Definition - What does Downy Mildew mean?
Downy mildew is a common term used in viticulture to describe a water mold that infects grapevines, it is also known as plasmopara viticola. Water molds infect grapevines during the spring and early summer when there is heavy rainfall, or it can affect them when moisture is trapped on vines that are overgrown and not being managed properly. Downy mildew appears on the grapevine leaf as yellow patches and turn reddish-brown as the mold grows and harms the grapevine.
WineFrog explains Downy Mildew
Downy mildew is caused by the spores of the water mold, plasmopara viticola. Downy mildew is most common during the warm rainy months of spring. It can also be caused by saturated soil, and improper irrigation practices that splash water onto the vine. Once downy mildew infects the vine, it grows and releases spores that spread throughout the vine and other vines; it can also spread itself through water, which is how downy mildew can become epidemic in a vineyard in wet conditions.
The leaves of the vine are the first to show signs of infection. Initially, the downy mildew appears as yellow spots and gradually change to shades of red and brown; it can cause necrotic spots on the leaves. Downy mildew interferes with the vines ability to photosynthesize and will lead to leaf drop, which means the plant loses energy and nutrients, the mildew can also infect the grapes and lead to low yields and poor grape quality.
Keeping downy mildew out of the vineyard is difficult, as vineyard managers cannot control rainfall, however, they can implement good viticulture practices to reduce the risk of infection. Minimizing downy mildew requires effective canopy management and ensuring that water is not splashed from the ground onto the vine. The mold spores can exist in the ground and withstand the cold temperatures of the winter. Spores germinate in the spring and that is the time of greatest risk of infection.