RÉSUMÉ
Jackfruit (Artocarpus heterophyllus) is widely cultivated in the tropical areas in the world. Jackfruit bark split disease occurred in the large-scale plantations of 18 cities and counties surveyed in Hainan since 2021, among which the incidence rate of serious orchards reached about 70%, and the mortality rate reached about 35%. Jackfruit bark split disease mainly harms tree branches and trunks, manifested as water stains, bark gumming, bark depression, bark cracking, and ultimately plant death. To identify the pathogen, Four samples with jackfruit bark split disease symptoms were collected, sterilized with 75% ethanol for 30 s, then soaked in 2% sodium hypochlorite (NaClO) for 5 mins, and finally rinsed continuously with sterilized distilled water. The sterilized tissues were placed on LB agar medium and incubated in illumination incubator at 28 â. Four milky white, round with neat edges, convex and smooth, translucent colonies were obtained. All isolates (JLPs-1 to JLPs-4) were Gram-negative, negative for oxidase, catalase and gelatin liquefaction. Amplification and sequencing of 16S rDNA gene from 4 isolates were conducted with the universal primers 27f /1492r (Lane et al. 1991). The BLASTn analysis of obtained JLPs-1 and JLPs-3 sequences (GenBank accession nos. OP942452 and OP942453) showed an identity percentage of 98.99% and 98.93% with Pectobacterium sp. (CP104733), respectively. Phylogenetic analysis based on 16S rDNA gene using the neighbor-joining method with MEGA 7.0 software revealed that JLPs-1 and JLPs-3 were clustered together with P. carotovorum reference strains. The four housekeeping genes gyrA, recA, rpoA and rpoS were partially sequenced for JLPs-1 isolates using primers gyrA1/gyrA4, recA1/recA2c, rpoS1/rpoS2 and rpoA F1/rpoA R1 (Loc et al. 2022), respectively. Multilocus sequence analyses identified the isolates from jackfruit as P. carotovorum. To further confirm the identification of Pectobacterium carotovorum, pelY gene, P. carotovorum subsp. Brasiliensis 16S-23S intergenic region (Pcb IGS) and P. carotovorum subsp. carotovorum (Pcc) specific fragment were amplified with primers Y1/Y2 (Darrasse et al. 1994), BR1f/L1r (Duarte et al. 2004) and EXPCCF/EXPCCR (Kang et al. 2003), respectively. A 540 bp target fragment was successfully amplified from JTPs only by EXPCCF/EXPCCR and there no bands for the other two primers. Pathogenicity test was performed in the field, and all the inoculated trees were 2-3-year-old 'Qiong Yin No.1' variety. Dense small holes were pierced with sterilized inoculation needle on four healthy jackfruit trees. Then punctured wounds were spraying-inoculated with bacteria suspension of JLPs-1 (108 CFU/ml), and finally wrapped with plastic wrap to moisturize. Two trees inoculated with sterile distilled water served as negative control. Typical symptoms of bark gumming, bark depression, bark cracking were observed on all of the inoculated trees at 17 dpi which just similar to those originally caused by P. carotovorum in the field, whereas negative control trees remained asymptomatic. The strains were re-isolated successfully from symptomatic jackfruit trees and were consistent with the biological and molecular biological characteristics of original strains, confirming that the pathogen of jackfruit bark split disease was Pectobacterium carotovorum. To our knowledge, this is the first report of P. carotovorum causing bark split disease on jackfruit in China.