Photograph of Downy Mildew
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| Squash Cucumis pepo - Courtesy University of Georgia |
Natural History
During cool wet weather, downy mildew can occur very rapidly and is difficult to control. Some of the greenhouse crops prone to infection include: snapdragon, salvia, alyssum, pansy, rose, rosemary, and ornamental cabbage. Perennials susceptible to downy mildew include aster, coreopsis, geranium, geum, lamium, potentilla, veronica and viola.
Primary infections in the field or garden generally come from sporangia (spores) produced on southern-grown crops and carried progressively northward on moist air currents during the spring and summer.
The sporangia are disseminated locally from plant to plant and from field to field by splashing rains, moist air currents, insects, tools, farm equipment, the clothing of workers, and through the handling of infected plants. Heavy dews, fogs, frequent rains, and high humidity favor infection and rapid multiplication of the pathogen
Symptoms can be confused with other plant problems. Leaves may become mottled and yellowed, resembling nutritional deficiencies. On some plants, downy mildew infection may look similar to injury from foliar nematodes. In both cases, angular lesions are bounded by leaf veins. However, downy mildew infection results in a soft, fluffy gray, brown or purple fungal sporulation developing on the underside of leaves. On coreopsis, the fungal sporulation is white.
Symptoms can also vary depending upon the host plant. On snapdragons, infected plants are yellow and stunted. You may see downward leaf curling on the young seedlings. On salvia, angular yellow blotches can be seen between the leaf veins. On pansy, leaves turn mottled and off-color with purple blotches. Pale green or yellow patches develop on geum. Purplish patches occur on lamium and veronica. On roses, leaves develop angular dark purple to black areas and may turn yellow and drop.
Management Strategies
To manage downy mildew, first “manage the moisture”: space plants to ensure good air circulation and rapid drying of foliage after irrigation.
Choose planting sites with good air movement and without shading. Avoid overhead irrigation in early morning when leaves are wet from dew or late in the day when leaves will not have an opportunity to dry before dew forms. Maintain ample but not excessive nitrogen fertility.
Practice good sanitation techniques. Discard all diseased plants as well as plant debris that may harbor spores, and control weeds that may be another source of downy mildew inoculum.
Protect susceptible crops during cool, wet weather with preventive fungicides. Active ingredients labeled for control of this disease include azoxystrobin, Bacillus subtilis, copper, dimethomorph, fosetyl-Al, kresoxim-methyl, mancozeb,phosphite, trifloxystrobin, and triadimefon + trifloxystrobin (Armada). To reduce the possibility of the development of fungal resistance to some of the newer systemic fungicides with single or few modes of action, rotate these chemicals with protectants such as mancozeb.
Avoid the sole use of any fungicide for extended periods of time when other reliable products are available, and refer to label for timing, host plants, and rates.
Additional Information
Plant and Pest Advisory: Downy Mildew in Ornamental Plants - Rutgers University
Cucurbit Downy Mildew Forcasting - North Carolina State University
Botrytis or gray mold - Penn State University
Downy Mildew - University of Connecticut
Downy Mildew - University of llinois
Fun Facts about Fungi - Utah State University
