the verb 'to be' in a sentence using plural and singular

I have a funny sentence that usually has people sat on one side or another but don't know which is correct. The sentence is e.g.

"Each of the fifteen surveys given in Appendix A are to be undertaken over a period of seven days."

Because we use 'each', suggesting we are taking in the singular, should the 'are' be 'is' or stay as it is? Or is the subject still plural?

[Tdol]

It would be better with 'all 15', but I'd use the singular with 'each'.