Hello,
it doesn’t look like a disease (i.e., black rot) also if it was black rot the leaves should had necrotic areas.
From the picture it does look like sun burn (my first guess) or physical damage, for example berries injured during management operations. Also the shoot in picture looks broken.
I would exclude a physiological /ripening disorder, called berry shrivel, because the symptoms show up immediately following veraison, and your Valliant grapes are still green.
Similarly the symptoms of another shriveling disorder called bunch-stem necrosis (BSN) develop weeks after veraison with necrotic lesions on the rachis.
You wrote that this happen every year. I was wondering if the affected clusters keep ripening (accumulating sugar) to harvest or not.
I hope this helps
Michela Centinari
Assistant professor of viticulture
Penn State University
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