These are four year Marquettes. They've had two sprays of Dithane/Sulphur/Sevin and a third spray of Captan/Sulphur/Sevin. Leaves are prestine but all clusters have brown spots as in the pictures. Am I screwed already in June?
If those had black rot showing (I don't think so) the leaves would also show it. To me it looks like spray damage probably from the captan. Had you applied an oil spray of any type early season with a month or so of the captan? If so that is likely what caused it. A dormant spray of stylet oil or a lime sulfur mix would do that followed by the captan. Also only two sprays into the season probably was not enough for having grapes that size already. Early season is the most critical time to spray to prevent many diseases.
My frontenac look almost identical to that, and mine is from a hail storm we had just over a week ago. I went out the next morning and didn't see hardly any damage, but with every passing day, it showed up more. It surly thinned the clusters for me!
If not spray damage, I would also say this looks like hail damage. The few split berries lead me to lean more towards hail damage than chemical damage from spray -- assuming you had rain recently, of course, because if you didn't then it could not possibly be hail damage.... .
To compare, here is heavy hail damage from last July at my vineyard:
Yup, that's exactly what mine look like, but not quite that bad. It really didn't last long enough to knock any leaves off, but it sure bruises and busts up the berries! I'd post a pic of mine, if I was smart enough to know how to do it!
Grapeman Can you clarify something that has been said here? Are you saying that if dormant oil or lime-`sulfur spray is used in early spring; let's say within a month of leaf break. That Mancozeb and Captan shouldnt be used until 5-6 weeks after to avoid spray damage?