Grape Leaf Problem

Some of our vines have been displaying this type of problem on its leaves:

We are thinking that this is not a fungal disease or a disease that is caused by insects. We are leaning toward a disease that is caused by nutritional deficiencies. There is some information on the Internet, appropriately titled Grapevine Problems: Leaf Spots Not Caused by Insects or Disease. I also read another document on the Internet called Grapevine Growth Distortions: A Guide to Identifying Symptoms authored by V. Walton, P. Skinkis, A. Dreves, C. Kaiser, S. Renquist, S. Castagnoli, and R. Hilton and published by the Oregon State University Extension. This publication has a flowchart that might help in narrowing down what the cause of the leaf problem can be.
There are only a few plants that appear to have this problem and they seem to be located in the same area. I am going to take a count of these plants and their location and plot it to see if I can see a pattern. I'll post my plot later.