Potato (Solanum tuberosum L.) stem canker caused by Rhizoctonia solani occurs in potato-growing regions all over the world and can result in severe losses in crop yield and quality. In late July 2011, potato subterraneous stems with stem cankers composed of brownish, sunken lesions were observed at 15% incidence in seven sites in Jilin Province, northeast China. Samples were collected, and stem pieces (each 5 mm long) taken from the margins of the healthy and diseased tissues were surface-disinfested with 0.5% NaOCl for 2 min, rinsed with sterilized water, dried, then placed on potato dextrose agar at 25°C in the dark. Three (designated JL-3, JL-5-1, and JL-6) of seven Rhizoctonia isolates that developed from single hyphal tip transfers were identified preliminarily as binucleate Rhizoctonia (BNR) isolates (teleomorph Ceratobasidium Rogers). The colonies were white or light gray with fluffy aerial hyphae and no sclerotia after 14 days in culture. Hyphal cells were binucleate when stained with 4′-6-diamidino-2-phenylindole. Average hyphal diameters (mean ± standard deviation) of isolates JL-3, JL-5-1, and JL-6 were 4.8 ± 0.5 μm (range 4.1 to 5.6 μm), 4.4 ± 0.4 μm (range 3.9 to 5.2 μm), and 4.5 ± 0.3 μm (range 4.0 to 5.0 μm), respectively. The internal transcribed spacer (ITS) region of ribosomal DNA was amplified from genomic DNA with primers ITS1 and ITS4 and sequenced. BLASTn analysis indicated that the resulting sequences (GenBank Accession Nos. JX885459, JX885460, and JX885461 for JL-3, JL-5-1, and JL-6, respectively) were 100% identical to that of a Ceratobasidium sp. AG-A isolate CHR08-10 (HQ270171). So the three isolates were identified as BNR AG-A based on morphological and molecular characteristics. To determine pathogenicity of the BNR isolates, potato seed tubers (cv. Favorita), each with 3- to 5-mm-long sprouts, were inoculated with wheat seeds (sterilized by autoclaving twice at 121°C for 1 h with a 24-h interval between autoclavings) colonized with each isolate (1). One sprouted potato tuber was planted in a plastic pot with a single colonized wheat seed placed 10 mm above the uppermost sprout tip in a sand/sawdust mixture (1:2 v/v). Plants were incubated in a glasshouse at 25 to 27°C, and assessed after 21 days. The test was performed on 20 plants/isolate and the experiment was repeated. The incidence of plants inoculated with JL-3, JL-5-1, and JL-6 that developed stem canker symptoms averaged 11.1, 23.5, and 28.6%, respectively, whereas all control plants inoculated with sterilized wheat seeds remained asymptomatic. Rhizoctonia spp. were not reisolated from the control plants, whereas BNR isolates were reisolated consistently from symptomatic stems of the inoculated plants, and the identity confirmed by morphological and molecular characteristics as described above, fulfilling Koch's postulates. BNR AG-A has been reported to be pathogenic on soybean (Glycine max), pea (Pisum sativum), snap bean (Phaseolus vulgaris), and pak choi (Brassica chinensis) in China (4). Isolates of R. solani AG-3 are most often associated with potato stem canker (2), although unidentified BNR isolates were reported to cause mild symptoms on potato sprouts in Finland (1), and small lesions on potato roots and stems in the United Kingdom (3). To our knowledge, this is the first report of BNR AG-A causing potato stem canker in Jilin Province, one of the main potato-producing areas of China.
References: (1) M. J. Lehtonen et al. Plant Pathol. 57:141, 2008. (2) L. Tsror. J. Phytopatology 158:649, 2010. (3) J. W. Woodhall et al. New Dis. Rep. 23:31, 2011. (4) G. H. Yang et al. J. Phytopathology 153:333, 2005.