Biological characteristics and isolation of Pythium ultimum causing rot of Chinese cabbage

Soft rot, a major bacterial disease affecting the production of Chinese cabbage ( Brassica rapa L. ssp. pekinensis ) in China, has been widely attributed to Pectobacterium carotovorum subsp. carotovorum. Outbreaks of a new disease of Chinese cabbage caused considerable economic losses in many varieties from June to July of 2018 and 2019 in Harbin, China. This new disease had symptoms similar to those of soft rot but was different from bacterial rot caused by Pcc . Plants affected by this disease showed rot at the midrib of their leaves and stumps with short hyphae. A fungal strain named DBC755 was isolated from plant samples exhibiting soft rot to determine the relevant pathogen. The pathogen had a radial colonial morphology, white hyphae, and luxuriant branches in potato dextrose agar culture medium. The isolate was identified as Pythium ultimum var. ultimum on the basis of its morphology, biological characteristics, internal transcribed spacer, β-tubulin gene and coxII gene sequence analyses. A pure fungal culture was inoculated on Chinese cabbage, and rot symptoms were consistent with those observed in the field to satisfy Koch’s postulates. The discovery and biological characteristics of P. ultimum var. ultimum causing rot in Chinese cabbage provides a deeper understanding of soft rot and has great importance for the breeding of Chinese cabbage with resistance to this disease.