Client's Question and Request:
I'm in central county and growing Zinfindel Grapes in my backyard garden. The vines are now several years old and producing fruit this year. The grape leaves have now developed “blotches” and the grapes look “cloudy” and not very healthy. What's the problem and what can I do about it? The pictures below show the leaf damage and what the grape bunches look like.
MGCC Help Desk Response and Advice:
Based on the grape samples and the photos you provided, the problem with the grapes appears to be powdery mildew.
Powdery mildew is a common fungal disease on grapes. It first shows up as faint white powder on the grapes but later can progress to cause brown russeting on the developing grapes. That russeting was somewhat apparent on the grape samples you brought in. Affected fruit cannot ripen normally and may crack as it grows.
Shady conditions and lack of good air circulation favors the development of the disease on grapes. When the vines are pruned iduring dormancy so that shoots are positioned in the next growing season, try to prune so that the plants will allow exposure of the developing fruit to sunlight and good air circulation. Avoid overhead watering of the vines which can spread the fungal spores to new locations.
Next growing season, watch closely for the appearance of powdery mildew. Early signs of a developing problem include young emerging leaves being deformed or showing a puckered condition. Prune out such areas as soon as they appear and it may help to prevent new infections.
If you lose a large percentage of the grapes this season, you may also want to consider the use of fungicides to prevent a recurrence next year. Take a close look at this UC website which gives detailed information about different types of fungicide that can be used and includes directions on how and when they should be applied: http://www.ipm.ucdavis.edu/PMG/PESTNOTES/pn7494.html
We hope this information is helpful. You're welcome to contact us again with any further questions
Help Desk of the Master Gardeners of Contra Costa
Note: The Master Gardeners of Contra Costa's Help Desk is available year-round to answer your gardening questions. Except for a few holidays, we're open every week, Monday through Thursday for walk-ins from 9:00 am to Noon at 75 Santa Barbara Road, 2d Floor, Pleasant Hill, CA 94523. We can also be reached via telephone: (925) 646-6586, email: firstname.lastname@example.org, or on the web at http://ccmg.ucanr.edu/Ask_Us/