Sang-Sik Nam

Physicochemical Characteristics of Sweetpotato (Ipomoea batatas (L.) Lam) Starch Depending on Cultivation Periods

This study was performed to understand the effect of cultivation periods on the physicochemical characteristics of the starch of five sweetpotato cultivars, cultivated in Muan, Korea. Starch, protein, and ash contents increased with increased cultivation period, whereas amylose content decreased. Rapid viscosity analysis showed that the pasting temperature, peak viscosity, breakdown, setback and final viscosity increased with increased cultivation period. However, trough and final viscosity decreased. Although the onset temperature and peak temperature values increased, the conclusion temperature did not show any consistent patterns by differential scanning calorimetry. X-ray diffraction showed that the starch samples had C-type crystallinity irrespective of the cultivation period and cultivar. The starch granules were dominantly round and oval, or polygonal irrespective the cultivation period. The bigger the particle size was, the longer the cultivation period was.

Fusarium root rot caused by Fusarium solani on sweet potato (Ipomoea batatas) in South Korea

Abstract Fusarium species are important post-harvest pathogens of sweet potato worldwide. A root rot disease was observed on sweet potato (Ipomoea batatas) at storage houses in different locations in South Korea during 2016–2017. The disease was characterized by circular lesions with dark and light brown concentric rings along with cavities in the inner part of the root. Seven isolates were recovered from diseased samples. Two representative isolates were cultured on potato dextrose agar and the morphology was studied. Macroconidia were cylindrical and slightly curved with blunt and rounded apical cells. The size of the macroconidia was 28.3 (15.0–44.3) × 5.8 (4.2–8.3) μm. The microconidia were fusiform to ovoid, variable in size and aseptate or rarely with single septa. Microconidial size varied from 9.2 (5.2–16.1) × 3.6 (2.6–4.5) μm. The fungal isolates were identified as Fusarium solani (Martius) Appel & Wollenweber emend. Snyder & Hansen on the basis of the morphological and cultural characteristics. The isolates were confirmed as F. solani on the basis of molecular identification by rDNA–ITS and translation elongation factor (EF 1-α) gene sequence analysis. The pathogenicity of the organism was confirmed by fulfilling Koch’s postulates. To our knowledge, this is the first report of F. solani root rot of sweet potato in South Korea.

Phylogenetic Placement and Morphological Characterization of Sclerotium rolfsii (Teleomorph: Athelia rolfsii) Associated with Blight Disease of Ipomoea batatas in Korea

Abstract In this study, we aimed to characterize fungal samples from necrotic lesions on collar regions observed in different sweetpotato growing regions during 2015 and 2016 in Korea. Sclerotia appeared on the root zone soil surface, and white dense mycelia were observed. At the later stages of infection, mother roots quickly rotted, and large areas of the plants were destroyed. The disease occurrence was monitored at 45 and 84 farms, and 11.8% and 6.8% of the land areas were found to be infected in 2015 and 2016, respectively. Fungi were isolated from disease samples, and 36 strains were preserved. Based on the cultural and morphological characteristics of colonies, the isolates resembled the reference strain of Sclerotium rolfsii. Representative strains were identified as S. rolfsii (teleomorph: Athelia rolfsii) based on phylogenetic analysis of the internal transcribed spacer and large subunit genes along with morphological observations. To test the pathogenicity, sweetpotato storage roots were inoculated with different S. rolfsii strains. ‘Yulmi’ variety displayed the highest disease incidence, whereas ‘Pungwonmi’ resulted in the least. These findings suggested that morphological characteristics and molecular phylogenetic analysis were useful for identification of S. rolfsii.