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.
By
07/13/2009
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.
By
07/09/2009
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.
By
07/08/2009
BTW, this is a picture of the "powdery white substance"