Some of my tomato plant leaves have turned dark brown and have rings on them. Is this blight?
Yes, you are no doubt suffering from early blight, which comes on in humid weather. It’s a fungus that can last the winter on diseased plants. After you harvest the tomatoes (assuming you get fruits on these vines), destroy or discard all the infected plants. Meanwhile, you can spray them with a fungicide that won’t cure the disease but will protect any new foliage. Ask at your nursery for the fungicide that will work best in your area.