Virginia Grape Disease Updates

At Winchester, night time temperature in recent few days is getting pretty low to the point that relative humidity during the night is close to 100%. In addition, we are looking at a thunderstorm forecast tonight that will bring high relative humidity. These conditions favor downy mildew development because downy mildew pathogen prefers to produce spores under dark, humid conditions. At this point, you do not need to worry about downy mildew infection on clusters; however, they can still infect leaves. Often time, you will initially see infection on the top of the canopy because younger leaves are more susceptible than older ones. Losing the top leaves and laterals are not a big deal; however, once the infection gets severe, it can defoliate many leaves rather quickly (as in the picture above), and that can affect maturing process. As usual, it is much better and easier to have a preventative program than trying to play the catch-up game. There are many good protective mater...