April showers bring May fungi to vines

Grape growers get back into the fields to apply
fungicides to keep spores at bay
by Dawn Pillsbury, Staff Writer
With and early bud break and the cold spring rains, county grape
growers are concerned about a variety of fungi, including powdery
mildew.
When the rain stopped Sunday, growers took to the vineyards to
stop the pesky spores from getting a foothold.
“I had to get back in the vineyard at the first opportunity as
soon as the rains stopped,” said Richard McDowell, vineyard manager
for Toad Holllow Vineyards on Westside Road.
McDowell spent Monday driving a spray rig through the vineyards,
applying two fungicides to counteract mildew and botrytis.
Many growers had applied sulfur during the past three weeks to
battle the fungi, but the rains washes it off, said McDowell.
“They’re out there doing it again these last few days,” he
said.
Nick Frey, executive director of the Sonoma County Grape Growers
Association, said last week that many growers were just waiting for
the rains to stop to start preventative measures against powdery
mildew and other fungi.
Vicky Robinson, executive director of the Alexander Valley
Winegrowers, said growers have seen some evidence of phomopsis and
shoot blight, but that overall the vines are healthy.
“It’s a good thing we went from cold rains to this warm, dry
weather,” she said. “It’s when it’s tropical that it’s a
problem.”
Because the weather hasbeen cool, the spores that rains release
on the vines hadn’t had the opportunity to take off yet, said Frey,
but as soon as the weather warms up, the multi-million wine grape
crop could be threatened.
“Powdery mildew attacks leaves and later the fruit,” said
Frey.
Other fungi that could threaten the crop because of the wet
weather are botrytis, which causes earthy flavors in wine, and
phomopsis.
“I’d be a lot more worried about phomopsis than mildew,” said
Kate Burroughs, co-owner Harmony Farm Supply in Graton. “It’s
another fungal disease that attacks grapes. It makes funny little
splotches on leaves and can infect the way the grape bunch attaches
to the vine so that it gets rotten and falls off.”
The worst the mildew can do is create off-flavors in the wine,
said Burroughs, unless it goes untreated. If untreated, it can kill
all a vine’s foliage, stopping grape production.
Burroughs said the fungal threat was the hot topic of discussion
at the Pest Management Alliance Field Day in Sebastopol on April
25.
“A lot of people were talking about it,” she said. “This is one
of the wettest Aprils we’ve had in quite a while.”
Both Burroughs and Frey said the thing for winegrowers to do is
wait for fields to dry off — which will take longer for fields
without cover crops — then apply liquid or dust sulphur, Kaligreen
— a potassium bicarbonate fungicide or an oil, such as neem
oil.
Burroughs added that there are two biological control agents for
powdery mildew, competing organisms that don’t harm the plant. All
the treatments for powdery mildew should affect the botrytis and
phomopsis as well, she said.
Forestville vineyard consultant Mike Porter agreed that
phomopsis is the bigger concern until the weather gets warmer.
“Powdery mildew has the same optimal growth conditions as the
vines — pleasantly sunny — which is what makes it such a
nuisance,” he said. Warm weather will prompt the release of
billions of fungus spores, propagating the disease over the vines,
he said.
The rains will spur diseases like phomopsis as well as soil
diseases in young vines, such as cylindrocarpon, one of the black
goo/young vine decline fungi that are present in nursery stock, and
phytophthora, a relative of the Sudden Oak Death pathogen. Those
soil fungi grow out of control in saturated soil, killing young
vine roots.
Porter said the solution to any of the diseases that thrive in
wet soil is improving drainage. He added that copper is an
elemental toxin to phomopsis, like sulphur is to powdery mildew —
meaning that the fungi can’t develop resistance to the
substances.
During the vine boom of the late ’90s, a lot of vineyards were
planted for quick sale without regard to soil conditions,
appropriate varietals or other considerations, said Porter.
“This is the year people find out what they bought,” he said.
Many new vineyard owners will have to install new drainage to get
water off of vine roots, he said.
“You’ll either have vines die or, if you’re lucky, just get your
tractor stuck in the mud,” he said.