On September 21st, 2013 Napa/Sonoma experienced a much greater total amount of rainfall than the meteorologists were projecting. Originally the reports were for up to 0.10" for the entire event. Surprisingly, the area experienced up to 1.0" of rain.
RAIN EVENT DETAILS:
During the early morning of September 21st, we had the first rainfall hit between 1:00am and 3:00am in the morning. This rain was minimal and we expect most of this moisture did not penetrate vine canopies.
The mid-morning of September 21st, it rained again around 8:00am. This rain was also minimal and we saw a rewetting of the ground and canopies. It was still insignificant rainfall totals in most areas.
In the late-morning between 9:00am-12:00pm we had heavy down-pouring rain. The clusters were soaked and ground was muddy. Totals seen from the Sonoma Valley and Napa Valley were 0.5"-1.0" of rain.
Shortly after the last rain fell in the late morning on September 21st, we had sun and wind come into the area. Clusters were dry by Saturday @ 5:00pm as long as they were not in low lying areas (or otherwise wind protected areas). 24 hour cluster wetness was avoided for most areas.
Chardonnay and Zinfandel blocks are the most sensitive to rot pressure. Maximum control over rot is achieved through canopy management, with the best results occurring when the canopies are leafed wide open before the event. In highly sensitive areas, Walsh leafed heavily before the rain.
Another approach is to spray botrytis materials that prevent rot. Walsh clients elected to do this in only the most sensitive areas. This effect is measurable, but not nearly as effective as opening the canopies.
(1) Most clusters did dry out by 5pm Saturday, before the critical 24 hour wetness, that can trigger the Botrytis life cycle.
(2) Phenology records indicate we are 120-130 days after bloom. Many blocks still have 10-25 days left to get fully ripe. As a result, grapes still have healthy protective skins, making them less susceptible to rot.
(3) The September 21st rain was our first significant moisture of the harvest season.
(4) Walsh does not expect rampant Botrytis activity in area vineyards. We will continue to monitor the situation with winemakers.
Click the link for a good research paper on botrytis control that supports our discussion on leafing to control botrytis.
As of Sunday, NOAA projecting high winds and fire danger by the end of the week. This will dry out any rot and likely lead to a greatly increased rate of ripening.