Well, maybe not the best but on my shelf it is giving Richard Smart's Sunlight Into Wine a run for the money. It's A Pocket Guide for Grape IPM Scouting in the North Central and Eastern U.S. It is clearly the best disease guide out there for our region. It would even be handy on the West Coast. The genius of this guide is its size...as the title implies you can slip it in your back pocket whenever you head out to the vineyard and you can diagnose ailments on the spot. Sometimes it's hard to tell your mildews or rots apart even if you are experienced.
This guide has good quality photos of multiple symptoms, a brief overview of each disease. It also covers insect injury, nutrient deficiencies and pesticide damage. There are other charts and images that come in handy, including photos detailing each stage of grapevine growth and berry development. I could have saved a few dollars in laboratory fees if I'd had this handy guide a month ago. I'm a big fan of the whole idea of a land grant university, and MSU's extension program is the original. I'm not surprised they're behind this clever little guide.