First Report of a Leaf Spot of Radermachera sinica in China Caused by Bacillus megaterium

Radermachera sinica is widely planted as an ornamental plant in homes, offices, and malls in China. A leaf spot of R. sinica occurred in Luoyang, China, from 2013 to 2014. Lesions mostly occurred in wounds and were irregular with light brown centers and purple borders. One or more lesions on a leaf sometimes covered the entire blade. Eighty plants were surveyed in Luoyang, with disease incidence of 17%. Five millimeter pieces from the borders of lesions were surface-disinfected with 75% ethanol for 30 s, 1% sodium hypochlorite for 5 min, washed three times in sterilized distilled water, placed on nutrient agar (NA) medium at 25°C in darkness, and incubated for 24 to 48 h. Four white, round, smooth, and shiny colonies were selected for further identification. All strains were gram-positive, aerobic rods with many peritrichous flagella, and could grow in medium containing 5% NaCl. The strains were positive for catalase, starch hydrolysis, liquefaction of gelatin, reduction of nitrate, acid production from glucose, mannitol, maltose, lactose, xylose, and pectinose. The strains were positive for phenylalanine deaminase, decomposition of tyrosine, and utilization of citrate. The strains were identified by biochemical tests as Bacillus megaterium (1). To confirm pathogenicity, the strains were grown on NA for 48 h and suspended in sterile distilled water to produce a suspension with a final concentration of 10⁸ CFU/ml. Healthy leaves of biennial R. sinica plants were sterilized with 75% ethanol and washed three times with sterilized distilled water. Fresh wounds were made with a sterile needle on the healthy leaves. Each of four strains was tested by spray inoculation with a bacterial suspension on three leaves. Sterile distilled water was used as negative control. Plants were enclosed in plastic bags and placed in a growth chamber at 28°C with 80% relative humidity. After 5 days, water-soaked lesions were observed. Two weeks later, lesions 4 mm in diameter turned light brown with purple borders, and most of lesions occurred in puncture wounds. Symptoms similar to those observed on field plants developed on all inoculated leaves, while no symptoms appeared on the control leaves. B. megaterium was re-isolated from the lesions of inoculated leaves, but not from the control leaves. To confirm the bacterial identification, PCR was performed on the 16S rDNA gene with P1/P2 (P1: CAGAGTTTGATCCTGGCT, P2: AGGAGGTGATCCAGCCGCA) (2) and 1,463 bp of the 16S rDNA gene (GenBank Accession No. KJ789369) showed 100% sequence identity to B. megaterium DSM 319 (NC_014103.1). To our knowledge, this is the first report of a leaf spot of R. sinica caused by B. megaterium in China as well as anywhere in the world. References: (1) P. Vos et al. Bergey's Manual of Systematic Bacteriology. Vol 3: The Firmicutes. Springer, 2009. (2) W. G. Weisbury et al. J. Bacteriol. 173:697, 1991.