A UD fact sheet, updated in February of 2017 gives timely info on leaf spot diseases of garden tomatoes. Fungal leaf diseases such as Septoria, bacterial leaf spot, and early blight can cause loss of leaves. 
Some leaf diseases such as anthracnose can lead to problems on ripening fruit. Click link below for fact sheet:
Tomato-Leaf-Spots-Fact-Sheet-Feb2017
