Uchida, Janice Y., Robert C. Anderson, Chris Y. Kadooka, Anne Marie LaRosa, and Cheresa Coles. 2008. Disease index for the rust Puccinia psidii on ohia (Metrosideros polymorpha) in Hawaii. CTAHR Plant Disease publication PD-38, Feb. 2008. College of Tropical Agriculture and Human Resources (CTAHR). Honolulu. 16 pp. illus.
A rust disease first found on potted ohia (Metrosideros polymorpha) on Oahu in 2005 has become a threat to Hawaii's forests. This plant composes as much as 80 percent of some of the remaining native forests, and its genetic diversity is highly valuable to these ecosystems. ohia is one of the first plants to colonize new lava flows and is adapted to both wet and dry environments from sea level to high elevations. As a keystone species in the forest, ohia provides habitat for numerous populations of native flora and fauna, including some endangered species, and helps maintain the hydrological cycle; without it, the entire ecosystem is at risk. The rust was identified as Puccinia psidii, which is known to have a wide host range within the Myrtaceae. Crucial to investigations of the rust is determining its distribution throughout the state. By 2006, it had spread to Hawaii, Maui, and Kauai. By 2007, it was also found on Länai and Molokai. However, the actual distribution on each island remains unknown. Many of the people who frequent Hawaii's forests could help map the distribution of this rust on ohia and its other hosts. In addition to identifying the rust and noting its location, it is also valuable to determine the severity of the disease. Because some other diseases and growths resemble the rust, complicating its identification, this document was prepared to illustrate the rust symptoms and their varying degrees of severity on ohia and to compare them with symptoms caused by other pathogens. This disease index thus aids a statewide program to monitor and document the incidence and severity of the rust Puccinia psidii on ohia. A companion index illustrates symptoms of P. psidii on another member of the Myrtaceae, rose apple (Syzygium jambos), a naturalized tree that is highly susceptible to the rust and serves as a reservoir of the pathogen. A third related index illustrates symptoms of diseases on other plants caused by algae and algae-like organisms that may be mistaken for P. psidii rust. All three publications will be issued in 2008 and posted at www.ctahr.hawaii.edu/freepubs under the category Plant Disease.