Efficacies of different inoculum forms of Rhizoctonia solani AG 2-2IIIB for resistance screening of sugar beet cultivars

Authors: BHUIYAN, ZIAUR - North Dakota State University; LUIS, DEL RÍO MENDOZA - North Dakota State University; Lakshman, Dilip; QI, AIMING - University Of Hertfordshire; KHAN, MOHAMED - North Dakota State University

Submitted to: Journal of Plant Pathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/11/2023
Publication Date: 9/21/2023
Citation: Bhuiyan, Z., Luis, D., Lakshman, D.K., Qi, A., Khan, M. 2023. Efficacies of different inoculum forms of Rhizoctonia solani AG 2-2IIIB for resistance screening of sugar beet cultivars. Journal of Plant Pathology. https://doi.org/10.1007/s42161-023-01485-z.
DOI: https://doi.org/10.1007/s42161-023-01485-z

Interpretive Summary: Sugar beet is an economically important crop in the USA and the soilborne fungal pathogen Rhizoctonia. solani AG 2-2 IIIB is the most damaging anastomosis group in sugar beet production. Cultivation of Rhizoctonia-resistant sugar beet cultivars remains the most practical disease management option. For evaluating cultivar resistance, investigations of various types and economical production of inoculums are critical factors. In this regard, investigations were carried out to determine the most suitable culture medium for the growth of the fungus and the development of sclerotia. Next, three types of Rhizoctonia inoculums, viz., mycelial plugs, sclerotia, and colonized barley grains, were evaluated for maximum disease pressure in greenhouse conditions. The CV8 medium was determined as the best for fungal growth and sclerotia development. Also, Rhizoctonia-colonized barley grain was found economical to produce and generate maximum disease pressure under greenhouse conditions, hence the most suitable inoculum type for germplasm screening experiments. Those findings will be useful to plant pathologists and breeders to screen commercial sugar beet cultivars for resistance against the Rhizoctonia pathogens to formulate effective disease management methods.

Technical Abstract: Sugar beet (Beta vulgaris L.) is one of the major sugar sources in the world. Rhizoctonia solani causes damping-off and crown and root rot that can result in significant yield and economic losses. R. solani AG 2-2 IIIB is the most damaging anastomosis group in sugar beet production. In this study, we evaluated three different types of inocula – barley grains colonized by fungal mycelium (CBG), agar plugs containing fungal mycelia (MAP), and sclerotia (SCL) for their ease of production and efficacy in inducing disease in sugar beet. First, the fungal growth rate and sclerotia production were compared on six types of media, clarified V8 [CV8], potato dextrose agar [PDA], metalaxyl benomyl vancomycin agar [MBV], yeast malt agar [YMA], corn meal agar [CMA], and oatmeal agar [OMA]. The fungus grew faster and produced more sclerotia in CV8 medium than in other media (P<0.05). The rate of fungal growth from CBG, MAP, and SCL was evaluated. The in vitro rate of growth of R. solani was faster when originated from MAP than from SCL (P<0.05) but equal to that from CBG. The different inoculum forms were then used to inoculate seeds at planting and 4-leaf stage sugar beet plants to evaluate the disease incidence and severity. R. solani on CBG caused greater severity. Overall, CBG was the best form of inoculum due to ease of inoculum production, low cost, and ability to consistently cause severe disease symptoms on sugar beet plants.