Root and crown rot of sugar beet, caused by the soil borne fungus Rhizoctonia solani AG2‐2, is one of the most important diseases of this crop causing significant yield losses every year worldwide. We investigated the reaction of four sugar beet genotypes, including Dorothea, Ramona, Sonja and Paulina, to infection with the pathogen in greenhouse conditions. The Plants of each cultivar were infected at 9 weeks after seedling emergence by placing a toothpick, which was colonized by the pathogen, next to the crown of each plant. Disease evaluation was carried out at 7 days after inoculation by estimating the lesion length. Data were analyzed statistically, using the Kruskall‐Wallis multiple comparison tests completed by a Mann‐Whitney comparison test at P = 0.05 using SPSS 12.0 for Windows. Our results revealed that Ramona was the most resistant, whereas Dorothea was the most susceptible cultivar to the pathogen. Using a pharmacological approach and thioglycolic‐acid assay, the host defense mechanisms associated with basal resistance of Ramona genotype was investigated. Accumulation of lignin in the cell walls was found to be one of the defense responses of sugar beet plants against R. solani. Higher level of lignification in the resistant Ramona cultivar was correlated with lower level of infection structure formation on plant surface by the pathogen. Lignin forms a structural barrier that limits the spread of pathogen and prevents the diffusion of extracellular enzymes and RS‐toxin produced by this necrotrophic fungus.

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