Questions have been coming in to Oregon State University Extension Ask An Expert online about how grapes  will be affected by the summer's cool weather.
"This was one of the coolest summers in more than 50 years for western Oregon," noted Steve Renquist, OSU horticulture specialist. "We need two to three more weeks of sunshine and temperatures in the high 70s for grapes to survive.”
The chilly, wet weather could eventually lead to Botrytis grey mold, and the grape skins will deteriorate from the rain, Renquist said.
"It is very unlikely that the grapes will gain more sugar if the rain does not stop over the next two weeks,” Renquist said. “If we get three to four days of sun strung together we could see a 1 percent to 2 percent sugar gain. Usually when it rains on maturing grapes, they will lose a small amount of sugar as the vine pulls some back into the plant and the rainfall uptake by the plant dilutes the sugars."
Home grape growers have nothing to lose by letting the fruit hang if the grapes are not sweet enough to use now, he said.
"But they won’t last more than three weeks hanging in bad weather this time of year,” Renquist pointed out. “Another problem when grapes hang for a long period is that birds and raccoons will eventually find them."
Information on Ask An Expert is online at: http://extension.oregonstate.edu/extension-ask-an-expert