Plant Gene and Trait 2024, Vol.15, No.5, 253-264 http://genbreedpublisher.com/index.php/pgt 258 markers associated with resistance to gray mold disease in onions has facilitated the selection of resistant lines, significantly reducing the breeding period (Scholten et al., 2016; Kim et al., 2021). Additionally, MAS has been successfully applied in breeding programs to develop varieties resistant to various diseases, such as Fusarium basal rot (Sharma and Cramer, 2023) and Botrytis squamosa (Scholten et al., 2016). Backcross breeding is another effective strategy for enhancing disease resistance. This method involves crossing a resistant donor parent with a recurrent parent, followed by repeated backcrossing with the recurrent parent while selecting for the resistance trait. Marker-assisted backcross breeding further refines this process by using molecular markers to track the presence of resistance genes, ensuring the retention of desirable traits from the donor parent while maintaining the genetic background of the recurrent parent (Pathania et al., 2017). This approach has been instrumental in developing disease-resistant varieties with improved agronomic performance. 5.2 Crossbreeding and selection Crossbreeding and selection involve hybridizing different plant varieties to combine desirable traits, such as disease resistance and high yield. This method has been particularly useful in introducing resistance genes from related species into cultivated varieties. For example, the introgression of resistance genes from Allium roylei and Allium fistulosum into onion cultivars has been facilitated by the identification and validation of SNP markers, enabling efficient selection of resistant hybrids (Scholten et al., 2016). This approach not only enhances disease resistance but also broadens the genetic base of the breeding population, contributing to the development of more resilient varieties. 5.3 Field Trials and performance evaluation Field trials and performance evaluation are critical components of the breeding process, providing essential data on the effectiveness of selected traits under real-world conditions. These trials involve testing the performance of breeding lines in various environments to assess their resistance to diseases and overall agronomic performance. For instance, artificial inoculation and mature bulb screening have been used to evaluate the resistance of onion cultivars to Fusarium basal rot, leading to the identification of advanced selections with improved resistance (Figure 2) (Sharma and Cramer, 2023). Such evaluations ensure that the developed varieties not only possess disease resistance but also meet the agronomic and quality standards required for commercial production. Figure 2 Artificial inoculation method for selecting Fusarium basal rot (FBR) resistant onion bulbs (Adapted from Sharma and Cramer, 2023) Image caption: (A) Culture of Fusarium oxysporum f. sp. cepae (FOC); (B) Artificial inoculation process where PDA (potato dextrose agar) medium containing spores is inserted into the transversely cut basal plate of the onion bulb; (C) Incubation process of the inoculated onion bulb, with high humidity conditions within 24 hours promoting disease development; (D) A susceptible onion bulb scored at 9, showing pronounced symptoms of basal rot (Adapted from Sharma and Cramer, 2023)
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