First Report of Bacterial Black Spot Disease in Watermelon Caused by Acidovorax valerianellae in Korea
Authors: Han Y.-K. , Han K.-S. , Lee S.-C. , Kim S. , Lee J. 
Published: January 2012

Watermelon (Citrullus lanatus (Thunb.) Matsum. & Nakai), an important member of the Cucurbitaceae family, is cultivated on 21,000 ha that produces 850,000 t in Korea. In April 2011, we received grafted watermelon with necrotic leaf spots from a commercial watermelon grower in Andong, Korea. Black spots were observed on cotyledons of the plants in seedbeds, and approximately 9% of watermelon plants were infected with the disease. Initial symptoms on the seedling were black, greasy spots sometimes surrounded by a halo of discoloration. Younger leaves usually showed symptoms later than cotyledons. Bacteria isolated from the infected plants were gram-negative, motile, straight rods with a single flagellum and 0.84 to 0.89 μm wide and 1.54 to 1.69 μm long. They formed rough colonies with a white-cream color after 48 h of incubation on Luria-Bertani (LB) agar at 28°C. Colonies of isolates were nonfluorescent, smooth, and white on King's medium B. On YBGA (7 g of yeast extract, 7 g of bactopeptone, 7 g of glucose, 15 g of agar, 1,000 ml of distilled water; pH 7.2) colonies are circular, raised with an entire margin, and white to cream. Pathogenicity tests were conducted with potted, greenhouse-grown watermelon plants. Bacterial colonies grown on LB medium for 48 h at 28°C were suspended in sterile distilled water, and the suspension (1.0 × 108 CFU/ml) was infiltrated into mesophyll of watermelon leaves with a syringe as previously described (2). Inoculated plants were maintained at 28°C and 90% relative humidity in a growth chamber with a daily 12-h photoperiod of fluorescent light. Five plants were used for inoculation. Sterilized distilled water was used as a control. The bacterial isolates induced necrosis in the infiltrated area within 3 to 5 days. Typical water-soaked spots appeared after 3 days of incubation and became gray to black after 6 days. The bacterium was successfully reisolated from the diseased lesions, thus completing Koch's postulates. A cell suspension (50 μl of 1 × 106 CFU/ml) was infiltrated with a syringe into the intercellular spaces of tobacco leaves to determine the hypersensitive reaction (HR). A typical HR developed 20 h after leaf infiltration. The 16S rDNA region of the isolates, amplified by using universal PCR primers, shared 99% sequence identity with an Acidovorax valerianellae strain (GenBank Accession No. AJ431731) (1). The resulting sequences of 1,424 bp were deposited in GenBank (Accession No. JN983471). The isolates we obtained in this study clustered with A. valerianellae on a phylogenetic tree generated by the neighbor-joining method implemented in MEGA Version 4.1. In the Biolog Microbial Identification System, Version 4.2 (Biolog Inc., Hayward, CA), all isolates were 63 to 77% similar with a match probability of 100% to A. konjaci. Fatty acid composition analysis of isolates based on the MIDI Library version TSBA 5.0 and Library Generation system software version 5.0 showed that the isolates were 52 and 72% similar to an Acidovorax sp., respectively. To our knowledge, this is the first report of bacterial black spot disease in watermelon caused by A. valerianellae in Korea. A. valerianellae is a causal agent of bacterial black spot in corn salad and is transmitted by inoculated seeds (3). Further studies are required to determine whether it is seed transmitted in watermelon. References: (1) L. Gardan et al. Int. J. Syst. Evol. Microbiol. 53:795, 2003. (2) C. Grondeau et al. Plant Pathol. 56:302, 2007. (3) C. Grondeau et al. Plant Pathol. 58:846, 2009.