Black measles.

What is happening with this bunch of Cabernet Sauvignon grapes has me both excited, and sad. Excited because I have never seen this particular grapevine disease before.  And I must admit that it is extremely interesting to see it in person, rather than in the pages of a text book. However, it also makes me sad because it means this grapevine (located upvalley in a favourite vineyard) is infected with a disease that will seriously impact its longevity: Black Measles. Measles refers to the purple/black blemishes found on the fruit.  Look at the spots on those grapes.  Grab the calamine lotion, quick!
The cause of black measles, sometimes called Apoplexy (severe infections), or Esca (in France), is not fully understood.  But it is has been established that the disease is closely correlated with vascular fungal infections and pathogens that cause other vine-trunk diseases/rot.  All vines are prone to disease, but black measles starts to show symptoms in infected vines that are 10 years old, or older. Whilst this particular vine was not exhibiting foliar symptoms; i.e., interveinal chlorosis (in white grapes), or reddening (in black grapes) followed by necrosis, a nearby vine had a couple of shoots that were entirely dried up and had raisined fruit.  The economic impact due to crop loss could prove devastating to a grape grower.
There really is no treatment for a measles infected vine.  Applications of sodium arsenite, a recognised carcinogen, seemed to keep the spread of black measles in check, but its usage is no longer acceptable.  Now, with infections becoming more widespread in grape growing regions across the globe, researchers are studying possible causes, such as bad pruning practices, poor propagation of plant material (in grapevine nurseries) and plain, old-fashioned water stress.  Farming is hard.
Like I said, I have mixed feelings about witnessing this particular grape malady, literally, in the flesh.