Efficacy of different inoculum forms of Rhizoctonia solani AG 2-2IIIB for resistance screening of sugar beet varieties

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 inoculums, namely 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 its ease of inoculum production, low cost, and ability to consistently cause severe disease symptoms on sugar beet plants.