Ceratocystis lukuohia-infested ambrosia beetle frass as inoculum for Ceratocystis wilt of ʻōhiʻa (Metrosideros polymorpha)

Abstract

Metrosideros polymorpha (‘ōhi‘a) trees in Hawaiʻi are dying from two distinct diseases, collectively referred to as rapid ‘ōhi‘a death (ROD), caused by Ceratocystis lukuohia and Ceratocystis huliohia. Boring dust (frass) released when ambrosia beetles attack and colonize infected trees has been suspected as a transmission source. We sampled ambrosia beetle frass from six locations on Hawaiʻi Island and screened samples for Ceratocystis DNA and fungal viability. Ceratocystis DNA was detected in 79% of frass samples and 61% were viable. To assess the infectivity of C. lukuohia-colonized frass, M. polymorpha seedlings were wound-inoculated with frass in growth chamber trials. Wilt incidence was 40% in the first trial and 15% in the second. Frass particles naturally infested with C. lukuohia were treated with a cytoplasmic stain and microscopically examined; thick-walled chlamydospores were found in all samples. The chlamydospores appeared to be derived from aleurioconidia. Ceratocystis survival in frass was assessed in a baiting experiment conducted under varying environmental regimes; viability decreased with increasing temperature, frass age and decreasing humidity. After 6 months, 90% of the samples exhibited viable C. lukuohia at the lowest temperatures. Results confirmed that C. lukuohia chlamydospores are the fungal inoculum within ambrosia beetle frass and can induce wilt when introduced to M. polymorpha wounds. Although ambrosia beetles or other insects may serve as vectors, the present work supports the hypothesis that Ceratocystis species may be dispersed in frass, which can inform ongoing efforts to manage ROD across Hawaiʻi's native forests.