Bruce Bordelon, Professor, Department of Horticulture and Landscape Architecture, Purdue University
Downy mildew is a serious disease of grapes in the Midwest. Plasmopora viticola (Berk. & Curt.) Berl. & de Toni, the cause of downy mildew is an obligate parasite belonging to the Oomycete class of fungi. This class of fungi is unique in that it produces motile zoospores (spores with flagella) that are released into a film of water on the plant surface and “swim” until they find a suitable spot to infect. In this case, the spores specifically infect grape leaves, flower clusters and shoot tips through stomata, the natural openings in the plant epidermis that allow exchange of gases (CO2 in, O2 out). The fungus develops intercellularly within parasitized tissue. Asexual reproduction occurs by formation of sporangiophores that emerge from the stomata on the lower surface of leaves (Picture 1). Each sporangium contains up to 10 motile zoospores. Sexual reproduction occurs in late summer by fusion of specialized hyphal structures to produce oospores, thick walled resting spores that overwinter in fallen leaves until the next season.
Downy mildew occurs every year in Indiana. It is most prevalent in years with plentiful rainfall and warm temperatures. Last year downy mildew was at epidemic levels by mid September and it looks like this year may be similar thanks to frequent rainfall this summer. Some cultivars of grapes are moderately resistant, but most can be infected if conditions are conducive. Typical symptoms include yellow spots on the upper leaf surface, especially along the main leaf veins and often angular in shape, being delineated by smaller veins (Picture 2.) Some cultivars are highly susceptible and can be significantly damaged if the disease is not controlled (Picture 3). Downy mildew can directly infect fruit in some years, but the damage is usually caused by premature defoliation, which leads to poor fruit quality, poor winter hardiness and reduced vigor.
Management of downy mildew in commercial vineyards starts with early season fungicides applications, especially broad-spectrum products like mancozeb and captan that are applied for Phomopsis and black rot. Once we get past bloom, however, growers switch from mancozeb to sterol inhibitor products primarily for black rot and powdery mildew. The sterol inhibitor fungicides are totally ineffective against downy mildew. That is when downy mildew can become problematic. Strobilurin fungicides are effective against downy mildew and most other diseases and have been more frequently used by growers. Phosphorous acid products are also effective and widely used. However, there is concern about development of fungicide resistance to both these classes of fungicide. There is evidence of resistance to strobilurin fungicides in the Mid Atlantic states so it is likely to happen here as well. There are a number of newer fungicides available to the commercial growers that appear to be very effective. Rotating chemistry is especially important to avoid resistance development. Growers should pay close attention to label requirements for sequential sprays and total number of sprays each year. The Midwest Small Fruit & Grape Spray Guide provides a detailed description of fungicide classes and strategies to manage fungicide resistance.