Neem oil needs a little soap with it to mix with water. I use Murphy's Oil
Soap. Castille soap and Seventh Generation dish soap have also been
recommended (
http://www.seventhgeneration.com/Dish-Soap ).
Daconil (chlorothalonil) is a preventative. If your plants already have
Early Blight or any other fungal infection, it won't go away after spraying
with Daconil, that will only slow down the spread of it to uninfected parts
of the plant.