Evaluation of the resistance of foreign sugar beet cultivars to Rhizoctonia solani induced root and crown rot under artificial inoculation conditions

Introduction
Rhizoctonia root and crown rot, caused by the soil-borne fungus Rhizoctonia solani, is one of the major constraints to sugar beet (Beta vulgaris L.) production in Iran. Worldwide, more than twenty fungal species have been identified as causal agents of sugar beet root rot, whereas more than fourteen species have been reported in Iran. Although integrated disease management practices, including crop rotation and fungicide application, can reduce disease incidence and severity, substantial yield losses continue to occur in many sugars’ beet-growing regions. Consequently, the development and cultivation of resistant cultivars have become one of the most effective, economical, and environmentally sustainable strategies for managing Rhizoctonia root and crown rot, providing a practical alternative to intensive chemical and cultural control measures.
Materials and Methods
Six sugar beet cultivars were evaluated under controlled microplot conditions over two consecutive growing seasons (2022 and 2023). The plant materials consisted of four foreign cultivars (F-21444, F-21446, F-21448, and F-21439), one resistant control cultivar (F-21436), and one susceptible Iranian control cultivar (Sharif). The experiment was conducted in a randomized complete block design (RCBD) with four replications. In April of each year, seeds of each cultivar were sown in four 2-m-long rows within the microplots. At the 6-8-leaf stage, plants were artificially inoculated with Rhizoctonia solani isolate 133 using sterilized corn kernels colonized by the pathogen as the inoculum. Approximately one month after inoculation, when disease severity in the susceptible control reached a level suitable for evaluation, roots were harvested and assessed. Disease severity was recorded, and the disease index (DI) was subsequently calculated for each cultivar.
Results and Discussion
According to the analysis of variance, differences among the sugar beet cultivars in root number, disease index (DI), and harvest index (HI) were not significant during the first growing season because of the high disease severity. In the second growing season, root number also did not differ significantly among cultivars, whereas significant differences were detected for both the disease index and harvest index at the 1% significant level (P< 0.01). The mean disease index across all cultivars was 3.60 in the first year and 2.65 in the second year. Among the evaluated cultivars, F-21444 exhibited the lowest disease index, with values of 5.50 and 1.93 in the first and second growing seasons, respectively. In contrast, the susceptible check, cultivar Sharif, showed the highest disease index (6.81 and 3.93), representing approximately 25% and 50% higher values than those of the most resistant cultivar in the first and second growing seasons, respectively. Regarding the harvest index, cultivar F-21446 recorded the highest value in the first year (24.1%), whereas cultivars F-21444 and F-21436 exhibited the highest val ues in the second year (79.6% and 71.9%, respectively). Based on the SIIG index, cultivar F-21444 ranked first in the first growing season, whereas cultivars F-21439 and F-21446 showed the highest SIIG values in the second growing season. Conversely, cultivar F-21448 consistently exhibited the lowest SIIG values in both growing seasons, indicating its high susceptibility to Rhizoctonia solani.
Conclusion
The results showed that the mean disease index across all evaluated cultivars was 3.60 in the first growing season and 2.65 in the second. In the second growing season, significant differences among cultivars were detected for both the disease index and harvest index (P< 0.01). In contrast, no significant differences were observed in the first growing season, most likely because the high disease pressure limited discrimination among cultivars. The susceptible control cultivar, Sharif, exhibited the highest disease index and the lowest harvest index, confirming its high susceptibility to Rhizoctonia solani. Based on the SIIG index, cultivar F-21444 ranked first in the first growing season, whereas cultivars F-21439 and F-21446 achieved the highest SIIG values in the second growing season, indicating superior overall performance under disease pressure. Conversely, cultivar F-21448 consistently recorded the lowest SIIG values in both growing seasons, reflecting its high susceptibility to R. solani. Overall, cultivars F-21444 and F-21439 demonstrated stable performance across growing seasons and may therefore be considered promising sources of resistance for cultivation and future sugar beet breeding programs in areas affected by Rhizoctonia root and crown rot.