SE Minnesota. I have grown grapes on my site over 20 years. This is the first time I lost two vines over the winter, and had what I would call severe die back on several others. I have noticed lighter color to leaves on many vines of my 16. This does not seem to be variety specific. I have Concord, Niagra, and Catabwa. The vines are mulched with shredded oak bark, which has been in place and renewed many years. Perhaps I overpruned? REPLY WELCOME