Vacuum steam treatment of Metrosideros polymorpha logs for eradication of Ceratocystis huliohia and C. lukuohia

Abstract

A new and devastating disease, rapid ohia death (ROD), in Hawaii led to a state quarantine that regulates inter-island transport of ohia wood and plant material to prevent spread of the causal pathogens. Heat treatments of ohia logs in commercial trade were considered for phytosanitary treatment. Vacuum steam (VS) was evaluated for its ability to eradicate the pathogens, Ceratocystis lukuohia and/or C. huliohia, in main stem logs from ROD-affected forest trees. Replicate loads of three de-barked logs (24 to 43 cm diameter; 1.7 to 2.0 m long) were VS-treated at 56° C for 30 min (5 loads) or 60° C for 60 min (4 loads) at a sapwood depth equal to 70% of log radius. Percent isolation of Ceratocystis from VS and ambient temperature logs prior to treatment and summarized by source tree ranged from 12 to 66% and 6 to 31% based on carrot baiting assays of tissue taken from outer and inner sapwood, respectively. No viable Ceratocystis was detected in either sapwood locations for the 60° C/ 60 min schedule or for the inner locations for the 56° C/ 30 min schedule following treatment. Only one subsample (0.48%, n=208) of the latter schedule treatment yielded Ceratocystis. Time required for treatment ranged from 7.4 to 15 h for the 56° C/ 30 min schedule and from 8.6 to 19.2 h for the 60° C/ 60 min schedule. These results demonstrate VS is an effective and efficient method for treating large diameter ohia logs that mill owners and regulatory plant pathologists may consider for use in Hawaii.