Winchester area had thunderstorm yesterday from 12:40 PM to 3 PM (0.27 in), the average temperature was around 72 F.  The relative humidity were low after the storm, thus it accounted for about 3.5 hours of wetness.  Then, another drizzle hit us during 3:20 AM to 4:20 AM (0.02 in), and the RH was high (>90%) until 6:20 AM.  The average temperature during this 3 hour wetness event was 59F.  Since both were relatively short and relatively warm, the risk of downy mildew is the major concern.

As I noted yesterday, we are having hot humid early summer which is a conducive condition for powdery mildew.  Here are today’s pictures from our untreated vines that started to show the development of powdery mildew (click the picture to see a larger image).  Please scout your vineyards especially the location where leaves are shaded.  Powdery mildew pathogen prefer to thrive under diffused sunlight conditions.

This is a close-up of powdery mildew pathogen (Erysiphe necator).  The round egg-like structure is a conidia (spore) of this fungus.  Only one per branch (a conidiophore) is shown, but it will form a chain of conidia over time.