It is a phosphorous acid based spray. At one point I used one of those on grapes. It is an eradicant spray, it will kill when you spray but will not prevent. It is supposed to be pretty good on downy mildew, but I found it did not work well at all on its own and my standard spray became fosphite (the brand of phosphorous acid spray I was using) plus 1/2 strength copper. Those together worked reasonably well on downy mildew. There are probably other better options for downy if you are using synthetics.

First step though is figure out what fungus you have, if its black rot or powdery phosphorous does little for those. Grape sprays (as well as most fruit sprays) are very much per-disease.

Scott