Bacterial wilt caused by plant pathogenic Ralstonia spp. is one of the most important diseases affecting the production of many important crops worldwide. In China, a large scientific community has been dedicated to studying bacterial wilt and its causative agent, Ralstonia pseudosolanacearum and R. solanacearum. Most of their work was published in Chinese, which has hindered international communication and collaboration in this field. In this review, we summarize the status of knowledge on geographical distribution, diversity, and host range of Ralstonia spp., as well as, the impact of bacterial wilt on important crops and disease control approaches, in China. We present areas of research and publications by Chinese scientists and propose the promotion of collaborative research within China and with the international community.

A putative circular single-stranded DNA (ssDNA) virus was recovered from Hypericum japonicum collected in Vietnam. The viral isolate was tentatively named Hypericum japonicum-associated circular DNA virus (HJasCV). HJasCV shares 58.7-65.4% nucleotide sequence identity with Sclerotinia sclerotiorum hypovirulence-associated DNA virus 1 (SsHADV-1) and SsHADV-1-like viruses. Like this group of viruses, the genome of HJasCV (2 200 nt) has two large ORFs, one in the virion-sense and the other in the complementary-sense DNA. The proteins encoded in the virion-sense and complementary-sense ORFs share 39-46 % and 45-67 % amino acid sequence identity with the putative capsid and replication-associated proteins (Reps), respectively, of SsHADV-1 and SsHADV-1-like viruses. The putative Rep of HJasCV contains all of the motifs related to rolling-circle replication. Its 111-bp intergenic region (IR) contains a hairpin structure with a geminivirus-like nonanucleotide sequence, TAATGTTAT, at the apex of the loop. Phylogenetic analysis revealed that HJasCV forms a monophyletic clade with SsHADV-1 and SsHADV-1-like viruses.

Tomato leaf curl Guangdong virus (ToLCGdV) is a begomovirus associated with a Tomato yellow leaf curl disease (TYLCD) epidemic in Guangdong province, China. Being the least conserved protein among geminivirus proteins, the function of C4 during ToLCGdV infection has not been elucidated. In this study, the infectious clones of ToLCGdV and a ToLCGdV mutant (ToLCGdV mC4 ) with disrupted C4 ORF were constructed. Although ToLCGdV and ToLCGdV mC4 could infect Nicotiana benthamiana and tomato plants, ToLCGdV mC4 elicited much milder symptoms compared with ToLCGdV. To further verify the role of C4 in viral pathogenesis, C4 was expressed in N. benthamiana from Potato virus X (PVX) vector. The results showed that ToLCGdV C4 enhanced the pathogenicity of PVX and induced more severe developmental abnormalities in plants compared with PVX alone or PVX-mC4. In addition, ToLCGdV C4 suppresses systemic gene silencing in the transgenic N. benthamiana line 16c, but not local gene silencing induced by sense GFP in wild-type N. benthamiana plants. Moreover, C4 suppresses transcriptional gene silencing (TGS) by reducing the DNA methylation level of 35S promoter in 16c-TGS N. benthamiana plants. Furthermore, C4 could also interact with the receptor-like kinase (RLK) BARELY ANY MERISTEM 1 (BAM1), suggesting that C4 may suppress gene silencing by interfering with the function of BAM1 in the cell-to-cell spread of RNAi. All these results suggest that C4 is a pathogenic determinant of ToLCGdV, and C4 may suppress post-transcriptional gene silencing (PTGS) by interacting with BAM1.

Ralstonia solanacearum species complex is a devastating phytopathogen with an unusually wide host range, and new host plants are continuously being discovered. In June 2016, a new bacterial wilt on Cucurbita maxima was observed in Guangdong province, China. Initially, in the adult plant stage, several leaves of each plant withered suddenly and drooped; the plant then wilted completely, and the color of their vasculature changed to dark brown, ultimately causing the entire plant to die. Creamy-whitish bacterial masses were observed to ooze from crosscut stems of these diseased plants. To develop control strategies for C. maxima bacterial wilt, the causative pathogenic isolates were identified and characterized. Twenty-four bacterial isolates were obtained from diseased C. maxima plants, and 16S rRNA gene sequencing and pathogenicity analysis results indicated that the pathogen of C. maxima bacterial wilt was Ralstonia solanacearum. The results from DNA-based analysis, host range determination and bacteriological identification confirmed that the 24 isolates belonged to R. solanacearum phylotype I, race 1, and eight of these isolates belonged to biovar 3, while 16 belonged to biovar 4. Based on the results of partial egl gene sequence analysis, the 24 isolates clustered into three egl- sequence type groups, sequevars 17, 45, and 56. Sequevar 56 is a new sequevar which is described for the first time in this paper. An assessment of the resistance of 21 pumpkin cultivars revealed that C. moschata cv. Xiangyu1 is resistant to strain RS378, C. moschata cv. Xiangmi is moderately resistant to strain RS378, and 19 other pumpkin cultivars, including four C. maxima cultivars and 15 C. moschata cultivars, are susceptible to strain RS378. To the best of our knowledge, this is the first report of C. maxima bacterial wilt caused by R. solanacearum race 1 in the world. Our results provide valuable information for the further development of control strategies for C. maxima wilt disease.