Zucchini Pest

I went out to the zucchini yesterday and found that there were several plants that were cut in half.  The damage almost looked like vine borer, but there was no sign of the borer or the tunnels through the plants.  Some have suggested that cut worm is a problem, but usually we've seen that on the young plants but not the old.  The plants are otherwise very healthy and vigorous looking. I'm stumped. It's always something.  Just as production is hitting its stride and zucchini is going to the farmers co-op a couple times a week, bang! I get a crop issue.  Anybody know what this might be?