If your area had particularly wet weather in spring and early summer, it's possible you have a fungal disease attacking your plants. It could be gray mold (Botrytis) or blight (Phytophthora); the symptoms and treatment for the two are similar. Remove the infected plants and all plant debris on the ground. Exposure to cold winter weather will kill any lingering pathogens in the soil, especially if you can turn it over a couple of times between freezes. If you can brew or buy compost tea, flush the soil with it. Compost tea is loaded with beneficial organisms that can clean up pathogens living in the soil. You could also flush the soil with a fungicide if you wish; but don't use it in conjunction with the compost tea as it'd probably kill the beneficial organisms as well as the pathogens.