What Grapevine Disease Is That?

While spraying a week or so ago, I came across a Cabernet Sauvignon vine with some shoots that had very strange looking leaves at the growing end.
They were quite deformed and shriveled with distinct yellow spots.
On inspection there was no evidence of any insect activity so it was into the text books to try to identify what the problem was.
It was the only vine in the one hundred in the block that demonstrated this phenomenon. 





















My grapevine disease/pest 'bible' is Diseases and Pests edited by Nicholas, Magarey and Wachtel.
From the text there would appear to be three possibilities.
Two are herbicide damage with Glyphosate (Roundup®) and 2,4-D the culprits. We don't use the latter so it's possible that the former is a problem. I spray the undervine rows with Roundup® for weed control but am very careful about preventing foliage overspray. However in vineyards with sandy soils where roots are near the surface (that's us!) Roundup® can cause these observed symptoms through root absorption.
The other possibility is some sort of mutation (genetic changes in buds) which occur on single shoots.
It does appear however whatever is happening is not a major problem.