Potato blight fungus sporangium. Coloured scanning electron micrograph (SEM) of a sporangium (spore-producing structure) of the potato blight fungus (Phytophthora infestans). The sporangia form on the mycelium of the mould. They contain infectious zoospores that are released when mature. These swim in water films on potato plant leaves infecting new plants and potato tubers alike, causing the tissues to decay. This mould was responsible for the great Irish Potato famine of the mid-19th century.