Landscape - Powdery Mildew on Annuals and Perennials

The following is a good article on powdery mildew on annuals and perennials in the landscape.

Powdery mildew fungi attack many different kinds of herbaceous annual and perennial plants in the landscape. The white, dusty appearance which indicates the presence of powdery mildew on these plants results from fungal hyphae growing on the surface of infected leaves, shoots, and flowers while producing massive amounts of spores, or conidia. In addition to these fungal signs, powdery mildew can cause infected leaves to turn yellow, wither and die, or it may cause leaves and shoots to be abnormally curled and distorted. Powdery mildews generally do not kill their hosts, but infected plants are less productive and not aesthetically pleasing.

Although all powdery mildew fungi growing on plant surfaces tend to look alike, many are fairly host specific i.e., dogwood powdery mildew does not affect zinnia, or phlox powdery mildew does not affect rose. However, some powdery mildew species affect several herbaceous ornamentals and some plants are attacked by more than one species of powdery mildew. Examples of the powdery mildew fungi that attack flowering and foliage plants in the garden include Erysiphe cichoracearum, E. polygoni, Microsphaera begoniae, Oidium begoniae, O. cyclaminis, O. spp., O. verbenae, Sphaerotheca fuliginea, and many other species.

Conditions favoring infection. Powdery mildew fungi are obligate parasites and thus require a living plant host to complete their life cycle. Spores are wind dispersed and when they land on a plant they can germinate and cause infection. Powdery mildew infections are favored by moderate to cool temperatures and high relative humidity. Powdery mildew spores rarely germinate when leaves are wet. As the disease develops on the plant, large numbers of spores can be produced in a matter of a few days, quickly resulting in secondary infections. Continued infections are favored by a humid or almost moist microclimate at the plant surface. Shaded leaves are sometimes more heavily infected because the microclimate favors disease development. Vigorously growing plants tend to be more susceptible to powdery mildew infection than plants growing under stressful conditions. Powdery mildew-infected weeds may be a source of inoculum for garden annuals and perennials.

Powdery mildew diseases may begin early in the season on many flowering plants in the garden. By mid-season many of these plants are practically defoliated by the disease. Reduced photosynthesis by infected or lost leaves reduces flower production. Annuals and perennials that often suffer from powdery mildew in Kentucky gardens include begonia, chrysanthemum, cosmos, crape myrtle, dahlia, delphinium, hollyhock, hydrangea, impatiens, monarda (bee balm), ornamental pepper, primrose, pansy, petunia, phlox (Figure 1), snapdragon, rose (Figures 2, 3, & 4), rudbeckia, verbena, viola and zinnia (Figure 5).

Disease management. Powdery mildew management typically involves an integrated approach to reduce the effects of the disease.

1) Choose powdery mildew resistant or tolerant types. Sometimes seed and nursery catalogs will list plants with powdery mildew resistance or tolerance. There are powdery mildew resistant cultivars of crape myrtle, monarda (bee balm), pansy, viola, and zinnia, for example. Extension publications listing resistant varieties are another good source.

2) Use optimal plant spacing so that plant crowding in the beds doesn’t result in humid conditions favoring powdery mildew.

3) Provide ventilation and sunlight penetration into the foliage by pruning out shading vegetation from nearby plants or from within the mildew-susceptible plants.

4) For some kinds of plants, fungicide applications may be needed. Fungicides for powdery mildew which contain ingredients such as myclobutanil, triforine, thiophanate-methyl, or sulfur would be examples.

Powdery mildew on Gerber Daisy. Photo by Mary Ann Hansen, Virginia Polytechnic Institute and State University, Bugwood.org.

Article reprinted from "Annuals and Perennials Are Susceptible to Powdery Mildew" by John Hartman in the current edition of the Kentucky Pest News from the University of Kentucky.