Tomato (Solanum lycopersicum) is one of the most economically important vegetables worldwide, and its production is directly affected by several bacterial diseases (Singh et al., 2017). During a disease survey in 2020, pith necrosis-like symptoms, commonly caused by Pseudomonas spp., were observed in two commercial greenhouses in PyeongChang and Gyeongju, South Korea. Disease incidence ranged from 8 to 10%, and infected plants showed wilt symptoms, brown stem discoloration, leaf blight, and corrugated pith tissues (eXtra Fig. 1). Symptomatic stem tissues were surface disinfected, cut into small pieces, and macerated in sterile water. The resulting suspension was spread on nutrient agar, and incubated at 28°C. The dominant bacterial colony types were round, mucoid, and frequently produced yellow to brown pigments. Four bacterial colonies (CPB20664 - CPB20667), each from a different diseased plant, were selected for further study. All isolates were Gram-negative and did not produce fluorescent pigments on King's B medium. Biochemical profiles of the isolates were determined by the API20NE (Biomerieux, Durhan, NC, USA) and LOPAT test (eXtra Table 1). The bacterial isolates were further identified by PCR amplification of partial 16S rRNA, gyrB, and rpoD genes using primers 27F/1492R, UP-1E/AprU, and 70F/70R, respectively (Lane 1991, Yamamoto et al., 2000). The resulting sequences were deposited in GenBank under accession numbers MW602997 to MW603000 for 16S rRNA, MW602987 to MW602990 for gyrB, and MW602991 to MW602994 for rpoD. These sequences exhibited 99-100% nucleotide similarities with multiple Pseudomonas mediterranea sequences in Genbank. Additionally, the isolates were subjected to PCR assays using the P. mediterranea specific primers PC5/1-PC5/2 and the P. corrugata specific primers PC1/1-PC1/2 (Catara et al., 2002). All isolates produced a specific 600-bp band with the P. mediterranea primers, but did not produce any bands with the P. corrugata specific primers. The PCR amplicons were sequenced and BLAST queried against GenBank database. All isolates shared 100% identity with the type strain P. mediterranea DSM 16733 (acc No. LT629790.1). These results indicated that the bacteria isolated from the tomato plants with pith necrosis were P. mediterranea. Pathogenicity tests were conducted on 2-week-old tomato seedlings (cv. Yekwang) by wound inoculations. Single colonies were picked up using sterile toothpicks, and the stems of tomato seedlings were stabbed below the second leaves. As a negative control, a sterile toothpick was dipped in sterile water and used in the same manner. After inoculation, the plants were kept in a humidity box for 48 h, then moved to a plant growth room. After 15 days, light brown lesions had developed at the stab sites, and pith necrosis and slight wilting of plants were observed at 30 days (eXtra Fig. 1). Control plants remained asymptomatic. P. mediterranea was re-isolated from infected plants, fulfilling Koch's postulates. Five species of Pseudomonas are known to cause tomato pith necrosis (Alippi and Lopez, 2010, Cañizares and García-Pedrajas., 2015, Ruan et al., 2018) including P. corrugata previously reported from Korea (Choi and Han, 2004). This is believed to be the first report of P. mediterranea as the cause of tomato pith necrosis in Korea. Tomato pith necrosis disease reduces the quality and yield of tomato production and appropriate management strategies should be investigated to control this disease.
Keywords: Pith Necrosis; Pseudomonas mediterranea; Tomato.