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It's hard to say why they areIt's hard to say why they are dying. Do you have any frass? It's possible that it's the squash vine borer pest, especially if the soil's been too wet. Take a knife and slit the stem lengthwise and look for those grubs. If they're there, you need to quickly pull those plants as it is too late for treatment. In the future, row covers may help.

2015 Special Edition Garden GuideCooking Fresh with The Old Farmer's AlmanacThe Almanac Monthly Digital MagazineWhat the heck is a Garden Hod?