First report of bacterial soft rot on Epipremnum aureum caused by Pectobacterium aroidearum in Taiwan.

ABSTRACT

Pothos (Epipremnum aureum) is an Araceae foliage plant with great ornamental values, which has long been enjoyed by consumers (Chen et al. 2010). In September 2021, pothos showing soft rot symptoms were found in 2 nurseries in Taichung, Taiwan. The petioles of the infected plants were macerated; some lesions extended to the leaves (Figure S1). The disease incidence was 50% in one nursery and 37.5% in the other; two and three plants were respectively collected from the two sites. Macerated tissues were homogenized in 10 mM MgCl2 and the samples were observed microscopically without dyeing. Motile, rod-shaped bacteria were observed in the samples, and the bacteria were isolated onto nutrient agar (NA) and grown at 28°C for 2 days. Fast-growing, round, creamy colonies were isolated from all 5 plants. One strain was isolated from each plant and the strains were named Ea1 to Ea5. The bacteria could ferment glucose and induce maceration on potato tuber slices (Schaad et al. 2001), but did not produce indigoidine on NGM medium (Lee and Yu 2006) and were tested negative for phosphatase activity (Schaad et al. 2001). The bacteria's DNA samples were tested using primers specific to Pectobacterium (Y1/Y2; Darrasse et al. 1994). The expected 434-bp amplicon was amplified in all five strains. Multilocus sequence analysis was conducted as previously described (Portier et al. 2019). A concatenated sequence (1,592 bp) comprising partial dnaX (492 bp), leuS (452 bp) and recA (648 bp) sequences was obtained for each strain. Two genotypes were detected among the strains; Ea1 and Ea2 belonged to one genotype (i.e., they had identical sequences), while Ea3, Ea4 and Ea5 belonged to the other (GenBank accession nos. OK416015-OK416020). Phylogenetic analysis was conducted using these data and those of representative strains of known Pectobacterium species (Klair et al. 2022). A maximum-likelihood tree showed that Ea1 to Ea5 clustered with P. aroidearum CFBP8168T (Figure S2). Sequence comparison (Table S1) showed that the similarity between the two genotypes' concatenated sequences was 99.1% (Ea1 vs. Ea3; 1,578/1,592 bp); Ea1 and Ea3 shared 99.2% and 99.3% sequence similarity with P. aroidearum CFBP8168T, respectively. The sequences obtained in this work were searched against GenBank and all of their top hits were those of strains belonging to P. aroidearum (supplementary information). Koch's Postulates were fulfilled by stab inoculating cutting-propagated pothos (8-cm tall) using toothpicks carrying bacteria grown on NA. The pathogen loads used were estimated by suspending cells (attached to individual toothpicks) in 10 mM MgCl2 and spread-plating them onto NA (after dilution); the loads were 5.5 x 106 - 2.2 x 107 CFU. Three plants were inoculated for each strain (3 petioles per plant). Control plants were stabbed with sterile toothpicks. Each plant was then bagged and placed in a growth chamber (28°C; 14 h light). After 24 h, all inoculated plants produced symptoms resembling those found in the nurseries, and the controls did not. For every treatment group, a strain was re-isolated onto NA; each of them shared the same recA sequence with the original strain inoculated. This is first report of P. aroidearum causing pothos soft rot in Taiwan. Local nurseries often grow pothos and other Araceae plants together in humid areas. Since other Araceae species are also known to be susceptible to P. aroidearum (Xu et al. 2020), growers should be cautious of the pathogen's spread across hosts.