Molecular characterization of Acidovorax citrullistrain NIHHS15-280 causing bacterial fruit blotch disease in Korea and screening of resistance sources in melon

Bacterial fruit blotch (BFB), caused by Acidovorax citrulli, is an economically devastating disease in melon. Molecular characterization of causal strains and identification of sources of resistance are necessary to control this important disease. Many strains of BFB were identified by partial sequencing and documented in GenBank. Recently, there was an outbreak of BFB strain NIHHS15-280 in melon fields (Goksung, Jeonnam, Korea) that severely hampered melon production in Korea. For this reason, we characterized NIHHS15-280 by partially sequencing the 16S rRNA gene using PCR-based markers. The partial sequence of the 16S rRNA gene of NIHHS15-280 was much more similar to A. citrullistrains associated with melon and watermelon hosts than other cucurbit hosts. A PCR-based assay identified the strain (NIHHS15-280) as belonging to A. citrulligroup I, which predominantly infects non-watermelon cucurbits (virulent to melon). Thirty-five melon genotypes were screened against NIHHS15-280 in a detached-leaf inoculation assay. The lack of visible disease symptoms 11 days after inoculation in lines PI353814 and PI140471 indicated these to be highly resistant lines, whereas moderate resistance was observed in lines PI420145 and SCNU1154 and cultivars ME6 and ME5. Also, the most resistant lines were corroborated by a fruit inoculation assay. The characterized A. citrullistrain NIHHS15-280 (group I) and identified resistant lines will serve as a valuable resource for further genetic studies and improvement of BFB resistance in melon.