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.

Pathogenicity and colonization of Metrosideros polymorpha by Ceratocystis huliohia

“…The presence of C. huliohia and/or C. lukuohia in the frass of five ambrosia beetle species (Subtribe Xyleborina) has been documented (Roy et al., 2020) and the pathogens have been detected in airborne frass particles (Atkinson et al., 2019; Heller et al., 2023). Frass collected from the outer bark of C. lukuohia ‐killed trees was found to be pathogenic to ʻōhiʻa seedlings when introduced to stem wounds in growth chamber studies (Hughes et al., 2023). However, similar trials with C. huliohia ‐infested frass have not been conducted.…”

Ambrosia beetles (Coleoptera: Curculionidae) can directly transmit the fungal pathogens responsible for Rapid ʻŌhiʻa Death

“…The dispersal of ROD Ceratocystis inoculum in the form of contaminated ambrosia beetle frass and the subsequent landing of frass on wounds caused by extreme weather events, ungulates, and humans is the leading hypothesis for ROD pathogen transmission among ROD scientists (Cannon et al, 2022; Hughes et al, 2022; Perroy et al, 2021). However, the most likely method of spread in any given environment may vary depending on abiotic and biotic factors such as rainfall, temperature, and elevation, which can all affect beetle populations and fungal propagation (Fortini et al, 2019; Hughes et al, 2022; Roy et al, 2020). We demonstrated the increased attraction of ambrosia beetles to wounded ʻōhiʻa trees, highlighting the increased potential for direct transmission to stressed trees.…”

“…However, like other Ceratocystis fungal tree diseases, both C. lukuohia and C. huliohia can be transported in ambrosia beetle (Coleoptera: Curculionidae: Scolytinae) frass (i.e., wood particles, faeces, and beetle parts) (Atkinson et al, 2019;Barnes et al, 2018;Roy et al, 2019Roy et al, , 2020 and enter trees via open wounds created by mechanical means (i.e., feral ungulates, humans, extreme weather events) (Cannon et al, 2022). Frass particles contain long-lived resting chlamydospores (Hughes et al, 2022) and can spread in the wind (Atkinson et al, 2019;Roy et al, 2021), soil (via humans and ungulates; Yelenik et al, 2020), and potentially water (Harrington, 2013).…”

Unveiling Ceratocystis wilt disease: a review of cocoa’s unforgiving foe

“…Prior to wilting, no signs of hyphae and conidia in inoculated susceptible and resistant cocoa seedling, thus, reproductive structures of the fungal pathogen in planting materials like stem and trunk can only be noticed at the late stage of the infection process [8]. [41], and contaminated cutting tools [8,42]. The entry of infection for Ceratocystis spp is mainly limited to the xylem during the disease cycle, and some species break down the walls of the xylem vessels to spread nearby parenchyma cells, and often secrets structures of microsclerotia, chlamydospores, thick-walled mycelium and spore-bearing coremia in soil or dead plant tissues, which serve as their resting and overwintering stages [43].…”