This has been the perfect year for downy mildews in ornamental plants. The following is more information.
Symptoms of downy mildew most often appear on the upper leaf surface as a patchy yellow, purple, or brown discoloration bounded by leaf veins. Affected leaves may defoliate prematurely, and stunting may also occur in some species, such as snapdragon, and in seedlings and bulb crops. To see why the disease is called “downy mildew,” turn the leaf over. Evident on the lower leaf surface are the characteristic downy tufts of white to purple/grey fungal growth. This growth consists of spores (called sporangia or conidia) that are easily dislodged and disseminated by wind and splashing water.
Downy mildews develop on susceptible crops during cool (58 to 72ºF), fairly humid (> 95% RH) conditions when spores from previously infected tissue are splashed or blown by air currents to colonize new sites. As infection progresses, symptoms develop, and the downy tufts of spores associated with leaf lesions may appear on the lower leaf surface. Fungal development ceases for most downy mildews when weather becomes dry and warmer than 80ºF for 24 hours. In the absence of susceptible plant tissue, downy mildew pathogens in cold climates survive in plant debris, soil, or weeds. Watch closely for symptoms in susceptible crops during periods of cool, humid weather. When you see foliage that is discolored, turn the leaves over to check for mites or downy mildew sporulation. Symptoms of this disease can be confused with other fungal (such as Botrytis blight and powdery mildew) or bacterial diseases, foliar nematode, or some nutritional deficiencies.
To manage downy mildew, first “manage the moisture”: space plants to ensure good air circulation and rapid drying of foliage after irrigation. Avoid overhead irrigation when the weather is generally cool. In greenhouses, reduce relative humidity to less than 85% by, again, properly spacing plants, and use horizontal air flow fans to improve air circulation throughout the house. Fill the house with warmer, drier air by venting and heating two or three times per hour at dawn and at dusk. 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.
Information reprinted from "Downy Mildew in Ornamental Plants" by Ann B. Gould, Ph.D., Specialist in Plant Pathology, Rutgers University in the July 23 edition of the Plant and Pest Advisory; Landscape, Nursery, Turf edition. Information Source: Gleason, M. L., M. L. Daughtrey, A. R. Chase, G. W. Moorman, and D. S. Mueller. 2009. Diseases of Herbaceous Perennials. APS Press, St. Paul, MN.