This is the third season for our Valvin Muscat grapevines in the field. We had noticed “Muscat spot” on them each year. According to the New Yours State Ag Experiment Station at Geneva Food and LIfe Sciences Bulletin 161, 2006: “Symptoms of “rupestris speckle”, an apparent physiological disorder associated with grapevines related to Vitis rupestris, are commonly seen on Valvin Muscat. This disorder, also sometimes known as “muscat spot”, is characterized by circular to angular necrotic spots, under 2 mm, found especially on older leaves. The effects of this disorder on vine growth are apparently not significant.”
Here are some photos of the disorder. I think the spots are actually larger than 2mm, but from other photos observed, we are reasonably sure that we are looking at Muscat spot.