The characteristic pattern that develops on the leaves of affected vines.
In the parcels of Chenin Blanc we looked at I would say there is currently one or two vines in every row with the disease. I was surprised to learn that they are just left there until 90 - 95% of vines are affected. They can't be treated as there is no known cure (apart from sodium arsenite which is banned because it is so toxic to everything, not just fungal diseases...). Once the whole parcel is diseased the vines are grubbed out, burned, and fresh certified disease free stock planted. The vines here are being subjected to an outlandish sounding alternative treatment involving sound patterns and the affected plants are counted on an annual basis to see at what rate the disease is spreading.
A diseased vine.
For my previous post about esca, which goes into detail about the effect the disease has on the vines, see here.