First Report of Leaf Blight Wilt on Dracaena sanderiana by Pantoea stewartii subsp. indologenes in China

Abstract

Dracaena sanderiana is one of the important export ornamental plants in southern China. Zhanjiang district, Guangdong province, is one of the main areas to process and export D. sanderiana. The annual export value was $6.5 million in 2018. In Zhanjiang district, D. sanderiana was seriously affected by a disease in the field in 2017 with an incidence of 35 to 80%. Both young and older leaves developed necrotic lesions with small water-soaked spots at the tip or at the margin of the leaves, which then enlarged and were bordered by chlorosis. Diseased plants were collected in Zhanjiang city (110°50′ E, 21°54′ N), Nanning city (108°22′ E, 22°48′ N), and Haikou city (114°40′ E, 20°02′ N). Three diseased leaves from each province were surface disinfected in a 1% hypochlorite solution. Nine bacterial isolates were recovered from the samples. Colonies were raised and opaque with smooth margins. The bacteria were gram-negative, rod-shaped, 0.4 to 0.6 × 0.8 to 1.9 μm, had no endospore, and were facultative anaerobes. For molecular identification, the direct colony PCR method (Lu et al. 2012) was used to amplify the 16S rDNA, galE, pstC+pstS, and glmS using the primer pairs 16SF/16SR (Lu et al. 2012), #562/#564, #338/#339, and #356/#341 (Gehring et al. 2014). The resulting sequences were deposited in GenBank (MK321579, MK321583, and MK321587 for 16S; MK321580, MK321584, and MK321588 for galE; MK321581, MK321585, and MK321589 for pstC+pstS; MK321582, MK321586, and MK321590 for glmS). BLAST searches with the four gene sequences revealed the greatest similarity with the sequences of Pantoea stewartii. The sequences that matched with the four genes of the isolates in the genome of P. stewartii subsp. stewartii (CP017581.1) and P. stewartii subsp. indologenes (JPKO01000031.1 and JPKO01000005.1) were downloaded. The concatenated sequences were aligned by Mega 6.0 with the neighbor-joining method. Analyses showed that the concatenated sequences were more similar to the sequence of the P. stewartii subsp. indologenes than that of P. stewartii subsp. stewartii. Amplification using subspecies-specific primers also proved that the three isolates were P. stewartii subsp. indologenes (Gehring et al. 2014). Three isolates were selected for pathogenicity tests according to Koch’s postulates. Six plants at the five- to seven-leaf stage were inoculated with each isolate separately, and three to four leaves of each plant were inoculated. Leaves were inoculated with the isolates by hand infiltration (Choi et al. 2013) and suspension injection (Paccola-Meirelles et al. 2001). A ∼10⁸ CFU/ml bacteria suspension was prepared. Sterile distilled water was used as a control. Inoculated plants were placed in an incubator at 25°C, and 80% humidity under a 12-h light/dark cycle for 7 days. After 3 days of incubation, water-soaked yellow spots at the infiltration or injection sites were observed in all the inoculated plants, except the negative control. The water-soaked yellow spots enlarged and became necrotic, usually bordered by a yellow halo that matched those seen in fields. The bacteria were then reisolated from the lesions and found to have the same morphology of the colonies and 16S rDNA sequences of the inoculum. According to morphological and sequence analysis, the pathogen was identified as P. stewartii subsp. indologenes. P. stewartii caused disease on imported D. sanderiana in Korea (Choi et al. 2013), as reported previously. This is the first report of disease of D. sanderiana caused by P. stewartii subsp. indologenes in China. Identification of the causal organism of this disease is essential for the development of economical management practices.