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OSU Plant Path – Grape PM update

Ed Hellman
North Willamette Research & Extension Center
Oregon State University
May 22, 2000


OSU Plant Path – Grape PM update

We have found the first powdery mildew colonies in our vineyard.
Before you fire up the sprayer let me explain some more. We found these
first colonies on vines that had not been sprayed for powdery mildew all
last year. That’s right, high inoculum conditions that do not exist in most
of our commercial vineyards. Well, it shouldn’t exist in our vineyards.

What does it mean? It means that the potential for powdery mildew
to get started early in Oregon has already occurred. It seems to be
following what is observed in other grape growing areas. There were no
flag shoots observed in or near these vines. This suggests that these are
ascospore infections from overwintering cliestothecia on the trunks or
cordons. According to one of the powdery mildew models there have been
3 light, 4 medium and 2 severe ascospore release and infection events
since grapes broke bud last month.

Are there tons of colonies in your vineyard already? Most likely
not for our well maintained commercial vineyards. However, abandoned
grapes or unsprayed grapes in the neighborhood (backyards or as living
decorations around the winery) could easily be sources now for everyone.
Some of you might have traditional, overly vigorous, hot spots that show
powder mildew every year. These might be places to concentrate early
control efforts.

As for other sources, we do not have a lot of wild grapes or grape
relatives in the fence lines and woods around many of our vineyards to
worry about. Also, the powdery mildew that you might find on hops,
apples or roses will not infect grapes.

Go ahead and fire up the sprayer so you can get it calibrated. It
looks like a long season ahead. (BTW – the proverbial “we” includes my
program at OSU and Walt Mahaffee’s program with the USDA. Many
thanks to Julie DiLeone for scouting these vines.)

**** This Information provided (on May 19) by Dr. Jay W. Pscheidt,
OSU Extension Plant Pathology Specialist, (E-Mail)
pscheidj@bcc.orst.edu, (voice) 541-737-3472, (mobile) 541-740-6621
(FAX) 541-737-2412.