Leaf infected with potato blight, coloured scanning electron micrograph (SEM). Lower surface of a leaf from a potato plant infected with late blight, showing stomata (circular), leaf hairs (long, pointed) and blight hyphae (lighter, thin strands). Late blight is caused by the water mould Phytophthora infestans. The spores of the mould lie dormant on tubers under the soil during the winter, but spread rapidly in warmer damp conditions. The infection ultimately leads to the collapse of the whole plant.