Recent advancement and perspectives of molecular markers for resistance against purple blotch disease in onion (Allium cepa)

Purple blotch, caused by Alternaria porri (Ellis) Cifferi, is one of the most destructive fungal foliar disease of onions globally. Development of cultivars exhibiting enhanced genetic resistance to biotic stressors is the most effective, economical, and environmentally sustainable way of management. Nonetheless, very few efforts have been undertaken to identify or delineate a purple blotch resistant region within the onion genome. A single dominant gene, named ApR1, has been identified as responsible for conferring resistance to purple blotch disease. Markers like SSRs (AcSSR7) STS (ApR-450), and SNPs (ApRsnip5, ApRsnip21, and ApRsnip23) have been documented being associated with resistance genes across various mapping populations. Genetic markers associated with the disease resistance gene enhance the breeding process, facilitating the implementation of Marker Assisted Selection (MAS) to achieve comprehensive purple blotch resistance in onions. For the complete understanding of the disease resistance, transcriptomic investigation is necessary to identify purple blotch resistant genes by gene annotation and mining, for crop enhancement.