Sunday, September 12, 2010

Disease risks from today's rain

Winchester area received much needed rain this morning!  It started around 12:40 AM and lasted until 6AM (0.43 in), then the RH was above 90% until 11AM or so.  Thus it accounted for about 10.5 hours of wetness with an average temperature of 63F.  It is long and warm enough for downy mildew.

It is so close to the harvest (we are harvesting Tony's Cabernet Sauvignon this weekend) that you may not think of the disease management.  If you did a good job of maintaining vines clean earlier in the season, you probably do not need to be concerned.  However, if you have issue with downy mildew this year, it is important for you to manage even after harvest.  Your vines need carbohydrate stored in trunks in order to survive the winter.  A severe downy mildew infection can reduce photosynthesis by reducing the leaf area with leaf lesions, and also by defoliating leaves.  Thus, if you know there is downy mildew in your vineyards (it often shows up on younger leaves on the top or on laterals first), and it is increasing in intensity, then you may need to apply downy mildew material after harvest to keep your leaves clean.