Root Rot of Rose Caused by Pythium helicoides in Korea

Abstract

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