Mapping quantitative trait loci (QTLs) for resistance to downy mildew and powdery mildew in cucumber (Cucumis sativus L.)

Cucumber (Cucumis sativus L.) is a significant vegetable with a small genome size (367 Mbp/ genome). Cucumber is susceptible to various bacterial, fungal and viral diseases with downy and powdery mildew being the two most economically significant foliar diseases worldwide among all biotic stresses. Genetic resistance is considered one of the most eco-friendly and cost-effective approach to disease management in crop plants. In this review we have summarized all the QTLs or genes identified for downy (DM) and powdery mildew (PM) resistance linked with numerous genetic loci distributed completely within the seven chromosomes of cucumber. Chromosome 5 has been identified as a hotspot for both DM and PM resistance genes/ QTLs based on previous studies. Most DM and PM resistance genes are closely linked wherein PM resistance genes are potentially being crucial contributors to DM resistance. Through fine mapping, transcriptomics, and proteomics studies, a number of candidate genes, related proteins and various molecular processes related to DM and PM resistance have been discovered in cucumber. Recessive inheritance was commonly described for both DM and PM resistance, often associated with loss of function in plant susceptible (S) genes. Identifying more closely linked markers, flaking markers and QTLs/ genes could be a useful tool for recessively inherited resistance to downy and powdery mildew diseases in cucumber for marker-assisted breeding programs.