First Report of Ceratocystis manginecans Causing Udeuid (Manilkara udoido) Wilt in Palau

Udeuid (Manilkara udoido) is a culturally and ecologically important forest tree endemic to Babeldaob Island, Palau (Kitalong et al., 2014). Extensive mortality of mature udeuid trees was noted in late 2021. Dying trees had wilting and yellowing leaves, with main stems showing necrotic inner bark, black-stained xylem, and attacks by bark and ambrosia beetles. Losses exceeded 50% in several areas within the core range of udeuid. Because symptoms were suggestive of Ceratocystis wilt, samples of necrotic and stained inner bark and xylem, as well as beetle frass, were placed between fresh carrot discs and incubated at RT (Moller and DeVay, 1968). Ceratocystis-like perithecia formed after 7 days, individual ascospore drops were transferred to agar media, and colonies were hyphal-tipped. From January to July 2024, 23 isolates were obtained. Cultures on malt extract agar at 18⁰C formed black perithecia with globose bases, long necks, and sticky drops of hat-shaped ascospores. Chains of cylindrical conidia from tapering phialides, doliform conidia from wide-mouthed phialides, and darkly pigmented aleurioconidia were also produced. This morphology and DNA sequences of four gene regions were consistent for the expanded concept of C. manginecans, a Brazilian species that has been associated with major disease outbreaks in Asia and elsewhere, sometimes listed under the name of C. fimbriata (Harrington et al., 2024). Direct sequencing of ITS-rDNA PCR products resulted in overlapping electropherograms, suggesting intragenomic variation among rDNA repeats; partial sequences (GenBank nos. PV978653−PV978655; ITS types 6 and 7b) were typical for C. manginecans isolates from Oman, Pakistan, India, and other Asian countries (Harrington et al., 2014; 2024). All isolates had mating type gene sequences (MAT1-1-2, PV567692; MAT1-2-1, PV567694) of 100% identity with C. manginecans from Eucalyptus (KF482985 and OP921563, respectively) and various other hosts in Brazil, South Africa, India, and China (Harrington et al., 2024). Ceratoplatanin sequences (PV942056) were 100% identical with the CP1a sequence from Punica in India (OR654997) as well as Mangifera and other hosts in Pakistan (OR654996) and Oman (Harrington et al. 2024). In each of two trials, eight wild, udeuid seedlings (50 to 120 cm tall) in containers were inoculated with either isolate DS02-PIII or 5759, and three seedlings served as controls (Hughes et al., 2020). Whatman™ filter paper disks were soaked in a conidial suspension, incubated on potato dextrose agar plates for 3 days at 24⁰C, and inserted into a shallow incision into the xylem. Control seedlings received a sterile disk. A wet cotton ball was wrapped around the wound before sealing with Parafilm®. Plants were held in a covered nursery (28 ± 3⁰C, 80-90% RH) for 33 days in the Trial 1 (DS02-PII) and 37 days in Trial 2 (5759). In Trial 1, 75% of inoculated seedlings wilted or died, compared to 62% in Trial 2. Controls showed no symptoms. Lesions in inoculated seedlings ranged from 8.1 to 32.6 cm long vs. 0.4 to 3.2 cm in controls. The pathogen (with the MAT sequences of the originals) was recovered from all inoculated seedlings but none of the controls, thus fulfilling Koch’s postulates. This is the first report of C. manginecans on the genus Manilkara and its first detection in the Pacific Islands. Typhoon Surigae caused extensive damage to large udeuid trees in April 2021 and may have contributed to the outbreak by creating wounds for infection. Udeuid mortality has continued in Palau, and the pathogen poses a serious threat to native forests and cultivated crops in Oceania.