نوع مقاله : مروری
نویسندگان
1 مؤسسه تحقیقات اصلاح و تهیه بذر چغندرقند، سازمان تحقیقات آموزش و ترویج کشاورزی، کرج، ایران
2 استادیار مؤسسه تحقیقات اصلاح و تهیه بذر چغندرقند سازمان تحقیقات، آموزش و ترویج کشاورزی، کرج، ایران.
3 بخش تحقیقات چغندرقند، مرکز تحقیقات و آموزش کشاورزی و منابع طبیعی خراسان رضوی، سازمان تحقیقات آموزش و ترویج کشاورزی، مشهد، ایران
4 بخش تحقیقات چغندرقند، مرکز تحقیقات و آموزش کشاورزی و منابع طبیعی همدان، سازمان تحقیقات آموزش و ترویج کشاورزی، همدان، ایران
چکیده
کلیدواژهها
موضوعات
عنوان مقاله [English]
نویسندگان [English]
Introduction
Rhizoctonia root rot is one of the most important diseases limiting sugar beet production, particularly in spring-sown sugar beet in Iran. The disease is mainly caused by the soil-borne fungus Rhizoctonia solani Kühn, especially anastomosis group AG-2-2 IIIB. Its long-term survival in soil as sclerotia and mycelium, together with its broad host range, makes effective disease management difficult. Depending on environmental conditions and pathogen pressure, disease severity may range from minor damage to more than 60% reduction in root yield loss and a 5–10% reduction in extractable white sugar. Under severe disease pressure, the disease can lead to complete crop failure. Symptoms include seedling damping-off during early growth stages and crown and root rot in later growth stages. Therefore, the development and deployment of genetically resistant cultivars represent an essential component of sustainable disease management.
Materials and Methods
A comprehensive review and synthesis of research conducted in Iran and other countries on the genetic resistance of sugar beet to R. solani were undertaken. Studies involving conventional breeding, artificial inoculation, field and greenhouse screening, microplot evaluation, genetic mapping, QTL analysis, and molecular marker development were critically examined. Particular attention was given to the inheritance of resistance, resistant germplasm, evaluation methodologies, development of O-type and pollinator lines, hybrid breeding, and the application of molecular markers for marker-assisted selection. The optimized field microplot method, involving artificial inoculation with R. solani- colonized maize kernels and disease assessment using a 1–9 rating scale, was considered as a reliable approach for screening breeding materials for resistance to Rhizoctonia root rot.
Results and Discussion
Resistance to R. solani in sugar beet is generally considered a quantitative and polygenic trait, with incomplete dominance and the involvement of at least two major genes together with minor genes having variable effects. QTL mapping studies have identified important resistance loci on chromosomes four, five, and seven, which together account for approximately 71% of the observed phenotypic variation. In addition, the SNP marker RsBv1 on chromosome six has been validated across different genetic backgrounds. An allele-specific molecular marker associated with resistance has also recently been developed in Iran, providing an opportunity to accelerate the selection of resistant genotypes.
Reliable and standardized phenotyping is essential for successful resistance breeding. Conventional field trials based on artificial inoculation are affected by environmental variation and are generally limited to one evaluation cycle per year. Greenhouse screening methods, including toothpick inoculation and the use of infected millet or barley seeds, provide controlled conditions for resistance as sessment. Among field-based approaches, the optimized microplot method has demonstrated particular value for precise screening. In this method, plots are inoculated with infected maize kernels colonized by R. solani, regularly irrigated, and evaluated approximately two months after inoculation using a 1–9 disease severity scale. Disease and harvest indices can subsequently be calculated, with roots scoring 1–3 generally classified as resistant. This method is routinely applied at the Hamedan and Kermanshah research stations for screening O-type and pollinator lines.
Breeding efforts in Iran have resulted in the development and identification of resistant full-sib (S1) pollinator lines from resistance-gene-carrying populations available in the gene bank of the Sugar Beet Seed Institute. Screening programs have identified superior resistant lines and base populations. Resistant O-type lines have also been identified under artificial inoculation and incorporated into the production of maternal single crosses. Resistant hybrids are generally developed through three-way crosses involving a resistant pollinator parent and a resistant or partially resistant maternal single-cross parent. The combination of resistance in both parental components is expected to provide more stable resistance in commercial hybrids. Given the diversity of fungal and fungus-like pathogens responsible for root rot in Iranian soils, identifying lines resistant to Rhizoctonia could provide a basis for developing lines with resistance or tolerance to other soil-borne pathogens as well.
Several Iranian cultivars with relative resistance to R. solani have been released, including Ekbatan, in which resistance was mainly contributed by the pollinator parent, as well as Sina, Dena, and Donya. The latter cultivars combine resistance to rhizoctonia root rot and rhizomania with improved yield performance. Because rhizomania is widely prevalent in Iran, cultivars possessing resistance only to rhizoctonia, such as Ekbatan, have experienced limited adoption. Consequently, simultaneous incorporation of resistance to both pathogens is considered an important breeding objective. Among licensed imported cultivars, approximately 22% (60 cultivars) possess resistance to rhizoctonia root rot, and most of these also carry resistance to rhizomania.
Conclusion
The availability of resistant germplasm, standardized phenotyping methods, particularly the microplot technique, and molecular markers has created favorable conditions for the development of rhizoctonia-resistant sugar beet cultivars. Integrating molecular markers with conventional selection can accelerate the identification of resistant genotypes and reduce the time required for cultivar development and release. Given the increasing challenges associated with drought and climate change in spring cultivation and the expansion of autumn sowing, particularly in Khuzestan where late-season root rot has become increasingly important, resistance to R. solani should be incorporated into breeding programs for both spring- and autumn-sown sugar beet. The use of resistant commercial cultivars as sources of resistance genes and the establishment of appropriate resistant gene pools are recommended as key strategies for future breeding programs. Genetic resistance should also be complemented by integrated disease management practices, including appropriate crop rotation, avoidance of highly susceptible preceding crops such as soybean and maize, timely chemical control, and biological control
کلیدواژهها [English]
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