New <i> Ceratocystis </i> species associated with rapid death of <i> Metrosideros polymorpha</i> in Hawai`i

The native 'ōhi'a lehua (<i>Metrosideros polymorpha</i>) has cultural, biological and ecological significance to Hawai'i, but it is seriously threatened by a disease commonly referred to as rapid 'ōhi'a death (ROD). Preliminary investigations showed that a <i>Ceratocystis</i> species similar to <i>C. fimbriata</i> s.lat. was the cause of the disease. In this study, we used a combination of the phylogenetic, morphological and biological species concepts, as well as pathogenicity tests and microsatellite analyses, to characterise isolates collected from diseased 'ōhi'a trees across Hawai'i Island. Two distinct lineages, representing new species of <i>Ceratocystis</i>, were evident based on multigene phylogenetic analyses. These are described here as <i>C. lukuohia</i> and <i>C. huliohia. Ceratocystis lukuohia</i> forms part of the Latin American clade (LAC) and was most closely associated with isolates from <i>Syngonium</i> and <i>Xanthosoma</i> from the Caribbean and elsewhere, including Hawai'i, and <i>C. platani</i>, which is native to eastern USA. <i>Ceratocystis huliohia</i> resides in the Asian-Australian clade (AAC) and is most closely related to <i>C. uchidae</i>, <i>C. changhui</i> and <i>C. cercfabiensis</i>, which are thought to be native to Asia. Morphology and interfertility tests support the delineation of these two new species and pathogenicity tests show that both species are aggressive pathogens on seedlings of <i>M. polymorpha</i>. Characterisation of isolates using microsatellite markers suggest that both species are clonal and likely represent recently-introduced strains. Intensive research is underway to develop rapid screening protocols for early detection of the pathogens and management strategies in an attempt to prevent the spread of the pathogens to the other islands of Hawai'i, which are currently disease free.