Grape disease?

[NorCal]

I've got a hunch I know what's going on, but would like your opinion.

 

[garymc]

Scroll down and there's a forum on Grape Growing and Vineyards. My opinion is that you might get answers and a lot of questions there.

 

[grapeman]

You have a case of powdery mildew going on. I assume you rarely see this where you are because it is normally dry.

 

[balatonwine]

Yes, as already said, powdery mildew. It does not need free water to cause infection, simple high humidity is enough. Which is why it is not uncommon in much of California's wine regions when conditions are right. Preventive spraying often needs to start just before bloom to prevent berry infection. I suspect you know this, but to give a complete response, either UC Davis or the UC Statewide Integrated Pest Management Program is the place to go for information on management (i.e. when and if you need to spray), control and eradication options for California growers:

http://ipm.ucanr.edu/PMG/r302100311.html

And below is a very useful guide for major grape vine diseases for future reference (plenty of comparison pictures, and basic environmental factors that cause each disease):

http://www.agr.gc.ca/resources/prod/doc/sci/pub/pdf/id_guide_major_diseases_grapes_e.pdf

 

[NorCal]

Thank you for the responses. Yea, that is what I figured. We have a spray program, but it doesn't look like it's been all that effective. Will spraying for it now eradicate the problem?

 

[grapeman]

Powdery mildew is sensitive to sunlight so opening the foliage up will help get rid of it and you can beef up the protective sprayings as needed. Wayne Wilcox at Cornell has done studies showing how sunlight is effective at killing PM.

 

[balatonwine]

Will spraying for it now eradicate the problem?

Most sprays are just preventative. After infection, as per the first link I provided, only oil is an effective eradicate. And you can not use oil for two weeks after applying sulfur based dusting or sprays.

Canopy management is also good practice to help contact sprays reach the fruit and otherwise reduce disease incidence.

How many clusters are affected will dictate the cost/benefit ratio of implementing oil spraying or just improve canopy access to contact sprays and continue just with preventative spraying for the unaffected grapes.

 

[balatonwine]

Wayne Wilcox at Cornell has done studies showing how sunlight is effective at killing PM.

Canopy management is indeed important in disease control. A link to Wilcox's study (it is worth reading for anyone with a vineyard):

https://grapesandwine.cals.cornell....u/files/shared/documents/Research-Focus-2.pdf

For CA growers, I recommend following UC recommendations for canopy management in general, as they are more specifically tailored for that region where light intensity and temperatures are different than in NY. Too much opening of the canopy around the fruit can cause a different set of problems in CA.

 

[Kiwi]

Down in NZ I get this a lot with a high rainfall over our summers. If you are interested in playing with a few bunches / plants try using a milk solution. I have had some success with this even after the fungus has started. 1 part milk / 9 parts water. I use low fat milk to help keep the smell down. It is the proteins that is thought to impact. It will not fix the problem but might give you more product than doing nothing.
https://www.thespruce.com/how-to-use-milk-to-control-mildew-1402739
https://www.growveg.com.au/guides/using-milk-to-prevent-powdery-mildew/