My zucchini plants produced beautifully for the first part of the season,...

Hi,

Your description of problem is a bit too general for a single diagnosis. The following links may be helpful for matching to your specific symptoms:

The Penn State Extension and the University of Minnesota Extension have overviews that may be of help:
http://extension.psu.edu/plants/vegetable-fruit/news/2015/why-are-my-cucurbits-wilting
http://www.extension.umn.edu/garden/diagnose/plant/vegetable/summersquash/leaveswilting.html

More information on bacterial wilt: http://u.osu.edu/vegetablediseasefacts/cucurbit-diseases/bacterial-wilt/

On squash vine borer: http://articles.extension.org/pages/65684/biology-and-management-of-squash-vine-borer-in-organic-farming-systems

Once a specific diagnosis is made, control measures may be recommended.

I hope this is helpful.