First Report of Potato Stem Canker Caused by Rhizoctonia solani AG-5 in China.

Potato (Solanum tuberosum L.) is grown worldwide as a major food crop. Potato stem canker is an important disease mainly caused by Rhizoctonia solani AG-3 (4). In 2011, samples of potato stem canker were collected from 26 sites in Heilongjiang Province, northeast China. Stem fragments taken from the margins of the healthy and diseased tissues were surface disinfected with 0.5% NaOCl for 2 min, rinsed with sterile water, then placed on potato dextrose agar (PDA) at 25°C in the dark. Twenty-two fungal isolates taken from single hyphal tips were identified as R. solani based on morphological traits. Colonies were light brown with abundant growth of mycelia and produced brown, irregular sclerotia after 20 days on PDA. Mycelium was branched at right angles with a septum near the branch and a slight constriction at the branch base. Hyphal cells were determined to be multinucleate (five to 13 nuclei per cell) when stained with 4'-6-diamidino-2-phenylindole (DAPI). Anastomosis groups were determined by pairing with reference strains (kindly provided by N. Kondo, Hokkaido University, Japan) (1), and six out of 22 isolates anastomosed with R. solani AG-5. The internal transcribed spacer (ITS) region of rDNA was amplified from genomic DNA of the AG-5 isolates with primers ITS1 and ITS4. The ITS sequences (GenBank Accession Nos. JQ946291 to JQ946296) were 99% identical to R. solani isolate AG-5 ND1 (GenBank Accession No. HQ629863). Therefore, based on molecular characteristics and the anastomosis assay, these six isolates were confirmed to be R. solani AG-5. To determine the pathogenicity of R. solani AG-5 isolates, potato seed tubers (cv. Favorita) with 3- to 5-mm sprouts were inoculated with wheat seeds (sterilized by autoclaving twice at 121°C for 1 h with a 24-h interval) colonized with each isolate (2). Wheat seeds were placed 10 mm above the uppermost sprout tip (one seed per sprout). Plants were incubated in glasshouse conditions maintained at 25 to 27°C. After 3 weeks, all inoculated plants showed symptoms of potato stem canker disease, whereas control plants inoculated with sterilized wheat seeds remained healthy. R. solani AG-5 was consistently reisolated from symptomatic stems, and the identity was confirmed by morphological and molecular characteristics as previously described, fulfilling Koch's postulates. Potato stem canker caused by R. solani AG-5 was previously detected in Australia, South Africa, Finland, and Japan (3). However, to our knowledge, this is the first report of R. solani AG-5 on potato in China. Besides previously reported AGs 1, 3, and 4 implicated in Rhizoctonia disease in China, AG 5 should also be taken into account when designing programs for disease management in potato. References: (1) W. C. Kronland and M. E. Stanghellini. Phytopathology 78:820, 1988. (2) M. J. Lehtonen et al. Plant Pathol. 57:141, 2008. (3) M. J. Lehtonen et al. J. Agric. Food Sci. 18:223, 2009. (4) L. Tsror. J. Phytopathol. 158:649, 2010.