Light micrograph of a fungus (Trichoderma asperellum, blue) attacking a Phytophthora ramorum spore (green circle). Phytophthora ramorum is a pathogenic oomycete (fungus-like organism) that causes the disease sudden oak death (SOD). Once trees are infected with SOD, patches of dead tissue called cankers appear on their bark. This is typically followed by leaves falling off, which can eventually kill the infected tree. Trichoderma asperellum fungus is being investigated as a means of biologically controlling the spread of Phytophthora ramorum, as it parasitises and kills the spores.