The warm and wet spring following the earlier wet winter has created perfect conditions for a very uncommon appearance of Downy Mildew in vineyards of both San Luis Obispo and Santa Barbara Counties. Downy Mildew is a very aggressive disease that has the potential to cause large amounts of damage if not controlled.
The weather forecast for possible light precipitation over the next several days will create conditions conducive to rapid spread of infections. Growers in the coastal areas should pay extra attention to scouting for this disease and treat it accordingly if found. Infections have also been observed in other areas including Paso Robles, thus growers throughout the Central Coast should be on the lookout for this disease.
For basic biology and treatment information, see the UC IPM website:
http://ipm.ucanr.edu/PMG/r302101111.html
Note that the fungicides listed on the above website may not include some of the more recently available products. For details on newer products see pages 30-34 of the document at the link below from Wayne Wilcox at Cornell University:
http://www.fruit.cornell.edu/grape/pdfs/Wilcox-Grape%20Disease%20Control%202016.pdf
The images below were taken on May 11, 2017 in a Chardonnay vineyard in the Santa Maria area. They provide an idea of the appearance and potential severity of infections at this time.
Downy Mildew can infect all green tissues of the vine. Infected leaves will have yellowish spots on the top sides and the characteristic white fluffy growth on the undersides (Figures 1 and 2 below). Infections of the flower clusters can lead to complete necrosis (Figure 3). Infected shoots can have partial infections or become fully infected and collapse (Figures 4 and 5).
