My trellis is covered in unripe green grapes. It looked as if we were going to have a bumper crop. One morning I saw that most of the grapes were covered in a powdery white substance. Over the next few weeks the grapes shriveled up. I have no idea what happened here. The previous year we had beautiful, sweet purple grapes.
The white powder is a fungus called oïdium. Here is a natural remedy that should stop the fungus propagation : spray leaves and grapes with 0.5% sunflower oil emulsified in water.
It looks sort of like what happened to my fig tree several years ago. The white spread to the bush itself and ended up killing it, we thought. A nurseryman told me later that it was Bordeaux fungus. It affects grapes, figs, some other things. I believe that Ortho makes a spray for Bordeaux fungus, just read some of the labels. Ask at a farm or nursery center what they have.
If spraying with liquid soap/water doesn't work you might consider that it isn't mildew.
http://www.backyardgardener.com/tv/mildew.html or I make a spray with liquid soap in kitchen; 1 table spoon of soap to a quart spray bottle of water, from Dollar store. Use it at least once a week, re apply after rain, good luck.
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