Irish Blight/Potato Blight of potatoes and tomatoes is caused by the fungus-like oomycete pathogen Phytophthora infestans. It can infect and destroy the leaves, stems, fruits, and tubers of potato and tomato plants. Severe late blight epidemics occur when P. infestans grows and reproduces rapidly on the host crop. Reproduction occurs via spores that are produced from infected plant tissues and is most rapid during conditions of high moisture and moderate temperatures. Spores disperse to healthy tissues via rain splash or on wind currents.