Unusual weather may hurt grape crop

The wintry month of May, with average temperatures ten degrees
cooler than usual and more than twice the typical rainfall, has
left the county’s grapevines open to potential problems. Wine
grapes are currently flowering and setting fruit: a sensitive
process that can be jeopardized by wet, cool weather.
“The biggest concern with flowering underway is whether you will
get good pollination, good fruit set,” said Nick Frey, president of
the Sonoma County Winegrape Commission. “I think going into
flowering, if we’d had normal weather we had the potential for a
normal or above average crop. Now we may be looking at an average
or below average set.”
A below average set could be good news for grape growers without
long term contracts; if yields are lower than expected, wineries
may seek out extra grapes come harvest time. But for those who
contract out their acreage, it would mean simply mean less
income.
May’s cool, wet weather deviated significantly from the 30 year
average. In Healdsburg, rainfall for the month totaled 2.24 inches,
compared to a 30 year average of 1.06 inches. And the rain didn’t
arrive in one hard storm followed by warm, sunny weather; nearly
one out of every three days produced precipitation. Eighteen days
of the month were overcast and lacked strong sunshine.
“There isn’t a lot of good or bad with these rains,” Frey said.
“We’ve got a full soil profile from the winter rain so we don’t
really need the water too badly, and since a lot of it comes in
small amounts it’s probably not washing off your protective sprays
for mildew.”
Frey noted that it’s difficult to isolate the effects of rain
and cold weather on grapevines, since the two usually go hand in
hand, but that cold weather generally has a more significant impact
on bloom than the rain.
Residents kept scarves and warm hats handy, with good reason;
temperatures for the month were unusually cold. Last month’s
average low was 41 degrees Fahrenheit, 9 degrees cooler than the 30
year May average. The average high was 70 degrees, 10 degrees
cooler than the 30 year average.
“Probably anything that is flowering this week and last week is
most at risk, so the early varieties in the warmer appellations are
likely to be affected,” Frey said. “On the other hand if it does
warm up, those later varieties that haven’t started flowering yet
would be in a better position.”
During bloom, which began at the end of May and will continue
through early June, grape clusters transform from tiny, tight buds
into open flowers. Green petals peel back, and five stamens unfurl.
In a warm, dry year, the petals—fused together, known as a cap or
calyptra—fall easily off the berry.
But when the weather is cool and wet, the cap tends to
stick.
Stuck caps are associated with poor fruit set. They can
interfere with pollination or result in berry shatter. If hot
weather spurs sudden growth and the cap is stuck, the berry can
swell up against the tough cap and rupture.
Moist, stuck caps—which are essentially dead plant tissue
pressed up against the berry—also provide a breeding ground for
botrytis. A latent botrytis infection can remain dormant until
early fall rains, when it can reemerge and cause berries to
shrivel.
Botrytis is also known as noble rot; under ideal conditions,
botrytis fruit can make a fine dessert wine. But in Sonoma County,
botrytis isn’t a good thing. “Not many people in California
specialize in botrytis wines,” long time grape grower Bob Hopkins
noted. “And if it stays cold and damp, secondary infections like
aspergillus can move in.”
Hopkins also noted that the current cool weather can affect more
than this year’s harvest; it can decrease next year’s yield, too.
“Buds develop best under sunny conditions,” he said. The bud
determines the following year’s potential fruit set. “The initial
burst of growth comes from stored carbohydrates in the wood,”
Hopkins explained.
Besides treating vines for mildew and botrytis, there’s little
grape growers can do besides wait and watch the weather. Hand or
automated leaf pulling can help loosen stuck caps, but will not
entirely mitigate the meteorological impacts.
“I have to admit that yesterday [Thursday] morning I went from
slightly concerned to extremely concerned when it felt like the
middle of winter and it was cold and pouring and downright awful,”
Kendall Jackson Winemaster Randy Ullom said. But Ullom wasn’t about
to get swept up in doom and gloom forecasts just yet. “But the
rains come and go, and it’s a beautiful day and it’s pretty much
forecast to be nice in perpetuity,” he said.
Frey agreed that the situation was concerning, but not dire. “I
think it’s wait and see for now,” Frey said. “I think the wineries
are waiting to see before they make decisions on grape purchases
this year. It will be important to see what the set is. We’ll know
in the next couple of weeks.”
Don Schukraft, CEO and Certified Consulting Meteorologist of
Western weather Group, Inc, predicted a cooler than average June.
“Right now, there’s no indication that we’re going to get into any
heat spells of significance. Overall, it looks like below normal
temperatures for the next few days, then warming up to near to
slightly above normal, and then cooling off again around the 13,
14, and 15.”
Rain is a possibility mid-month as well. Long-range forecasts,
which were originally predicting a hotter than average summer, have
recently adjusted temperature predictions downward. And while all
this cool weather and late rain may seem abnormal, just ask a
farmer or grape grower: they’ll tell you, there’s no such thing as
a normal year.