Plant Gene and Trait 2024, Vol.15, No.5, 253-264 http://genbreedpublisher.com/index.php/pgt 255 Figure 1 Inoculation method for Fusarium basal rot (FBR) resistant onion bulbs (Adapted from Cramer et al., 2021) Image caption: (a) FOC (Fusarium oxysporum f.sp. cepae) culture inoculated after cutting the basal plate of the onion bulb; (b) Special potato dextrose agar (SPDA) medium containing FOC conidia suspension; (c) Onion basal plate inoculated using an SPDA plug; (d) Onion bulbs initially incubated under high humidity conditions; (e) Visual assessment of FBR infection after 20 days, categorized into four levels based on infection severity: 0%, 11-20%, 41-50%, and over 70% (Adapted from Cramer et al., 2021) Similarly, gray mold disease caused by Botrytis species results in reduced productivity and storage life of onions, leading to financial losses for farmers. The difficulty in controlling this disease through conventional methods further exacerbates the economic impact (Scholten et al., 2016; Kim et al., 2021). Downy mildew caused by Peronospora destructor is another disease with significant economic implications. The disease can cause severe yield losses, particularly in wet and humid conditions that favor its spread. The cost of managing downy mildew, including the use of resistant cultivars and fungicides, adds to the production costs for growers (Kim et al., 2016; Khrustaleva et al., 2019). Iris yellow spot virus (IYSV) also poses a significant economic threat, as it reduces plant vigor and yield, leading to lower marketable produce. The management of IYSV involves controlling the vector, onion thrips, which can be challenging and costly (Cramer et al., 2021). Overall, the combined impact of these diseases necessitates the development of effective disease management strategies, including the use of resistant cultivars and integrated pest management practices, to mitigate their economic impact on Welsh onion production. 3 Marker-Assisted Breeding Techniques 3.1 Principles of marker-assisted breeding Marker-assisted breeding (MAB) is a modern plant breeding technique that utilizes molecular markers to select desirable traits in crops. This method significantly accelerates the breeding process by allowing for the early identification of plants that carry beneficial genes, thus reducing the time and resources needed for traditional breeding methods. MAB relies on the identification of genetic markers that are closely linked to the traits of interest, such as disease resistance, yield, and quality. These markers can be used to screen large populations of plants, ensuring that only those with the desired genetic makeup are selected for further breeding. The principles of MAB involve several key steps: identifying and validating molecular markers associated with the traits of interest, developing high-throughput screening methods, and integrating these markers into breeding programs. For instance, in onion breeding, SNP markers have been identified and validated for their association with disease resistance, which can be used to introgress resistance genes from related species into onion cultivars (Scholten et al., 2016; Kim et al., 2021). This approach not only enhances the efficiency of breeding programs but also enables the development of new cultivars with improved traits in a shorter time frame. 3.2 Types of molecular markers Simple Sequence Repeats (SSRs), also known as microsatellites, are short, repetitive DNA sequences that are highly polymorphic and distributed throughout the genome. SSR markers are widely used in plant breeding due to
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