Comparison of different methods for evaluation of resistance to Rhizoctonia root and crown rot in selected genotypes of sugar beet

A comparative study was conducted to find a reliable and simple method for evaluation of resistance of sugar beet genotypes to Rhizoctonia root and crown rot. For this, 12 sugar beet genotypes were used in greenhouse, field and in vitro experiments. The field experiment was performed in micro plots (1×1×2 meter) using randomized complete block design with 4 replications. Prior to planting, the micro plots were disinfested with methyl bromide. Twelve weeks sugar beet plants were inoculated with a highly pathogenic isolate of R. solani AG2-2. Disease severity was measured 12 weeks after inoculation based on a 0-7 scale. Under the greenhouse conditions, evaluation of genotypes resistance at seedling and adult plant stages were conducted in two separate experiments. In the seedling stage, germinated seeds of the genotypes were inoculated using inoculum's layer technique and mortality percent of seedlings was recorded till 5 weeks after planting. In the adult plant stage, inoculation was carried out 8 weeks after transplanting sugar beet seedlings into large plots. The experiment was conducted in a completely randomized design with 3 replicates and disease severity was measured three weeks after inoculation. In the detached root method, the genotypes were sown under field conditions. Seven months after planting, 30 roots of each genotype were randomly selected and detached root slices of one cm in diameter from each root were prepared in laboratory conditions. The detached roots were put in sterilized 10 cm Petri plates and then inoculated by 8 mm pieces of the actively growing colony of Rhizoctonia isolate. Seven days after inoculation, the diameter of rotting tissue on each detached root was measured. The in vitro test was performed in a completely randomized design with 5 replicates using 10 cm petri plates. For this, germinated seeds of the genotypes were placed on the periphery of an actively growing colony of R. solani on Water Agar (WA) medium and five days later the disease severity of seedlings was measured on the basis of a 0-4 scale. In order to assess the correlation among the five evaluation methods, Pearson Correlation Coefficient (PCC) was calculated. Results of the seedling-stage method in greenhouse as well as detached root method showed poor correlation with the other evaluating methods. The results of field, greenhouse and in vitro experiments exhibited positive and significant correlations. Significant correlations were also found between in vitro test with greenhouse (0.862) and field evaluation (0.685;), respectively using PCC test. Similar results were obtained from greenhouse and field evaluation (0.912) methods. Based on the results, the in vitro test is recommended as a simple and reliable method for screening sugar beet germplasms against R. solani root rot.