Decline in the vine.

Although tourists may enjoy seeing red leaves on grapevines this time of year, red leaves are not normal and are, in fact, an indication that something is wrong with the vine. Several problems can cause red leaf symptoms in grapevines.  For example, red leaves are one symptom of Syrah Decline (SD), but they are not the only symptom.  And certainly, Syrah is not the only variety to show red leaves.
Several of Vinoland's Syrah vines display varying levels of red colouration in their dying leaves in the autumn.  (Curiously, there are fewer vines displaying redness this year than previous years.)  In the above photograph, the yellowing vine on the right is shutting down correctly - from green the leaves turn yellow, then brown, then drop to the ground - the reddening vine is shutting down not so correctly.  I needed to take a closer look, so out to the vineyard I went. A quick once over of this vine revealed, or not, as the case may be, a few things to me.
The culprit could be SD (more commonly referred to as Syrah Disorder in California), a syndrome that is said to affect particular clones of Syrah more than others.  The graft union of this particular Syrah vine shows no evidence of cracking or pitting (which are two symptoms of SD).  In fact, I'd have to say this particular vine has the prettiest of graft unions.
It can't be Leafroll disease as there is no evidence of, well, leafroll.
It isn't Fanleaf Degeneration as the leaves show no signs of deformation - as in looking like a fan.
Could it be Pierce's Disease (caused by the bacterium Xylella fastidiosa)?  Well, there is some leaf scorching, and there are a few petioles that have remained attached to the cane after the leaf blade has separated, but this vine shows no other symptoms.  No stunted shoot growth - this particular vine has shoots that are up to 8 feet in length.  No raisining of fruit - the vine supported a fantastic crop this year.
So, in conclusion, I don't know what is wrong with this vine.  If I really wanted to know what was going on I could have the tissue tested.  Then again, I could not.  Perhaps it is a hitherto unknown pest...that'd just be my luck. Ho hum.