FIRST REPORT OF RHIZOPUS SOFT ROT OF SWEET POTATO (IPOMOEA BATATAS) CAUSED BY RHIZOPUS MICROSPORUS IN KOREA

Abstract

Rhizopus soft rot caused by Rhizopus spp. is one of the most important post-harvest diseases of sweet potato storage roots all over the world (Harter et al., 1921). In 2016, a soft rot diseased sweet potato was collected from a farmer’s field in Haenam region, Jeonnam province, Korea. Blackish sporangia along with fungal mycelia were observed at the surface of sweet potato roots. The fungus was isolated on PDA forming a white, dense colony with globose or subglobose sporangia that turned blackish upon sporulation. Sporangial size varied and the sporangiospores were uniform, oval with pointed ends and (3.8–6.5 × 3.3–5.8) µm (n = 20) in size. To confirm the identity, total genomic DNA was extracted and the ITS region of rDNA was amplified using the ITS5/ITS4 primer pair. The purified product was sequenced and the sequence deposited in the GenBank (accession No. KY606252). Maximum parsimony phylogenetic analysis clustered our isolate SPL16012 with the type strain of R. microsporus (CBS 699.68). Molecular and morphological observations described above suggested the microorganism was Rhizopus microsporus (Walther et al., 2013). Koch’s postulates were completed by inoculating two local sweet potato varieties with 5 mm agar plugs and three replications. A disease symptom similar to the original rot was seen on the inoculated sweet potatoes and the pathogen was reisolated. No symptoms were seen on the non-inoculated agar plug control. To the best of our knowledge, this is the first report of R. microsporus on sweet potato in Korea.