First Report of Fusarium tricinctum Causing a Stem Browning Disease on Asparagus kiusianus in Japan

Le résumé fourni par la source

Asparagus kiusianus, a rare and endangered wild species endemic to Northern Kyushu, Japan (Ito et al. 2011), was observed from 2022 to 2024 along the Nijinomatsubara coast (33°26′45.24″N, 130°00′10.44″E) in Saga Prefecture to exhibit irregular hazel-brown lesions on its stems (Figure 1). The disease incidence reached 30%, which poses a significant threat to the species. Diseased stem segments (1 cm) were surface-sterilised in 70% ethanol for 30 s, immersed in 5% sodium hypochlorite for 30 s, rinsed thrice with sterile water, and placed on potato dextrose agar (PDA) at 25°C for 7 days. Six representative isolates were obtained and labelled as: AkW22201, AkW22204, AkW22301, AkW22303, AkW22309 and AkW24110. These isolates exhibited diverse colony colours (yellow to pink) and dense, furry textures. Their microconidia were oval, aseptate, measuring 6.5–12.3 × 2.8–4.4 µm (n = 20), while macroconidia were falcate, with 2–4 septa, measuring 25.5–32.1 × 3.5–4.6 µm (n = 20) (Figures 2 and 3). All isolates were identified as Fusarium species. Fungal DNA was extracted using a DNeasy Plant Mini Kit (Qiagen, Germany). PCR amplification was performed targeting the internal transcribed spacer (ITS), translation elongation factor 1-alpha (TEF1) and beta-tubulin (TUB2) regions using primers ITS1/ITS4 (White et al. 1990), EF1/EF2 (Geiser et al. 2004) and Bt2a/Bt2b (Glass and Donaldson 1995), respectively. The resulting sequences were deposited in GenBank: ITS, LC868011–LC868016; TEF1, LC868018–LC868023; TUB2, LC868025–LC868030. Species identification was performed using BLASTn analysis of ITS, TEF1 and TUB2 sequences against the GenBank database. All six isolates showed 98%–100% identity and >95% query coverage with Fusarium tricinctum sequences, matching the ex-type strain ATCC 38183 (ITS, AB587028; TEF1, AB674264; TUB2, AB587077). A summary of the results is provided in Table 1. Pathogenicity tests were conducted on seven healthy A. kiusianus plants; one plant was inoculated with each of the six Fusarium isolates, and one served as an uninoculated control. Stems were washed with sterile water, and a conidial suspension (1 × 106 conidia/mL) from 14-day-old cultures was applied using a sterile brush, while the control received sterile water. Inoculated areas were wrapped with plastic film to maintain moisture. All plants were incubated at 25°C in a greenhouse, with the experiment repeated twice. After 10 days, hazel-brown lesions resembling field symptoms appeared on all inoculated stems, whereas the control remained symptom-free. Fusarium tricinctum was successfully reisolated from the inoculated plants, thereby fulfilling Koch's postulates. The symptoms induced by the six F. tricinctum isolates were indistinguishable. This study is the first to report Fusarium tricinctum as the causal agent of stem browning on Asparagus kiusianus in Japan. F. tricinctum has also been reported as a pathogen of A. officinalis (Enow et al. 2025), a commercially important crop cultivated in the Kyushu area, where A. kiusianus populations are also distributed. Since A. kiusianus is capable of hybridizing with A. officinalis (Iwato et al. 2014), the findings of this study are important not only for the conservation of this endangered wild species but also for preventing disease transmission in cultivated asparagus. This work was supported by JST SPRING, Japan Grant Number JPMJSP2136.

Ce résumé expose les affirmations des auteurs. BNTIC ne l’interprète pas comme une validation indépendante des résultats.