POWDERY MILDEW ON ORNAMENTAL PLANTS
Fig. 1. Powdery mildew fungal growth and spores on leaf surface.
Fig. 2. Powdery mildew on lilac
Powdery mildew, caused by a variety of fungi, is a common fungal disease on ornamental plants in Minnesota. This disease frequently infects rose, phlox, lilac, begonia, zinnia, alpine currant, and other shrubs. Powdery mildew does not usually cause serious damage to its host, but does derive nourishment from the plant, slowing down the plant's normal growth and reproductive rate.
Fungal spores produced on previously infected plant material are windblown in the spring to susceptible tissues. Spores landing on the surface of succulent leaf and stem tissues begin to grow and send small fungal structures into the epidermal layer of the leaf to act as anchors. After establishing itself, the fungus produces columns of spores capable of causing secondary infections (Fig. 1). Fungal growth and spores produced throughout the growing season give the leaf a white to light gray color (Fig. 2). The leaf may also appear powdery or dusty, since some of the spores can be rubbed off the plant tissue onto your fingers. Occasionally, leaves become puckered or may have necrotic (brown/dead) areas. Both temperature and relative humidity are important factors in initiation and disease development. Powdery mildew is most prevalent under conditions of cooler temperatures and high humidity. These conditions occur with poor air circulation or when cool night temperatures follow warm day temperatures. Consequently, in Minnesota we see increased levels of powdery mildew in the spring and fall of the year.
Since high relative humidity is an important factor favoring disease development, certain cultural practices can help prevent the disease or decrease its severity. Increase air circulation and light penetration. This can be accomplished by increasing ventilation and light intensity in greenhouses or homes. Trees and shrubs outside can be pruned and thinned to reduce overcrowding in the landscape. When planting new materials, select those which have resistance to powdery mildew and allow for adequate spacing of plants.
Powdery mildew seldom warrants chemical control in the home landscape. When control is necessary, wettable sulfur or thiophanate-methyl (Cleary's 3336) may be used. Do not use sulfur on sulfur-sensitive plants such as viburnum. Recommended fungicides for viburnum are thiophanate-methyl (Cleary's 3336) or chlorothalonil (Daconil 2787). Fungicides, as a general rule, prevent infections from occurring. They do not cure existing infections. Therefore, it is important to treat for powdery mildew as soon as it appears to prevent more infections from occurring. Apply additional sprays as needed, according to the label. For deciduous shrubs that have repeated, yearly infections, a dormant fungicide such as lime sulfur may be applied (do not use on viburnum). Apply in early spring before new growth starts. Check all chemical labels for recommended plants for treatment, before purchasing and using. Information on the label should be used as the final authority. Read all fungicide labels carefully and apply only as directed.
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Chad Behrendt, Crystal Floyd