Organic products that are labeled for late blight on tomato include Serenade®, Sonata®, Oxidate®, Sporan®, Sporatec®, and copper products. These products work preventatively; if late blight has been detected in your area, DON'T WAIT UNTIL YOU SEE SYMPTOMS TO SPRAY.
above from the following link...
http://www.ces.ncsu.edu/chatham/ag/SustAg/lateblight.html