Posted: 08/05/2013 at 10:23
Nicholas, I'd like a closer look but those darker "bullseyes" inside the necrotic (lighter) patches look like the early stages of a fungal problem.
"Blight" has become a generic term for any fungal problem. The most common fungal problems in the home garden are Early Blight and Septoria Leaf Spot. Neither will kill a tomato plant providing basic care is taken - nipping off the worst-affected leaves, etc. Without denuding the plant of foliage, obviously.
Unless you want to spray against fungal problems, you have to be rigorous with your preventive measures. Never ever water the foliage, keep sufficient distance - at least 3' - between plants to allow plenty of air circulation, and nip off excess branches and foliage (even healthy stuff) to avoid great clumps of foliage that work against air circulation through the foliage.
Clear air circulation is the bottom line. Still air, particularly if it's humid, and clumps of impenetrable foliage, are heaven for fungal spores. That's why I'd get them out from under the cloche.