Sounds like a leaf blight that is brought on by a virus or bacteria combined with damp foliage and warm weather. Not much you can do about it now but try to grow potatoes in well drained soil with good air circulation. I would need to see a photo of the foliage to make a more accurate guess on this infection. Some potatoes are virus free and better to plant in warm humid areas. Your local Master Gardener association would know the best varieties for your area. Keep growing, Marianne Binetti