First Report of Web Blight on Lavender Caused by Rhizoctonia solani AG-1-IB in Korea

Lavender (Lavandula officinalis L.) is an annual or short-lived perennial herb belonging to the mint family (Lamiaceae) that originates from eastern Africa, the Mediterranean, southwest Asia, to southeast India. Lavenders are cultivated extensively as ornamental plants for garden and landscape uses, for usage as culinary herbs, and commercially for the extraction of essential oils. In June 2017, approximately 5 to 10% of plants showed web blight symptoms in a private garden located near Gangneung, Gangwon Province, Korea. Specifically, leaf tissues were blighted, and white mycelial growth was seen on the stems. Symptomatic tissues were surface sterilized with 1% NaOCl, washed twice in sterilized water, blotted on sterile filter paper, and plated on 2% water agar at 25°C. After 2 days, hyphal tips were transferred to potato dextrose agar (PDA) to obtain pure cultures. A total of five morphologically similar fungal isolates (LWB001 to LWB005) were obtained from diseased samples; isolate LWB002 was selected for identification. The fungus produced a white mycelium when young but produced a brown mycelium with dark brown sclerotia on PDA at 20 ± 2°C for 10 days. Mature sclerotia (n = 20) were dark, spheroidal, with diameters ranging from 0.2 to 2.2 mm (average 0.7 mm). The young hyphae had acute angular branching near the distal septum of the multinucleated cells, and mature hyphal branches showed approximately 90° angles. Morphological characteristics were consistent with those of Rhizoctonia solani (Sneh et al. 1991). A representative isolate (LWB002) was deposited in the Korean Agricultural Culture Collection (KACC), National Institute of Agricultural Science, Rural Development Administration, Wanju, South Korea (KACC48671), to be used for further studies. The identification of the anastomosis group of the R. solani isolate was carried out on sterilized glass slides coated with 2% water agar medium placed on Petri dishes. The representative isolate was paired with tester strains of R. solani (AG-1-IA, AG-1-IB, AG-1-IC, AG 2-2-IIIB, AG 4, AG 7, and AG 11), which were collected from KACC (Ogoshi 1987) and examined microscopically. The fungal isolate anastomized only with tester strain AG-1-IB. To obtain a DNA sequence-based identification, a total DNA was extracted directly from the mycelium with a DNeasy Plant Mini Kit (Qiagen, Hilden, Germany). Following DNA extraction, the internal transcribed spacer (ITS) region of rDNA was amplified with primers ITS1/ITS4 and sequenced (White et al. 1990). BLASTn analysis of the 706 bp showed a 100, 99.71, and 99.86% similarity with sequences of R. solani AG-1-IB (teleomorph Thanatephorus cucumeris) in GenBank (JQ692293.1, GU585667.1, and KF907717.1, respectively). The nucleotide sequence has been assigned GenBank number MH122653. To confirm pathogenicity, three fungal mycelial discs (14 days old, 6.5 mm) were placed onto three wounded young plant stems on the soil line. Another three plants served as controls, for which only an agar plug was used on wounded plant stems. Then all the plants were incubated in a growth chamber (at 28 ± 2°C, 90% relative humidity, with 12/12-h light/dark). After 10 days, blight symptoms were similar to the original symptoms developed on inoculated plant leaves and stems, whereas no symptoms were observed on control plants. Five fungal isolates were reisolated from disease lesions of inoculated plants on PDA with the same morphological characters as before, and the identity was confirmed by microscopic studies, which fulfilled Koch’s postulates. Lavender web blight caused by R. solani AG-1-IB has been recorded in Italy (Garibaldi et al. 2013). To our knowledge, this is the first disease report of web blight on lavender caused by R. solani AG-1-IB in Korea, which poses a potential threat to lavender production, and our findings will be useful for the development of effective control strategies and further research.