Answer: Unfortunately you may have squash vine borers. These inch long white caterpillars bore into the stem of winter squash such as pumpkins. Unless you are watching for them, they can go undetected until the vines wilt. Watch for the entry holes at the base of the plant, and for frass, the yellow, sawdust like droppings of the borer. The best remedy is to carefully cut into the stem to remove the caterpillar inside. Then hill up soil around the cut stem to encourage new root growth.
Another cause of vine wilt is a disease carried by the striped and spotted cucumber beetles. These beetles have green or yellow wings and spread a fungal disease called bacterial wilt. Once infected the plant wilts and dies. To protect the plants, use floating row covers before they blossom (remove when blooms form to allow for pollination, or dust with pyrethrins. Choose cultivars resistant to common squash diseases. Rotate your crops. Provide for good air circulation. And be sure to clean up the garden in the fall so there is less possibility of disease and insects wintering over.
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