There are several controls. The most commonly used control is dusting sulfur and wettable sulfur. One needs to be a little careful with this during really hot weather. But I saw growers spraying it all the time in CA where it regularly hit 100+.

There are several systemic fungicides. These penetrate the plant tissues and provide control for several weeks.

All controls need to start early, May or June.

Your plants are not damaged, just the leaves and fruit. They should still produce a crop next year. If you think the fruit is a loss, I'd remove it to lower the drain on the plant.

NAU Grad, I also grow grapes near Boise and rarely have mildew problems. I find that if I take measures to prevent it (pruning, trellising, leaf pulling, keeping them dry, etc), most vines do not have issues with mildew here (because it is so hot and dry), even without spraying at all. I don't grow Thompson and I don't know what your red variety is, but the concord should not have mildew issues.

You said that they are in your yard... does that mean literally? Do you water with sprinklers? I'm wondering if that is the issue. I keep my vines very dry, they root very deeply and need very little water. If the foliage and especially the fruit is getting wet often, you will have issues and may have no choice but to spray. If you are able to keep them dry and still having issues, something like neem oil will probably work and is organic.

Also, an interesting control is to use milk... yes milk. I know of some commercial vineyards in California that use it for mildew. I've never tried it, but it is supposed to be effective when it is hot and you don't want to use oil or sulfur. Check out this link:

Here is a link that might be useful: Milk for Mildew Control

California has massive powdery mildew on grapes with no rain for 6 months. This is the driest summer ever here, about one inch. I have brutal PM on melons but none on grapes. I've never had any on grapes here or elsewhere in west Texas.

Yes, they do fight mildew in California. Most of the grape growing regions are not far from the coast and have some marine influences (such as fog) which increase pressure. I agree with you, the OP could have mildew issues without getting the vines wet - that just seems like a possibility for vines in a yard. There are other possibilities. If the vines are in a location with obstructed sunlight or airflow, that would increase pressure. Also, being in a watered lawn would greatly increase humidity around the vines and increase vine vigor which also increases mildew pressure (they don't need as much water as grass).

Based on my experience growing grapes in this climate, mildew can be avoided / greatly reduced with proper irrigation, trellising, pruning, and canopy management. We've had half of your rainfall this summer (0.48 inches since May), which is not that unusual. We typically get about an inch, with pretty much all of it in June. I do hit my vines and fruit trees with dormant oil and lime sulfur. I'm not even sure that is needed most years, but an ounce of prevention...

Fruitnut is correct, powdery mildew likes dry weather. I hardly ever get it here, its too rainy most of the time but in some hot dry spells it will come on. Pruning and canopy management do help a lot. What I would recommend first time you see it is to put down some horticultural oil, and keep doing the oil until its gone. Then mark your calendar next year for two weeks before you saw it this year, and put sulphur down at that point. Sulphur is a good preventative, less good at getting rid of it once you have it.

Scott

Figured I'd ask here as opposed to starting a new thread, as the issue is the same...

After a fair amount of research, I'm quite certain that our grapes, just two seedless Flames on a trellis, have the dreaded Powdery Mildew. I planted these nearly 10 years ago and experienced nice production, expect for the past two years. This year, all small and cracked...

I know that I must treat, and want to very much. Which begs the question... Can I do something now, this time of year, or must I wait until next Spring?

Thanks much.

canyon:

You'll need to treat next spring sooner rather than later. Once you see it there will likely already be quite a lot.

Here is a link that might be useful: grape powdery mildew control

fruitnut, thank for info and informative link!

While you said to wait until next Spring, which is what I had thought initially, within that link it seems that maybe I can begin treatment now?

"If eradication is necessary, a light summer oil may be used anytime in the season if there is no sulfur residue present..."

I have not done any sulfur treatments whatsoever, so it would seem that I can use this light summer oil now?

Thanks for the clarification.

canyon:

You can safely spray now but I don't think it will help next years crop. The disease will over winter no matter what. And the current leaves are about to fall or freeze anyhow. Certainly won't hurt to spray now if you haven't done sulfur. But even if you sprayed a systemic now I don't think it would help next season.

I'll refrain from now, and make sure to tackle this next spring though... Thanks again.

I live in a hot, dry area in California and powdery mildew hammered my vinifera grapes like Flame, Thompson, Ruby. Since I switched to labrusca grapes like Jupiter, Venus, Glenora, Steuben, Suffolk etc., powdery mildew has been non-existent.

any home-made organic methods to control the mildew? If I water a lot, would that help getting rid of the mildew?