Downy mildew initially causes light green spots on the upper leaf surface of infected plants. Spots may appear darkened or water-soaked with time. The diagnostic feature of downy mildew is the underside of these lesions. There, you see a grayish white, downy growth composed of hyphae and spores. The sporulation is distinct, and most labs can observe this growth with a compound microscope for rapid, positive identification of the disease. Usually oldest leaves are affected first. Infected leaves may become necrotic and drop from the plant.
The downy mildew fungus (Plasmopora sp.) overwinters as oospores in dead plant material or in the soil. In the spring, these spores are splashed to lower leaves. Another type of spore (sporangia) forms in the downy material on the leaves. These spores are spread by wind and water. To minimize disease spread, space plants properly, control weeds, and water early in the day to minimize periods when leaves are wet. Fungicides can be used as preventives.
Downy mildew development occurs in cool, wet weather. Recently, the Plant Clinic received samples of downy mildew on rudbeckia, geranium, and salvia. Downy mildews may occur on other garden plants, such as Artemisia, aster, cornflower, geranium, lupine, potentilla, rose, salvia, snapdragons, veronica, and pansy. An article on rose downy mildew can be found in issue no. 6 of this newsletter. There are two nice images of rudbeckia with downy mildew on the Internet at http://www.ipm.msu.edu/grnhouse04/G01-03-04downymildew.htm. This is a Michigan State University greenhouse alert page. There is also a University of Illinois Report on Plant Disease that may be helpful: no. 657, “Downy Mildew of Snapdragons,” available on the Extension site http://www.ag.uiuc.edu/%7Evista/horticul.htm.