First report of Dickeya dadantii causing bacterial soft rot of Scindapsus pictus in Taiwan.

Abstract

Scindapsus pictus (satin pothos or silver vine) is an evergreen climbing plant belonging to the Araceae family, subfamily Monstereae (Bown, 2000), which is also cultivated as a foliage ornamental (Masnira et al. 2019). In September of 2022, soft rot symptoms were observed on potted S. pictus plants grown in a greenhouse in Nantun District, Taichung, Taiwan, in which soft rot of another aroid (philodendron) has also been reported (Wu et al. 2023). The symptoms appeared on the petioles and most of them tended to extend to the leaf blades; the colors of leaf lesions ranged from dark brown to gray (Fig. S1). Some 70% of the plants in the greenhouse showed similar symptoms and losses were estimated to be 15-30%. Four symptomatic plants were sampled. Macerated tissues from rotting petioles were soaked in 10 mM MgCl2 and observed under a light microscope (Nikon, Japan) at 400 x magnification. Motile, rod-shaped bacteria were observed, and 1-2 loopfuls of undiluted sample suspension were streaked onto nutrient agar (NA; Gibco, USA). After culturing at 28°C for 1 day, all samples yielded round, creamy-white colonies (0.9 mm in diameter) and from each of the four samples a pure culture was obtained (Spi1-Spi4). All isolates exhibited oxidative and fermentative metabolism of glucose (Schaad et al. 2001). They caused pitting on crystal violet pectate agar, induced maceration on potato tuber and were tested positive for phosphatase activity and indigoidine production (Lee and Yu 2006; Schaad et al. 2001). Polymerase chain reaction tests using Dickeya-specific primers 5A and 5B (Chao et al. 2006) amplified the expected amplicon (0.5 kb) in extracted DNA samples of all isolates. Identification of the strains was achieved by amplifying and sequencing fragments of the housekeeping genes gyrB, recN, dnaA, dnaJ, and dnaX (Marrero et al. 2013); the lengths of the five gene fragments analyzed were 822, 762, 720, 672, and 450 bp, respectively (accession nos. OP985528-OP985532). The five sequences were concatenated for every isolate; the resulting 3,426 bp sequences were aligned with ClustalW and found to be identical. A maximum-likelihood analysis was conducted using the 3,426-bp sequences and those of known Dickeya species' type strains. Spi1 to Spi4 clustered with D. dadantii subsp. dieffenbachiae NCPPB 2976T and D. dadantii subsp. dadantii CFBP 1269T (Fig. S2) with sequence identities of 98.4 and 98%, respectively. To fulfil Koch's Postulates, stab inoculations of the four isolates into the petioles of cutting propagated, 38-day-old S. pictus plants (3 plants per isolate) were conducted using sterile toothpicks. The amounts of bacteria used was approximately 106 cfu per toothpick; the bacterial loads were estimated by suspending the cells in 10 mM MgCl2 and spread-plating diluted suspensions on NA. Sterile toothpicks were used as control. All tested plants were sealed in plastic bags (containing wet paper towel) and kept in a growth chamber (28°C; 12-h photoperiod). After 1 day, all isolates induced soft rot symptoms resembling those observed under natural conditions in the greenhouse. Bacteria were re-isolated, and they all shared the same dnaX sequence with strains Spi1 to Spi4. This is the first report of S. pictus affected by D. dadantii in Taiwan. Further investigation is needed to determine whether Spi1-Spi4 belong to D. dadantii subsp. dieffenbachiae. Dickeya dadantii has been found infecting different aroids (Lee and Chen 2021; Lin et al. 2012). The species has also been reported in Taiwan on poinsettia (Wei et al., 2019) and philodendron (Wu et al. 2023). Because these plants are often grown closely in the same facilities, growers should be wary of D. dadantii's spread among these plants. Reduction of environmental humidity and avoiding overhead irrigation may be effective in preventing the pathogen's transmission.