums and dahlias. How can I control powdery mildew on my garden phlox? Horticulturists with Iowa State University Extension and Outreach answer frequently asked questions about the diseases commonly found in Iowa perennial gardens. For example, if the site is too wet or too shady, this can promote disease problems. When disease issues are observed on perennials, the first step is to evaluate the growing conditions.
AMES, Iowa - Diagnosing problems in the perennial garden can be challenging.
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