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.

A word from a friendly, local winemaker.

It is not often that a winemaker will take time out of their busy, wine-filled day to address the idle inquisitiveness of a nosy blogger, in this case me.  But, that is just exactly what Chris Phelps, winemaker at Swanson Vineyards, did when I inquired as to why they had decided to rip out Swanson's Pinot grigio vines on the Oakville Crossroad.  Here is his reply in its entirety:

Dear Vinogirl,

Alas, we were disheartened, as well, to realize we had no choice but to pull some of the Pinot Grigio vines in our Oakville Vineyard.

Leafroll virus had taken a serious toll on the vines,  dropping yield down to an unsustainable 1.25 tons per acre, and preventing the crop from fully ripening.

We hope that your Pinot Grigio yields remain high, or at least sustainable, and may your vines continue to be virus-free and live forever!

We do remain dedicated to producing Pinot Grigio, in spite of the fact that the variety is notoriously low-yielding.

Thank you for your concern, and please do let us know if you have more questions.

Chris Phelps.

So, the culprit was leafroll virus, curses! 
Seeing as I can't do anything about it I have decided to open a bottle of 2009 Swanson Pinot grigio (a staple in Vinoland) to toast the memory of those grand old vines in Oakville.  And, instead of me waxing lyrical about the virtues of the wine in hand, I suggest you watch this clip of Mr. Phelps describing his own handiwork.
Thanks Chris.  

Going viral.

Long before viral became a marketing buzzword to describe how thoughts, information and trends move into and through a human population, the Napa Valley's grapevines were going viral without any help from social media.

The are many viruses that cause diseases in grapevines, and due to the woody nature of much grapevine tissue, it is often difficult to purify such tissue. New vineyards planted from infected cuttings or budwood will be diseased from the time they are established. That is why a programme such as the Foundation Plant Services (FPS) at UC Davis is so vital to the wine industry. FPS is a self-supporting service department in the College of Agricultural & Environmental Sciences and is very instrumental in the detection and elimination (using heat treatment techniques) of grapevine diseases.

Grapevine leafroll is probably the most wide-spread virus disease of grapevines and there are currently 9 different viruses associated with leafroll. Leafroll spreads slowly from vine to vine and impacts both vine health and grape quality, in some instances reducing yields as much as 50 percent or even more, depending on the severity of infection. Typical leaf symptoms include reddening of the leaves between major veins in red varieties. Thus, we can assume that the vivid dark red of the vine in the above photograph, and the reddening of its neighbouring vines on the Silverado Trail, indicates that it is indeed a red variety.

A green leaf is green because of the presence of a pigment known as chlorophyll. When chlorophyll is abundant in the leaf's cells, as they are during the growing season, the chlorophyll's green colour dominates and masks the colours of any other pigments that may be present in the leaf. Consequently, the leaves of summer are characteristically green, but in the autumn and at harvest time the entire vine takes on a reddish cast. It's very pretty, but it's very unhealthy.

When a virus is present it disrupts the normal physiological function of the vine's cells. A healthy vine will efficiently remove pigments leaving the dying leaf to become yellow, then brown, followed by the detachment of the petiole from the shoot and leaf drop. The red pigment in the leaf, anthocyanin, is usually the most difficult pigment to remove when the vine's phloem has been compromised by a virus. Exactly how leafroll affects the anthocyanin pathway through the vine's vascular system remains a mystery and is the subject of ongoing research. Curiously, a way of checking if a particular vine is infected with leafroll virus is to graft a bud from the suspect vine onto a healthy Cabernet franc vine. This vinifera cultivar is very sensitive to the disease, showing strong symptoms sometimes within as little as 18 months of grafting.

Because leafroll virus does not kill vines, but instead causes reductions in yield, maturity and quality, infected vines are rarely removed from otherwise healthy and productive vineyards. The only effective control of leafroll, or any other grape virus disease, is to ensure that healthy propagating material from virus-tested stock is used to plant vineyards.

Red coloured foliage, although the result of disease in a vine, really is very attractive. I, along with the valley's tourists, really enjoy the show.

My girdle is killing me.

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.