A plant disease is a dynamic process where a living or nonliving entity interferes with the normal functions of a plant over a period of time. Things that happen just once, like lawnmower blight or lightning strikes, are not considered diseases, but rather injuries. Plant diseases result in visible symptoms that can help diagnose the disease or disorder. Disease epidemics can also threaten entire plant species. Exotic pests can threaten the continued existence of a species. Historical examples of destructive plant disease epidemics include three American tragedies: American chestnut blight, Dutch elm disease and butternut canker. Along with these three diseases some other diseases like palm lethal yellowing, oak wilt, cypress canker and Xylella outbreak caused severe epidemic.