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Pathogenicity and host specialization of Ceratocystis spp. associated with rapid ˋōhiˋa death (ROD) in Hawaiˋi

Lisa Keith: USDA-ARS

<div>Two new species of <em>Ceratocystis</em> are attacking and killing ˋōhiˋa (<em>Metrosideros</em> <em>polymorpha</em>), Hawaiˋi’s most abundant and ecologically important native tree. On Hawaiˋi Island, hundreds of thousands of trees have died from these pathogens, a phenomenon known as Rapid ˋŌhiˋa Death (ROD). <span>We report here a study to determine the host specialization of these two pathogens. Hosts evaluated included <em>M. polymorpha, Syngonium podophyllum, Platanus</em> × <em>acerifolia</em> (London plane)<em>, Colocasia esculenta </em>(taro), <em>Coffea</em> <em>arabica</em> (coffee) and <em>Ipomoea</em> <em>batatas</em> (sweet potato). Pathogenicity tests were conducted using stem injection or a stem flap wound method with inoculum prepared from 7 day-old cultures. Most </span><em>M</em>. <em>polymorpha</em> plants inoculated with ˋōhiˋa isolates wilted 14 to 28 days post-inoculation (DPI) and died 28 to 365 DPI. In contrast, disease was not observed on <em>Syngonium</em>, London plane, taro, coffee or sweet potato plants inoculated with ˋōhiˋa isolates<em>. </em>Our results indicate that the two <em>Ceratocystis</em> spp. from ˋōhiˋa specifically cause disease on their respective host. However, it is not yet known if other plant species may serve as asymptomatic carrier hosts or if there is potential for the pathogens to develop new host specificities by hybridizing with other strains. In order to protect <em>Metrosideros</em> forests state-wide and throughout the Pacific, expanded investigation of the host range, genetic diversity and distribution of <em>Ceratocystis</em> species causing ROD is critical.</div>