In need of diagnosis

All varietals in our test vineyard look healthy for a freeze-damage year like this one, except for Chambourcin. This varietal has been especially peaked lately. "Like an influenza epidemic victim," as goes a line from a song that has been ringing through my head lately. Flushed red features, brown spots.

I don't believe the lesions and discoloring in these photos are the result of disease pressure, though...none of the other varietals sharing this same block are showing signs of mildew or rot with the exception of a couple phomopsis spots very early...but there were only a handful and we removed the leaves and added a couple interim sprays and that seems to have taken care of it.

I removed 60 leaf petioles and sent them to the University extension lab for analysis. I am gaining an increased appreciation for land grant universities. The petiole is the stem of the leaf, between the shoot and the blade. A lab test costs 25 bucks, and should be done on every varietal every year in commercial situations. I'm suspecting magnesium or potassium deficiencies, and I hope I can correct things with some foliar sprays and soil amendments.

Still, the copper-rust color, especially on the new leaves, is beautiful. If only it weren't the sign that something is amiss. The notion of mineral deficiencies is troubling as it just adds another item to the already daunting list of what can go wrong when growing grapes.

Ah, the romance of viticulture!