First report of Neofusicoccum parvum causing stem necrosis of 'ohi'a trees (Metrosideros polymorpha) on O'ahu, Hawai'i, USA

Authors: HUGHES, MARC - University Of Hawaii; HELLER, WADE - University Of Hawaii; FRIDAY, JB - University Of Hawaii; WEAVER, WILL - Koolau Mountains Watershed Partnership; Keith, Lisa

Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/24/2018
Publication Date: 10/23/2018
Citation: Hughes, M., Heller, W., Friday, J., Weaver, W., Keith, L.M. 2018. First report of Neofusicoccum parvum causing stem necrosis of 'ohi'a trees (Metrosideros polymorpha) on O'ahu, Hawai'i, USA. Plant Disease. 102(12):2660. https://doi.org/10.1094/PDIS-06-18-1002-PDN.
DOI: https://doi.org/10.1094/PDIS-06-18-1002-PDN

Interpretive Summary: ‘Ohi‘a is the most abundant and ecologically important native tree to the Hawaiian Islands. In November 2016 extensive ‘ohi‘a mortality was reported in the ‘Ewa Forest Reserve in the Ko‘olau Mountains of O‘ahu. The fungus causing extensive brown to black necrotic vascular discoloration was identified and its pathogenicity was proven. This is the first report of Neofusicoccum parvum causing stem necrosis on ‘ohi‘a in Hawai‘i.

Technical Abstract: ‘Ohi‘a lehua (Metrosideros polymorpha) is the most abundant and ecologically important native tree in the Hawaiian Islands (Friday and Herbert 2006). In November 2016 extensive ‘ohi‘a mortality was reported in the ‘Ewa Forest Reserve in the Ko‘olau Mountains of O‘ahu. A site visit revealed several (150+) ‘ohi‘a trees along the Poamoho Trail with branch dieback and defoliated crowns, often with epicormic sprouts emerging below dead branch terminals. Stem sections (> 2.5 cm dia.) containing the margin between healthy and necrotic xylem were collected from five symptomatic trees and sent to the USDA Agricultural Research Service in Hilo, HI for diagnosis. Woody stems were externally asymptomatic; however, when debarked, an extensive brown to black necrotic vascular discoloration was visible from the branch apex downward several centimeters on stems (Figure S1). Additional isolated lesions were found lower on the stems away from branch terminals. Sections of discolored xylem were surface disinfested in 3% sodium hypochlorite, rinsed in sterile water, placed on potato dextrose agar (PDA), and incubated at 25oC. Within four days, tissue from four of five trees yielded the same fungus containing white, aerial mycelium that later turned greyish-black (Figure S1). Isolates were obtained from two trees (P17-79 and P17-80). To identify the isolates, the internal transcribed spacer of the rDNA, partial ß-tubulin, and translation elongation factor 1-a gene regions were amplified by PCR and sequenced using primers (ITS1/ITS4), (Bt2a/Bt2b), and (EF1-728F/EF1-986R), respectively. Both fungal isolates were identical to each other and a nucleotide BLAST search revealed 100% similarity to several (10+) Neofusicocum parvum isolates for all loci examined. Sequences were deposited as GenBank accessions MF631021 and MF631022 (ITS), MF631023 and MF631024 (BT), and MF631025 and MF631026 (EF). Pathogenicity tests were conducted by inoculating 1-year-old ‘ohi‘a seedlings with PDA plugs containing seven-day-old mycelium into wounds made by a five-mm cork-borer and wrapped in Parafilm. Each N. parvum isolate was inoculated into five plants. Five negative control plants were inoculated with PDA plugs. Plants were kept in a greenhouse (21.8 ± 1.0 °C) with daily mist irrigation. After four months, inoculated stems were debarked, and extent of xylem necrosis measured. Necrosis in the xylem was identical to that observed in the field samples. This experiment was conducted twice. For both experiments combined, mean length of xylem lesions was significantly higher (P < 0.001) for N. parvum inoculated seedlings (P17-79 = 4.0 ± 0.5 cm, P17-80 = 3.7 ± 4.0 cm) vs. controls (mean = 0.6 ± 0.1 cm) (Figure S1). Fungi re-isolated from inoculated tissue were molecularly analyzed using ITS, confirming the fungi as N. parvum, thus completing Koch’s postulates. Control plants showed no symptoms. N. parvum is an endophyte or latent pathogen of woody hosts and has been reported in Hawai‘i on eucalyptus (Sakalidis et al. 2013). To our knowledge, this is the first report of N. parvum on ‘ohi‘a in Hawai‘i, and causing stem necrosis. We suspect N. parvum is acting secondarily as a stress-induced pathogen (Slippers and Wingfield 2007) and not a primary tree killer. Further investigation is needed to determine the interaction of N. parvum with other pathogens of ‘ohi‘a, including Austropuccinia psidii, which was detected from leaf samples of surrounding trees and could have been the cause of defoliation.