Multiple species of Streptomyces cause common scab disease in potato (Solanum tuberosum) (Kämpfer et al. 1991). Potato tubers (cv. Jinshu1 #5 and Longshu #6) with severe pitted common scab symptoms were observed from two farms in Chaozhou in Shanxi Province and in Tianzhu in Gansu Province during the national disease survey of bacterial diseases of potatoes in 2021. The disease incidence was around 30% on the 6.7 ha of the Chaozhou farm and 10% on the 0.7 ha on the Tianzhu farm. Three tubers with scab symptoms were surface disinfested with 3% sodium hypochlorite for 1 min. The symptomatic tissue was then ground in sterile water. Serially diluted ground samples were cultured on Streptomyces ISP Medium 5 agar plates (Shirling and Gottlieb 1966) and incubated at 280C for 5 days. Eight pure Streptomyces colonies were obtained and sequenced for identification using the universal 16S rRNA gene primers 27F (5'-AGAGTTTGATCMTGGCTCAG-3') and 1492R (5'-TACGGYTACCTTGTTACGACTT-3') (Monciardini et al. 2002) by colony PCR. Blast results of the sequences against the NCBI GenBank for the eight isolates, ZRIMU1508, ZRIMU1510, ZRIMU1511, ZRIMU1512, ZRIMU1514, ZRIMU1515, ZRIMU1516 and ZRIMU1530 (Accession numbers: OP941573 - OP941580), showed more than 99% sequence identity to S. niveiscabiei NRRLB-24457T type strain. Additionally, 12 housekeeping gene sequences, acnA (OP997624 - OP997625), atpD (OP997622 - OP997623), dnaN (OP997620 - OP997621), gap (OP997618 - OP997619), gyrA (OP997614 - OP997615), gyrB (OP997612 - OP997613), infB (OP997610- OP997611), mdh (OP997608 - OP997609), recA (OP997602 - OP997603), rplB (OP997600 - OP997601), rpoB (OP997598- OP997599), and trpB (OP997594 - OP997595), were extracted from the genome sequences of two strains, ZRIMU1510 and ZRIMU1530, and uploaded to GenBank. Genes for pathogenicity, txtA (OP997593 - OP997594), tomA (OP997596 - OP997597) and Nec1(OP997606 - OP997607), were also identified from the genome sequence and uploaded to GenBank. The housekeeping genes and the pathogenicity genes showed over 98% identity with S. niveiscabiei. Phylogenetic trees were constructed using concatenated housekeeping gene sequences (Kumar et al. 1994) and the cladogram showed that the isolates ZRIMU1510 and ZRIMU1530 grouped with the type strain NRRLB-24457T. Pathogenicity tests were done by drench application of 100 ml spore suspensions (104 CFU/ml) of ZRIMU1530, ZRIMU1510, or phosphate buffer into pots with potato plants (cv. Favorita) grown in potting mix. Five tubers were planted and inoculated with each pathogen or phosphate buffer as the negative control. The plants were then placed in a greenhouse with 12 h of light per day, irrigated regularly, and harvested after 3 months. The newly formed tubers were checked for disease symptoms. Tubers from pots inoculated with ZRIMU1530 and ZRIMU1510 exhibited typical symptoms of common scab with raised corky lesions with deep pits, but the negative controls remained asymptomatic. The pathogens were reisolated from the lesions and confirmed to be identical to the original isolates by 16S rRNA gene sequences, thus completing Koch's postulates. The pot experiment was conducted twice: first in May 2022 and second in February 2023. To our knowledge, this is the first report of S. niveiscabiei causing common scab of potato in Shanxi and Gansu, China. S. niveiscabiei was first reported in Korea (Park et al. 2003) and this report will draw attention to the study and management of scab pathogens in China.
Keywords: Crop Type; Field crops; Streptomyces niveiscabiei; common scab; potato.