evaluation of the resistance of sugar beet breeding lines to rhizoctonia root and crown rot

root and crown rot of sugar beet caused by rhizoctonia solani ag2-2 is a destructive disease in the main sugar beet growing areas. use of resistant cultivars is the most effective and environmentally safe way to control the disease. in order to find resistant genotypes, 70 sugar beet lines together with resistant and susceptible checks were evaluated against a highly virulent isolate of r.solani ag2-2 in the designed micro plots (1*1*2 meter) at ekbatan agricultural research station,hamadan. before seed sowing, manure and fertilizers were added to the micro plots, then disinfected by methyl bromide. the experimental design was a randomized complete block with 42 treatments and two replications. the fungal isolate was grown on corn kernels. ten-week old plants were inoculated with 6-8 corn infected kernels in crown area. the micro plots were watered immediately after inoculation. roots of sugar beet were harvested 80 days after inoculation and disease severity of individual roots were assessed according to a 1-9 scale where 1 represents the healthy plants and 9 represents the dead plants. mean disease ratings of plants in each plot was considered as disease index(di). results of two year mean comparisons and cluster analysis displayed that three lines with respect to disease resistance and 11 lines with respect to harvest index were superior to the resistant check (sb19) and other lines were at the same group as the resistant check. however, one of the lines had a higher disease index than the resistant check.