Symptoms of late blight on tomato leaves and stems are similar to those on potato. Irregularly shaped watersoaked lesions can be observed on young leaves at the top part of the plant. Under humid conditions, lesions become brown and pathogen sporulation can be seen. Eventually the leaves shrivel and become necrotic and die. Brown lesions can occur on stems and leaf pedicels. The pathogen can also infect tomato fruit and causes circular greasy lesions. The fruit remain firm but spots eventually become leathery and chocolate brown and can enlarge to cover the entire fruit.
Stem lesion (top left) and leaf lesion (top right) with sporulation on young tomato leaf and more necrotic lesion (bottom left) and tomato fruit lesion (bottom right) due to late blight. Leaf and stem images courtesy of Meg McGrath, Cornell University and tomato fruit image courtesy of Jean Ristaino, NC State University. View more images: NY.