What variety of grape do you have? It sounds like black rot. Start spraying Mancozeb, ziram, ferbam. Old standards, good control under most conditions. Do a "image" search online and type in your desciption and see if you can narrow it down. Concord types are suseptible.

Actually, I suspect it's powdery mildew or downy mildew. Black rot is just that, the entire fruit rots, dies, and shrivels away, often falling off. With the mildews, the skin is affected, but continues to live, with large gray-brown patches -- it can cover the entire grape, or only one side or part of it. The fruit under the skin continues to develop normally, but obviously, not good to eat like that.

Any of the fungicides mentioned above will work. Another good one is Bayleton. I don't know if it's technically labeled from grapes. I bought 3 quarts of it in ready to spray, hose end type sprayers on clearance at Lowe's last year in the spring for $1.25 a bottle. It was sold and labeled as "Lawn Disease Control," but I used it on summer and winter squash, tomatoes, grapes, peaches, and also on my lilacs, phlox, and monarada, and it did an absolutely fantastic job of keeping them clean and green all summer, no disease problems at all. If I ever found more of it at that price, I'd snatch it up in a minute.

Bayleton

I am not sure whether there are other kinds of Bayleton, but the one I found on the internet is for Turf and Ornamentals only. It is a systemic fungicide, that is absorbed into the plant, and as such probably not safe. In fact, from the label: "Edible portions of treated tress, such as nuts or syrup, should not be used for feed or food".

It may be one thing to use something which is labeled for one fruit tree on another kind (though still against the law), but I wouldn't risk using a product not designed for edibles on grape vines, fruit trees or whatever.

Kaig, good that you picked that up. Folks should seek advice and purchase sprays from a local nursery with expert staff.

The "expert staff" in my area nurseries know almost nothing about fruit growing unfortunately, so I stopped asking them questions. Its always worth a try but don't be surprised if you end up empty.

Bayleton is used widely for grapes, but not the turf formulation.

My advice in terms of labels is the sprays good for fruit trees are always so labeled if you read the full instructions. If fruits are not mentioned I would not spray it.

Scott

Scott,

You grow 400 varieties of fruit organically and you don't know a local expert on plant diseases? egads! A half a mile from where I grew up the local nursery always has a year round expert.

Ace:

There may be a local nurseryman around who knows something about plant diseases, but in over 30 years of growing fruit here I haven't found him yet. What I have learned has been from books, Nafex, the internet, and reading labels. Even so, I have to apply some judgment, and sometimes I am in the guesswork range.

If you have a nurseryman you can depend on, you are very lucky.

Don Yellman, Great Falls, VA

Don, In the county where you live (and most of the U.S.), there is a extension representative there to assist you for free. My cooperative extension works with Cornell on plant disease/insect diagnostics for a small fee. I want to try to fix a cankered apricot thats bearing and growing well. Maybe there is a surgery so it won't become firewood. I'll decide in a month when the Apricots are picked.

Here is a link that might be useful: Plant Disease Diagnostic Clinic