It is a sad day, both zucchini plants are sick and most likely dying. Both have been infected by Squash vine borer. While they are both under closer observation in the garden intensive care unit, their prognosis is not good.
Unfortunately, this is not the first year for this problem and unless I get better at controlling these pests in the future I am afraid it won’t be the last. Based on this information, “Often the first noticed sign of the problem is wilting of the vine, and it is too late for control efforts at that point” it seems as I should not have much hope for any more zucchini this year.
I’ve tried removing the dead and wilting stems and extracting the borers I could find. My fear is that for the three or four I was able to find, there are two to ten that I was unable to find.