In 1845, the late blight of potato destroyed the potato crop of Ireland was caused by

In 1845, the late blight of potato destroyed the potato crop of Ireland was caused by Phytophthora infestans.
Late blight, also called potato blight, disease of potato and tomato plants that is caused by the water mold Phytophthora infestans. The disease occurs in humid regions with temperatures ranging between 4 and 29°C.
The infection is caused by the zoospores found in the soil or that fall onto the tubers from infected foliage during harvest. Following germination, the zoospores penetrate the tubers through the "eyes", lenticels, growth cracks, wounds, or via the point of attachment to the plant.