Hi Havlikn,
Sorry I'm a little late to the party. I just now tripped across this post and thought I'd contribute since I'm in your neck of the woods. I too experienced this on my Frontenac but only on selected vines and not on my Frontenac Gris. I did lose some fruit clusters. I surmised downy mildew due to the excessive amount of rain we had this year. I thought this leave problem might be related. Just my theory. I don't use any fungicide. I'm judging based on last years harvest which was much better than this year. I just think it was not the best year for grapes this year in WI (or tomatoes either ironically) because of all the rain/moisture. I assume you know that the gaping holes in the leaves are probably Japanese Beetles. I battle those every year too. I've been applying Sevin about every two weeks since mid-July. My fruit is beginning to ripen now but several clusters have damage. I did not have this problem (fruit loss) last year. I just put up bird netting as the birds seem to be getting interested in the fruit now that it is sweetening up.
