BACTERIAL SOFT ROT OF EGGPLANT CAUSED BY PECTOBACTERIUM CAROTOVORUM subsp. CAROTOVORUM IN CHINA

Abstract

In December 2016, soft rot lesions on the stem and fruit of eggplants cv. Green Beauty were noted in greenhouses in Anshan, Liaoning Province, China. Disease incidence was estimated at 5-10% and resulted in a substantial yield reduction. Symptoms appeared as soft, dark green stem lesions that progressively turned brown with vascular bundle necrosis appearing. Fruits of infected plants initially had small water-soaked lesions that rapidly enlarged with a watery ooze. Infected tissues from stems and fruits of eggplant were macerated in sterile water, streaked onto nutrient agar (NA) and incubated at 28oC for 2 days. The resultant bacterial colonies were round, translucent, white, convex with smooth edges. All isolated bacteria were gram-negative rods, facultative anaerobes, non-fluorescent on King’s B media, produced a positive pectolytic response on crystal violet pectate agar and gave a positive hypersensitive response in tobacco plants. All were negative for sucrose reduction, sensitive to erythromycin (50 µg/ml), indole, oxidase and could grow at 37°C. PCR amplification of the pel gene using Y1/Y2 primers (Darrasse et al., 1994) produced a 430 bp fragment. Amplification of the 16S- 23SrRNA region by G1 and L1 primers (Toth et al., 2001) produced two main bands of 550 and 580 bp. 16S rRNA fragment of 1440 bp was amplified from bacterial isolates and the partial sequence deposited in GenBank (accession No. KY786125) showed 99% identity to the sequence of Pectobacterium carotovorum subsp. carotovorum strain Y57 (KP187518.1). No amplification products were produced for any isolate using P. atrosepticum and P. carotovorum subsp. brasiliensis specific primers (Eca1f/Eca2r and Br1f/L1r, respectively), whereas the P. carotovorum subsp. carotovorum primers EXPCCF/EXPCCR produced the expected 550 bp fragment (Kang et al., 2003). For pathogenicity testing, mature fruits and stems of Green Beauty were needle-stab inoculated with a 10 µl bacteria suspension (1×108 CFU/ml). Sterile water was used as a negative control. Inoculated plants were incubated at 28°C for 48 h, after which watersoaked and soft-rot symptoms were observed on all eggplant fruits and stems, but not on control plants. Bacteria with the same characteristics as those inoculated were reisolated from diseased tissues. To our knowledge, this is the first report of P. carotovorum subsp. carotovorum on eggplant in China.