First Report of Rhizoctonia solani AG 11 Causing Stem Rot of Phaseolus vulgaris in Idaho

Abstract

In July 2018, bean plants (cv. Viva pink) at one location in south central Idaho were observed to have reduced growth with approximately 20% of the plants affected over 1 ha. Visual assessment showed that the below-ground parts of stems consistently displayed brown lesions up to 2 cm in length, which sometimes girdled the entire plant. To determine the causal agent, affected material (approximately 5 mm³) was surface disinfested in sodium hypochlorite (2%) for 30 s, rinsed twice in sterile water, and plated onto water agar medium amended with penicillin G (0.2 g/liter) and streptomycin sulfate (0.8 g/liter). After 3 days, colonies of Rhizoctonia solani were identified based on the presence of relatively broad (8 to 12 μm), septate hyphae with right-angle branching. Hyphal tips of R. solani colonies were transferred to potato dextrose agar (PDA) and grown for 2 weeks at 21°C. DNA was then extracted from hyphae taken from a single representative isolate (DB23). Anastomosis group (AG) was identified by ribosomal DNA ITS sequencing of isolate DB23. DNA was extracted and sequenced as previously described (Woodhall et al. 2013). The resulting sequence (GenBank no. MT256181) was over 99% identical with isolates previously identified as R. solani AG 11 (LC215403). To confirm pathogenicity, 2-week-old pinto bean plants (cv. Windbreaker) were each inoculated by placing a fully colonized 10-mm² plug taken from a 2-week-old PDA culture of isolate DB23 next to the stem 10 mm below the soil surface. Sterile PDA plugs were placed next to the stems of control plants. Each treatment was replicated five times. Plants were grown in a greenhouse at 21°C in potting compost (Scotts, Marysville, OH) under light for 16 h a day. After 5 weeks, each plant was assessed for disease using a previously described stem disease severity scale (Carling and Leiner 1990). No symptoms were present in the control plants. Plants inoculated with isolate DB23 had an average stem disease severity score of 2.6 (from a maximum of 4), and R. solani was consistently reisolated, thereby confirming Koch’s postulates. To our knowledge, this is the first report of R. solani AG 11 in Idaho on any host. Previously, R. solani AG 11 has been reported on Phaseolus vulgaris in Turkey (Eken and Demirci 2004) and on lily in Japan (Misawa et al. 2017). AG 11 has also been associated with soybeans in Arkansas (Ajayi-Oyetunde and Bradley 2017; Carling et al. 1994) and Illinois (Ajayi-Oyetunde and Bradley 2017). AG 11 has also been reported on rice in Arkansas (Carling et al. 1994; Spurlock et al. 2016) and sweet corn in New York (Ohkura et al. 2009). In western Australia, it was found on wheat and lupin, and these isolates were pathogenic to potatoes in controlled environment studies (Carling et al. 1994). Idaho is a major producer of wheat, potatoes, and dry beans; the presence of soilborne R. solani AG 11 also needs to be considered when diagnosing and managing Rhizoctonia diseases of these crops.