What causes gs to rot on the vine? Disease or improper growing procedures?

most likely a fungal disease. Black rot is a common disease affecting gs on the East Coast, but it doesn't occur in gs on the West Coast. (There, the problem may more likely be caused by downy mildew, powdery mildew or botrytis.) Generally warm and humid or rainy weather aggravates the onset of disease, and the moisture helps in spreading spores to other places within the plant or other plants.