Plant Gene and Trait 2024, Vol.15, No.5, 253-264 http://genbreedpublisher.com/index.php/pgt 254 overview of the role of marker-assisted selection in enhancing disease resistance and yield, thereby aiding in the development of more resilient and productive varieties. 2 Overview of Welsh Onion Diseases 2.1 Common diseases affecting welsh onion Welsh onions (Allium fistulosum L.) are susceptible to a variety of diseases that can significantly impact their yield and quality. Among the most prevalent diseases is Fusarium basal rot (FBR), caused by the soil-borne fungus Fusarium oxysporumf. sp. cepae (FOC). This disease is a major concern for onion production worldwide, leading to severe losses in both yield and bulb quality. The infection typically starts at the basal plate of the onion bulb, progressing upwards and causing rot, which can be particularly devastating during storage (Cramer et al., 2021; Sharma and Cramer, 2023). Another significant disease is gray mold, caused by Botrytis cinerea and Botrytis squamosa. This disease affects the leaves and bulbs, leading to reduced productivity and storage life. Control of gray mold is challenging due to the difficulty in managing the pathogens through physical and chemical methods (Scholten et al., 2016; Kim et al., 2021). Downy mildew, caused by Peronospora destructor, is another critical disease affecting Welsh onions. It is considered one of the most destructive fungal diseases for bulb onions, leading to significant yield losses. The disease manifests as yellowish patches on the leaves, which eventually turn brown and necrotic, severely affecting the photosynthetic capability of the plant (Kim et al., 2016; Khrustaleva et al., 2019). Additionally, Welsh onions are also susceptible to Iris yellow spot virus (IYSV), which is transmitted by onion thrips. This virus causes necrotic lesions on the leaves and scapes, leading to reduced plant vigor and yield (Cramer et al., 2021). 2.2 Pathogens and infection mechanisms The pathogens responsible for these diseases have distinct infection mechanisms. Fusarium oxysporum f. sp. cepae (FOC) infects the onion bulbs through the basal plate, where it colonizes the vascular tissues, leading to rot and eventual plant death. The fungus produces conidia that can survive in the soil for extended periods, making it difficult to eradicate once established (Sharma and Cramer, 2023). Botrytis cinerea and Botrytis squamosa, the causative agents of gray mold, infect the plant tissues through wounds or natural openings. These necrotrophic pathogens thrive in humid conditions and produce spores that can spread rapidly, leading to widespread infection (Scholten et al., 2016; Kim et al., 2021). Peronospora destructor, the pathogen responsible for downy mildew, infects the leaves of Welsh onions by penetrating the stomata. The pathogen produces sporangia that are dispersed by wind and water, facilitating rapid spread under favorable conditions. The infection leads to the formation of characteristic downy growth on the underside of the leaves, which is a key diagnostic feature of the disease (Kim et al., 2016; Khrustaleva et al., 2019). Iris yellow spot virus (IYSV) is transmitted by onion thrips (Thrips tabaci), which acquire the virus while feeding on infected plants and subsequently transmit it to healthy plants. The virus replicates within the thrips and is introduced into the plant tissues during feeding, leading to the development of necrotic lesions (Cramer et al., 2021). 2.3 Economic impact of diseases The economic impact of these diseases on onion production is significant. Fusarium basal rot (FBR) can cause severe yield losses, with infected bulbs becoming unmarketable due to severe decay. This not only affects the current season’s harvest but also has long-term effects on storage and market sales, as infected bulbs may contaminate healthy ones during storage (Cramer et al., 2021; Sharma and Cramer, 2023). The costs associated with managing FBR, including the use of fungicides and soil treatments, add to the economic burden on growers. The inoculation method shown in Figure 1 provides an effective tool for screening disease-resistant varieties, simplifying the inoculation process and increasing spore inoculation efficiency, which accelerates the development of resistant varieties (Cramer et al., 2021). By utilizing this method, reliance on chemical pesticides can be reduced, enhancing the disease resistance of onions, thereby lowering the economic losses caused by disease and promoting the sustainability of the onion industry.
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