Blackleg on potato plants (Solanum tuberosum) is caused by Pectobacterium spp. and Dickeya spp. (Charkowski, 2018) worldwide. From June to August in both 2018 and 2019, cases of blackleg were inves...

Pectobacterium spp. and Dickeya spp. cause aerial stem rot on potatoes worldwide (Charkowski, 2018). Potato plants (cv. Xisen6# or Youjin) with aerial stem rot or blackleg symptoms (Fig. S1) were o...

Potato soft rot caused by Pectobacterium spp. are devastating diseases of potato which cause severe economic losses worldwide. Pectobacterium brasiliense is considered as one of the most virulent species. However, the virulence mechanisms and pathogenicity factors of this strain have not been fully elucidated. Here, through pathogenicity screening, we identified two Pectobacterium brasiliense isolates, SM and DQ, with distinct pathogenicity levels. SM exhibits higher virulence compared to DQ in inducing aerial stem rot, blackleg and tuber soft rot. Our genomic and transcriptomic analyses revealed that SM encodes strain specific genes with regard to plant cell wall degradation and express higher level of genes associated with bacterial motility and secretion systems. Our plate assays verified higher pectinase, cellulase, and protease activities, as well as fast swimming and swarming motility in SM. Importantly, a unique endoglucanase S specific to SM was identified. Expression of this cellulase in DQ greatly enhances its virulence compared to wild type strain. Our study sheds light on possible determinants causing different pathogenicity of Pectobacterium brasiliense species with close evolutionary distance and provides new insight into the direction of genome evolution in response to host variation and environmental stimuli.

Potato (Solanum tuberosum L.) common scab can be caused by multiple pathogenic Streptomyces spp. worldwide. Potato tubers (cv. Favorita) with severe pitted common scab symptoms were observed at a small farm (2 hectares) during harvest in Anshun, Guizhou province in early May 2020. The disease incidence was around 10%, and symptomatic samples were collected to isolate the pathogen. Two isolates, ZR-IMU141 and ZR-IMU146 (Accession number MW995958 and MW995959 respectively), showed more than 99% sequence identity to S. stelliscabiei sequences (Accession No. HM018085). Five house-keeping genes for multi-locus sequence analyze (MLSA) of Streptomycetaceae were amplified, sequenced and uploaded to NCBI: atpD (MZ343164 and MZ343165), gyrB (MZ343162 and MZ343163), recA (MZ343166 and MZ343167), rpoB (MZ343168and MZ343169) and trpB (MZ343170 and MZ343171). All the genes show over 98% identity with S. stelliscabiei. Phylogenetic trees of 16S rRNA gene sequence and multi-locus sequence analysis (MLSA) were constructed. The two isolates contain pathogenicity genes txtAB, nec1, and tomA, which was confirmed by PCR. To complete Koch's postulates, 9 potato seedlings (cv. Favorita, 15 centimeters high), were transferred to new pots and inoculated with spore suspensions of ZR-IMU141 and ZR-IMU146 (10⁴ CFU/ml), or water as a negative control. Two months later, potato tubers inoculated with either ZR-IMU141 or ZR-IMU146 exhibited typical symptoms of potato common scab, such as superficial or deep, raised, pitted, or polygonal lesions like the field symptoms, but the negative controls remained asymptomatic. The pathogens were reisolated from the lesions and confirmed identical to the original isolate by 16s rRNA gene sequences. To our knowledge, this is the first report of S. stelliscabiei causing potato common scab in Guizhou province, China. We believe that this report will draw attention to the study and management of the increased pool of scab pathogens in China.