Sclerotium rolfsii causes stem rot on Ixeridium dentatum in Korea
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Stem rot was observed on Ixeridium dentatum in Jinan-gun and Dangjin-gun, Korea during the growing seasons of 2016 and 2017. The first symptom was water-soaked lesions on the basal parts of stems. Lower leaves became wilted and blighted before the plants eventually died. White cottony mycelial mats and brown spherical or irregular sclerotia formed on the basal stem and adjacent soil surfaces. The optimal temperature for in vitro colony growth and sclerotia germination were 30 °C and 25 °C, respectively. The in vitro and morphological characteristics of the fungus were identical with those described for Sclerotium rolfsii. Phylogenetic analysis based on the internal transcribed spacer (ITS) region revealed that two isolates isolated from I. dentatum formed a monophyletic group with reference isolates of S. rolfsii. Koch’s postulates were satisfied for the same two S. rolfsii isolates, thereby confirming that the pathogen causes stem rot of I. detatum in Korea.
KeywordsIxeridium dentatum New report Sclerotium rolfsii Stem rot
Ixeridium dentatum, a perennial herb belonging in the family Asteraceae, is widely distributed throughout East Asia including Korea, Japan, and China (Yook 1997). The plant is well known as a medicinal crop with anti-cancer, anti-oxidative, and anti-allergic activities (Yi et al. 2002; Lee et al. 2014). Cucumber mosaic virus (CMV disease), Tomato spotted wilt virus (TSWV), and Puccinia lactucae-debilis (rust) have been reported to be pathogenic to I. dentatum in Korea (Anonymous 2009).
To observe colony morphology, agar block colonized by hyphae of two isolates (KACC48476 and KACC48477) were placed on PDA in 9-cm-diameter petri dishes and incubated at 10, 15, 20, 25, and 30 °C for seven days in the dark. To assay germination of sclerotia, the sclerotia were surface-sterilized with a 1% NaOCl solution for 1 min, rinsed three times with sterilized distilled water, and placed on PDA plate at 10, 15, 20, 25, and 30 °C for seven days. Colonies growing on PDA were white, cottony, often with fan-shaped sections and hyphae were 4.0–8.5 μm in diameter with clamp connections. Sclerotia started to develop in the colonies after 5 days at 30 °C, initially white then becoming brown with age, spherical or irregular, and 1.0 to 3.5 mm in diameter. The optimal temperature for colony growth and sclerotial germination was 30 °C and 25 °C, respectively. In vitro, all isolates were morphologically similar to description of S. rolfsii provided by Mordue (1974).
To fulfill Koch’s postulates, pathogenicity tests were conducted on the lower stem of three healthy I. dentatum 60-day-old plants by placing 6-mm-diameter mycelium plugs obtained from 2-day-old cultures. Non-inoculated plants were used as a control. All plants were kept in a dew chamber at 25 °C and relative humidity of >95%. Every inoculated plants showed symptoms such as water-soaked spots on the stem within two days followed by rotting, wilting, blighting, and eventually death, whereas non-inoculated plants remained healthy. Sclerotia developed on the basal stem and soil surfaces within six days (Fig. 1). The pathogenicity tests were similar to those reported by Kwon et al. (2017). S. rolfsii was consistently re-isolated from the symptomatic tissue to complete Koch’s postulates.
Based on these morphological features, sequence analysis, and the pathogenicity test, the Korean isolates KACC48476 and KACC48477 responsible for stem rot of I. dentatum were identified as S. rolfsii. Therefore, we conclude that S. rolfsii is the causal agent responsible for stem rot of I. dentatum in Korea.
This research was supported by the national joint agricultural research program (Project No. PJ01187802) of the Rural Development Administration, Republic of Korea.
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