Is that squash or zucchini? You have a squash vine borer at work - they take out plants fast.
I have been fighting with some all summer. I take a sharp short knife and a ketchup bottle (the restaurant type with a pointy nozzle) with water, a teaspoon or so of Bt (Bacillus thuringiensis) like Thuricide and a squirt of dish soap. I first squirt the liquid into the granular area that shows along the stem where the caterpillar has pulverized it, and then I carefully move up the stem a few nodes (past the leaf connections) and slice into it a little lengthwise and stick the point of the bottle through the slice and give the bottle a firm squeeze to push the Bt liquid up the hollow stem. If it reaches the caterpillar it will stop eating and then die. But I've had to do this several times this summer.
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The advice at the site where I got the photo is rather passive - I disturbed the soil incredibly before planting and it didn't keep them away. You just have to be diligent looking for these things, and I use a Bt drench (adding it to some Garrett Juice) and pour it over the base and stems of the plants when they're young trying to knock off the moths and worms before they get started. You can repel some pests, I am told, by grinding up bay leaves and sprinkling it around the plant. I've done that in the past but I forgot this year (and I have a bay tree in the yard!)