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

Authors: HUGHES, MARC - University Of Hawaii; ROY, KYLLE - Us Geological Survey (USGS); HARRINGTON, THOMAS - Iowa State University; BRILL, EVA - University Of Hawaii; Keith, Lisa

Submitted to: Plant Pathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/19/2022
Publication Date: 9/29/2022
Citation: Hughes, M.A., Roy, K., Harrington, T.C., Brill, E., Keith, L.M. 2022. Ceratocystis lukuohia-infested ambrosia beetle frass as inoculum for Ceratocystis wilt of 'ohi'a (Metrosideros polymorpha). Plant Pathology. 72(2):232-245. https://doi.org/10.1111/ppa.13653.
DOI: https://doi.org/10.1111/ppa.13653

Interpretive Summary: Metrosideros polymorpha in forests of Hawai'i Island and Kaua'i have experienced alarming levels of mortality due to Rapid 'Ohi'a Death, caused by the fungal pathogens Ceratocystis lukuohia and C. huliohia. The dissemination and infection processes of C. lukuohia and C. huliohia are not fully understood. However, similar to Ceratocystis spp. of other woody hosts, stem and trunk wounds are the most important infection courts for ingress into trees. Ceratocystis lukuohia and C. huliohia was recovered in the frass of ROD-infected M. polymorpha trees and due to the abundance of material released from heavily attacked trees in Hawai'i after infection, we hypothesize that this wood powder could serve as inoculum and a vessel for dispersion. Results of the island-wide survey indicate that frass collected from ROD-killed trees was frequently infested with Ceratocystis propagules. Results also suggest that the ambrosia beetle frass liberated from diseased 'ohi'a trees is serving as inoculum in the ROD pathosystem.

Technical Abstract: Metrosideros polymorpha ('ohi'a lehua) trees in Hawai'i are dying to a complex of two diseases called Rapid 'Ohi'a Death, caused by the Ceratocystis lukuohia and C. huliohia. How these pathogens move across the landscape is poorly understood; however, ambrosia beetles and the Ceratocystis-laden frass (boring dust) they release when tunnelling into diseased trees are suspected in transmission. Frass was surveyed from 118 diseased trees from six locations on Hawai'i Island for Ceratocystis DNA by qPCR and viability by carrot baiting and resulted in detection in 79% and 61% of the samples, respectively. Ceratocystis lukuohia and C. huliohia DNA was detected in 48% and 25% of frass, respectively, while 6% was co-infected. To assess pathogenicity, frass was collected from 16 C. lukuohia-infected trees and used to wound-inoculate M. polymorpha seedlings in growth chamber trials. Wilt incidence was 40% in the first trial and 15% in the second, and infection was variable within and between frass sources. Microscopy was used to visualize field-collected frass and fungal chlamydospores were found adhered to wood vessels in varying amounts. Fungal persistance in frass was assessed by a time series baiting experiment in three environmental regimes. Viability was negatively affected by increased temperatures, decreased relative humidity and time since initial collection, yet remained 90% viable at the coolest location until the conclusion of the 26-week study. These results suggest that Ceratocystis is readily found in frass from across the state and C. lukuohia can be pathogenic inoculum to M. polymorpha if introduced into exposed wounds.