Whilst walking through the vineyard late this afternoon, checking on how the Syrah vines are shutting down for the winter, I noticed a red leaf. I did not panic, instead I checked the leaf's underside.
A ha! As I expected I found insect damage: girdling of the petiole. Now in this neck of the woods there are two main suspects for such crimes against vines; the Buffalo Treehopper (Stictocephala bisonia) and the Three-cornered Alfalfa Hopper (Spissistilus festinus). Damage to the vine occurs when continuous feeding punctures, resulting in the interruption of the movement of materials to the leaf, reduce chlorophyll production and cause anthocyanins to become the dominant pigment remaining in the leaf. The reddish leaves look like those of Leafroll virus, hence my concern upon espying the offending leaf.
It is obviously not a big problem, seeing as this is the only red leaf I have seen in the vineyard, but it was a momentary cause for concern. After all, you need healthy vines to make yummy wines.