http://www.ct.gov/caes/lib/caes/documents/publications/fact_sheets/plant_pathology_and_ecology/septoria_leaf_spot_of_tomato_06-30-08r.pdf
And here is a brief summary:
-- This is disease that primarily harms tomato plants
-- It thrives (and spreads) in wet/humid conditions
-- Remove affected leaves and dispose of them
-- Consider a fungicide if needed
