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.