Looks like mold - too hot for them, maybe pick them a little earlier?

And as much as I hate to say it, they probably need more water than they got to be larger.

It could also be powdery mildew. Do you see symptoms on the leaves too?

It is powdery in nature... there is nothing on the leaves... they are fine.

I can't pick them earlier, as they aren't ready... I can always tells when they're ready to be picked... the birds let me know.

These are just so small that I'm leaning towards letting the birds have them this year.

Looking towards next year to see what I can do. Anyone have an idea of how often and how much water these should get? They face E and get direct sun from about 11am - 3pm

Thanks.

I would cut off the diseased parts and throw it away, so it doesn't spread.

Give the grapevine a shower every day if you can to keep the powdery mildew away. The spores cannot germinate in the presence of water.

Here is a link that might be useful: UofA article on powdery mildew

you have powdery mildew!
to get rid of it, i use an organic spray.
i use 2 cups of horticultural [ground whole corn]corn meal in a 5gal bucket of water ,let it soak over night strian with nylon hose & spray on plants & grape clusters.
its best to start in the spring when vines are starting to grow.i do this about once every 2 months until harvest
you can make your own corn meal by soaking cracked corn [sold at livestock feed stores] until soft & use a blender,or even pop corn works fine.
this treatment has worked for me.no more powery mildew!!!!
i am a subscriber ot the [www.dirtdoctor.com]

Yep its powdery mildew. You will need to cut them off and I am sorry but you are done for this year. I agree with Yuman at the first sign of the stuff you have got to spray. Mine don't get the mildew tell monsoon or some times not at all, but by then the grapes are done. Another thing to try to prevent this is get rid of some of your vines/leaves. I learned that from a winery in So.Cal. let the wind blow thru and sun shine thru (keeping the leaves dry)give those vines a hair cut. Plus this means all the good stuff will go to the grapes. I know they look great but those non producing vines just suck up the water. Look at pictures of vinyards and you will know what I am talking about. Grapes love heat as long as they are getting enough water at the base(only) I am sorry to say but my ruby reds did great this year and I know yours will do better next year. Campv

Thanks for the advice. I let the birds have them this year, and I'll give it another try next year.

I plan on cutting the grapes back a lot next year and hopefully running down wire like wine grapes. Right now, these just run up a trellis.

I can' figure out how to cut the branches though to get the arms. Anything I find online is very confusing.

pokerguard here is a talk on cornmeal by the dirtdoctor you should hear.it will help you with your grape problem.
go to utube. in the search bar type organic talk,feb.2003(segment2of3)
you can stop the mildew now!! fungus and molds.

Here's information on powdery mildew and treatment options. I've also included a link to growing grapes from the Yavapai County Extension office. Their elevation is higher but the information applies in all locations. http://ag.arizona.edu/yavapai/anr/hort/byg/archive/grapeculture.html

Here is a link that might be useful: Powdery Mildew on Grapes