The fungus Cylindrocarpon destructans is the cause of root rot in many ginseng production areas in Korea. A total of 57 isolates of C. destructans were recovered from diseased roots in a survey of ginseng-growing fields from 2011-2012. Among these isolates, 37% were classified as highly virulent (causing lesions on unwounded mature roots) and 61% were weakly virulent(causing lesions only on previously wounded roots). Radial growth of highly and weakly virulent isolates on potato dextrose agar was highest at 20 o C and there was no growth at 35 o C. Mycelial mass production was significantly (P = 0.05) lower at pH 7.0 compared with pH 5.0. To study the effects of pH (5.0 and 7.0) and wounding on disease development, ginseng roots were grown hydroponically in nutrient solution. Lesions were significantly larger (P < 0.01) at pH 5.0 compared with pH 7.0 and wounding enhanced disease by a highly virulent isolate at both pHs. In artificially infested soil, 2-yearold ginseng roots were most susceptible to Cylindrocarpon root rot among all root ages tested (1 to 4 years) when evaluated using a combined scale of disease incidence and severity. Root rot severity was significantly (P<0.05) enhanced by increasing the inoculum density from 3.5 × 10 2 cfu/g of soil to 2.0×10 3 cfu/g of soil.

Rose (Rosa hybrida Hort.) is the most important commercial cutflower plant in the world. Rose is mains cultivated in hydroponic system and propagated by cutting on rock-wool slabs in Korea. During the summers in 2007-2008, root rot symptom was found in cutting bed and as well in rock wool of hydroponic culture farms located in Goyang and Jeonju, Korea respectively. The initial symptoms showed wilting and yellowing. The stem and root had a dark brown or black water-soaked lesion, and the lesion gradually turned to brownish black color (Fig. 1A). As the disease progressed the infected plants rapidly lost vigor and finally wilted.Pathogens isolated from the roots rot of rose in Goyang and Jeonju and cultured on cornmeal agar added 0.01% wheat germ oil. The fungi produced cottony aerial mycelium. The optimum temperature for hyphal growth was 30-35 o C with a growth rate of 30 mm/24 hrs. The mycelia could grow on the media at 10-40 o C. Hyphae were 7.6-8.1 µm in width. Sporangia were terminal, ellipsoidal, papillate, proliferous, and 23-40×27-48 µm (av. 27.2-39.1 µm) (Fig. 1C). Zoospore was produced in a vesicle originating from a sporangium (Fig. 1D) and hypha were formed in vacant sporangium . Oogonia were terminal, lateral or intercalary, smooth, and 25-38 µm (av. 32.3 µm). Oospores were globose, aplerotic, and 21-32 µm (av. 30.2 µm) in diameter. Antheriada were lobulate and elogete, and 22-40 µm (av. 31 µm) in size (Fig. 1E).For molecular identification, their sequences of rDNA ITS and Cox II were analyzed. The fungi showed the highest similarity with Pythium helicoides strain (PPRI 8423, GenBank FJ415973) in rDNA ITS tree (McLeod et al., 2009) (Fig. 2) and the similarity was 99.7% (760/762). With Cox II gene, the fungi also showed the highest similarity with P. helicoides.A pathogenicity test was conducted using 30-day-old rooted cutting plants in greenhouse. Zoospore suspension (1×10 6 conidial/ ml) which prepared from 4-day-old cultured on cornmeal agar added 0.01% wheat germ oil was pipetted onto each of plant by 0.5ml. The infected cuttings showed symptoms of leaf yellowing and water-soaked rot in roots, followed by leaf blight and root dieback after 7 days (Fig. 1B).Based on the above characteristics, growth temperature, morphology, DNA sequence and pathogenicity test, this fungus was identified as P. helicoides Drechsler (Drechsler, 1930). Kageyama et al. (2002) reported that P. helicoides was causal pathogen of rose root rot in Japan and a high infection rate has been shown over 30 o