At this point, late blight has been confirmed on tomato plants in CT, PA, Long Island, and on potato in MI. Hopefully we’ll have lots of hot dry weather which limits spread of the disease but better to prepare for the odds that it will be here soon. We have had several false late blight alarms because other diseases, especially in the greenhouse, can look similar, such as gray mold and even leaf mold, see: http://www.extension.org/article/18337. There are excellent images and resources on late blight at Meg McGrath’s Cornell site: http://www.longislandhort.cornell.edu/vegpath/photos/lateblight_tomato.htm.
If you are in doubt send sample for positive ID to: Ann Hazelrigg, UVM Plant Diagnostic Clinic, 201 Jeffords Building, 63 Carrigan Drive, UVM, Burlington VT 05405. (802) 656-0493.
Your tomato and potato crops should be thoroughly and frequently scouted. Organic growers can apply copper hydroxide as a protectant (NuCop 50WP or ChampWG). Chlorothalonil is a conventional protectant fungicide; mobile fungicides for late blight are: Previcur Flex (FRAC Group 28), Ranman (Group 21), Revus Top, Revus, and Forum (Group 40), Curzate and Tanos (Group 27), Gavel (Group 22), Presidio (Group 43; tomatoes only), and Omega (Group 29; potatoes only). These fungicides need to be used in alternation to manage resistance.