Downy Mildew of Ornamentals
Several closely related fungi but most commonly the pathogens are in the genera Peronospora or Plasmopara.
Ornamentals are common hosts for downy mildew fungi. Common ornamental hosts include aster (Aster spp.), coreopsis (Coreopsis spp.), rose (Rosa spp.), rudbeckia (Rudbeckia spp.), snapdragon (Antirrhinum), viburnum (Viburnum spp.) and violets (Viola spp.). However, most downy mildew fungi have a restricted host range. For example, the species of downy mildew that affects roses (Peronospora sparsa) does not cause disease on any plant but roses. Other downy mildews such as Plasmopara halstedii can infect many members of the daisy family including rudbeckia, but it does not affect plants outside this family.
Downy mildew is most commonly found on plants in production in Oklahoma. Plants that have been recently purchased may have or develop symptoms before or after they are brought into the home landscape. These diseases are not generally a major problem for established plants in landscapes in Oklahoma because our weather conditions are unfavorable for the disease. However, infected plants may show severe symptoms in the spring on young plants.Symptoms vary by host, but there are some similarities. In general, dark spots may develop and they are often irregular in shape. The spots may appear angular and they are often delimited by veins. Multiple spots may develop on a leaf until the lesions coalesce. Infected leaves may drop.
With certain types of downy mildew, white or gray fungal growth may be visible on the underside of the leaves, beneath the discolored areas on the upper surface. Fungal growth may also be visible on the stems or canes. This may only be visible during periods of very high humidity.
The fungus overwinters as mycelium or as oospores within or on the plant. During cool temperature periods (50-75F) and high relative humidity (>85%), the oospores germinate to produce sporangia which contain zoospores. These zoospores can “swim” in films of water and initiate new infections on new plants or uninfected plant parts. The sporangia can also splash to nearby plants or plant parts to cause infections.In Oklahoma, most infections occur in the spring when temperatures are optimum for the disease. Plants in production are often tightly packed and may be maintained in covered green houses or high tunnels. Increasing air circulation by spacing plants and thinning (where possible) will reduce humidity and infections. Minimize periods of leaf wetness by watering in late morning or early afternoon. Drip or soaker hose irrigation is preferred to overhead irrigation.
Downy mildew epidemics end when temperatures increase and moisture decreases. Since Oklahoma is hot and dry for much of the year, the disease will seemingly “go away” as summer approaches. However, the oospores are long lived and the disease will simply go dormant until the next favorable weather period. Even in the home garden, downy mildew infections may be visible each spring.
Select plants that appear healthy and do not exhibit irregular leafspots or dieback. In established plants, remove infected stems or canes in late fall or winter. Discard plant debris in the trash. Prune plants to increase air circulation which decreases humidity. Fungicides are not recommended for the home garden, but they may be needed in nursery situations.