Septoria leaf blight on tomato

This fungal disease starts as distinct round lesions, often with a hint of purple to the round spot. Leaves turn yellow and eventually brown. Fruit is unaffected. Because all leaves can wither, this disease can kill plants unless new, uninfected tissue is protected with fungicides. 2009 has seen plenty of septoria, in particular because of the regular rains that encourage leaf wetness and the spread of fungal spores.