First Report of Causing Common Scab on Potato in China.

Severe typical deep-pitted lesions of Potato Common Scab (PCS) disease were observed in two locations in China, Dingxi, Gansu Province, and Shuozhou, Shaanxi Province, in 2021. Potato farms in Dingxi growing cultivar Huangxin 226 (26 hectares) exhibited a scab disease incidence of 10%, while cultivar Jinshu 15# (4 hectares) in Shuozhou showed a disease incidence of 30% (Fig. 1). During harvest, tubers displaying PCS symptoms were collected for pathogen isolation. To obtain pathogen isolates, surface-sterilized tuber tissue with scab lesions was ground in sterile water, serially diluted, and plated onto ISP5 agar medium plates (Handique et al. 2022). Five pure colonies of Streptomyces isolate were obtained, designated as ZRIMU1503, ZRIMU1502, ZRIMU1320, ZRIMU1321and ZRIMU791. Genomic DNA was extracted and sequenced using Illumina technology. Sequencing data of the 5 isolates were uploaded to NCBI GenBank and annotated (Accession numbers: JBBAYL010000000, JBBAYM010000000, JBBAYN010000000, JBBAYO010000000 and JBBAYP010000000, respectively) using the PGAP pipeline (Tatusova et al. 2016). Average Nucleotide Identity (ANI) values (97.52 %, 97.53 %,97.54 %,97.57 % and 97.52 %, respectively) indicated the identity of the five isolates to the type strain S. brasiliscabiei IBSBF 2867T. Additionally, pairwise comparisons of Digital DNA Hybridization (DDH) value (76.2%, 76.3%, 76.4%, 76.4% and 76.2% respectively) of all the Streptomyces type strains show the highest identity to S. brasiliscabiei IBSBF 2867T. Twelve housekeeping genes (acnA, atpD, dnaN, gap, gyrA, gyrB , infB, mdh, recA, rplB, rpoB, and trpB) were extracted from the genome sequence of the five isolates to construct a multi-locus sequence analysis (MLSA) tree. The evolutionary distance of the five isolates was constructed using MEGAX software (Kumar et al., 1994), along with other Streptomyces strains that are known to cause PCS. The resulting cladogram demonstrated the isolated strains clustered together with S. brasiliscabiei IBSBF 2867T (Fig.2). Koch's postulates were fulfilled by inoculating a perlite potting mix with spore suspensions of each isolate (104 CFU/ml), planting tubers (cv. Favorita), and reproducing PCS symptoms at harvest after three months. Negative control received water treatment. The plants were kept in greenhouse with 12 h of light per day and irrigated regularly. The experiment was repeated twice, once in April 2022 and again in April 2023. On harvest, all five isolates exhibited development of severe symptoms of PCS (Fig.1), while the negative controls had no lesions. The pathogen was reisolated from the lesions and confirmed to be identical to the original isolate by 16S rRNA gene sequences. To our knowledge, this is the first report of S. brasiliscabiei causing PCS in China. S. brasiliscabiei was identified as a new species to cause PCS in Brazil and was identified based on morphology, pathogenicity, and genetic features (Corrêa et al. 2021). Multiple pathogen-causing PCS has been recognized in China and a surge of disease incidence in potato fields has been reported (Handique et al. 2022; Wu et al. 2023). S. brasiliscabiei causes severe symptoms which makes potatoes unmarketable. The disease epidemiology of this pathogen needs to be investigated.