Leaf rot caused by Rhizopus oryzae on pak choy (Brassica campestris ssp. chinensis) in China

Pak choy (Brassica campestris ssp. chinensis L.) is the major vegetable crop cultivated in China. During December 2017, leaf rot was frequently observed on leaves of pak choy in greenhouses of Shanghai, China. Diseased leaf samples were plated onto acidified potato dextrose agar and fungal cultures were isolated and identified as Rhizopus oryzae, based on morphological features and molecular identification. Definitive identification as R. oryzae was based on the comparative molecular analysis of rRNA gene sequences. Blast analysis revealed 99% similarity with R. oryzae. Pathogenicity was determined by inoculating healthy pak choy leaves and plants with hyphal suspensions of R. oryzae. The fungus was re-isolated from developing similar lesions on the inoculated plants and identified as similar to the inoculated fungus, thus fulfilling Koch's postulates. This is the first report of fungal leaf rot caused by R. oryzae on pak choy in China.