Outbreak of Rhizopus Rot Caused by Rhizopus oryzae on Seedlings of Grafted Cucumber on Pumpkin Rootstock in South Korea

In 2013, an outbreak of Rhizopus rot caused by Rhizopus oryzae occurred in cucumber grafted onto pumpkin rootstock sampled from seedling farms in Changnyeong, South Korea. A water-soaked appearance of the affected tissue was the first symptom of this soft fungal rot in the seedling stems of grafted cucumber. Lesions at the graft sites softened and rapidly, rotted, and turned brown or dark brown. Measurements and taxonomic characteristics were most similar to R.oryzae. DNA sequencing and phylogenetic analysis of the internal transcribed spacer rRNA gene region confirmed that the isolates were indeed R.oryzae. Koch's postulates were supported by pathogenicity tests conducted on healthy plants. Based on mycological characteristics, pathogenicity test, and molecular analysis, the causal fungus was identified as R.oryzae Went & Prinsen Geerligs. To our knowledge, this is the first report of Rhizopus rot caused by R.oryzae in seedlings of grafted cucumber on pumpkin rootstock in South Korea.