ramorum

Ramorum most commonly denotes Phytophthora ramorum, an oomycete plant pathogen in the genus Phytophthora. It is not a true fungus, but a microorganism in the same broad group that includes many plant pathogens.

Host range and disease manifestations include both forest and ornamental impacts. In forests, Phytophthora ramorum causes sudden oak death, characterized by bark cankers and bleeding on trunks and branches, leading to branch dieback and tree death, particularly in oaks and tanoak. In ornamental horticulture, it causes ramorum blight on susceptible shrubs, with leaf spots, twig dieback, and dieback of plants such as rhododendron, camellia, viburnum, and pieris.

Life cycle and spread: the organism reproduces asexually by sporangia that release motile zoospores in wet conditions; sexual reproduction can occur when both mating types A1 and A2 are present, producing oospores. It spreads through infected plant material, rain splash and wind-driven spores, and contaminated irrigation water. Long-distance movement is frequently associated with the distribution of nursery stock.

Distribution and impact: Phytophthora ramorum was first identified in the western United States and has since been reported in parts of Europe. It has caused extensive forest mortality in California and Oregon and significant damage to nursery stock and ornamentals. It is subject to regulatory controls in many jurisdictions to limit spread.

Management and detection: surveillance and diagnostic tests, including PCR-based assays, are used to detect infection. Control relies on removing infected plants, sanitation, and restricting movement of host material; chemical controls such as phosphonate fungicides may be used protectively in nurseries and on high-value plants, but there is no curative treatment for infected trees. See also Sudden Oak Death.