Breeding and genetics of resistance to major diseases in Cucurbita—A review

Cucurbita crops, which include market types of pumpkin and squash, have unparalleled fruit variation and equally important economic value worldwide. Pumpkin and squash have versatile uses but are most popular in culinary, ornamental, snacking, and seed oil industries. The production of Cucurbita crops is hindered by diseases caused by fungal, oomycetes, and viral pathogens. Host resistance is an important component of integrated disease management for Cucurbita crops and is a major goal for plant breeders. This review addresses the major diseases of Cucurbita, including powdery mildew, downy mildew, Phytophthora rot, and aphid and whitefly transmitted viruses, with an emphasis on germplasm exploitation for the development of resistant cultivars. Resistance to powdery mildew derived from Cucurbita okeechobeensis subsp. martinezii (designated PM-0) has been extensively deployed in commercial cultivars and was recently mapped on chromosomes 3 and 10 of Cucurbita moschata and Cucurbita pepo, respectively. Resistance to Phytophthora crown rot is present across several Cucurbita species, including Cucurbita lundelliana, C. okeechobeenesis subsp. okeechobeenesis, C. moschata, and C. pepo. Mapping studies have identified significant loci associated with Phytophthora crown rot resistance on chromosomes 4, 11, and 20 of C. moschata and chromosomes 4, 5, 8, 12, 13, 16, and 19 of C. pepo. Sources of resistance to aphid-transmitted viruses exist in C. moschata, Cucurbita ficifolia, Cucurbita ecuadorensis, Cucurbita martinezii, C. ecuadorensis, Cucurbita maxima, and Cucurbita foetidissima. The availability of DNA markers linked to resistance against zucchini yellow mosaic virus and papaya ringspot virus in C. moschata and C. pepo has facilitated marker-assisted selection (MAS) in breeding programs. On the other hand, sources of resistance to tomato leaf curl New Delhi virus (ToLCNDV), a major whitefly-transmitted virus in Cucurbita, have been identified in C. moschata, C. lundelliana, and C. okeechobeensis. A major locus conferring resistance to ToLCNDV in C. moschata was recently mapped on chromosome 8 enabling the application of MAS with a prediction accuracy of 94.3%. Overall, the continued discovery and application of genomic tools for resistance breeding in Cucurbita will accelerate the rate of genetic gain while reducing costs associated with phenotyping.