Late Blight threatens Florida tomatoes, potatoes As if citrus greening disease and citrus canker weren't enough, now Late Blight, a highly contagious disease, is showing up on tomato and potato plants in Southwest Florida, NBC-TV affiliate WBBH has reported.   Researchers are concerned that the disease, which threatens local potato and tomato industries, might spread and devastate entire fields within weeks, the TV station reported.   "This disease has the potential to take out an entire field of tomatoes, and we have seen that," said plant pathologist Pamela Roberts. "It kills back the foliage. It turns a little bit yellow, and then almost immediately it turns brown and dies."   State scientists who have examined samples from farms in Collier and Hendry counties are convinced that the crops have Late Blight, but they are not sure what strain.   "About three years ago, there was a highly aggressive strain that showed up, and it was very difficult to control," Roberts said.   Although fungicides can fight the problem, some strains are becoming resistant.   Late Blight was the cause of the great potato famine in Ireland, the story said.