Novel Sources of Sclerotinia Basal Stalk Rot Resistance from Crop Wild Relatives of Sunflower (Helianthus annuus L.).

Sclerotinia sclerotiorum is the causal agent of a serious sunflower disease epidemic worldwide. Among Sclerotinia basal stalk rot, head rot, and mid-stalk rot, the former two are the most damaging, accounting for over 80% of the disease incidence. The genetics of resistance to basal stalk rot (BSR) is quantitative, requiring many genes, which complicates breeding efforts. There are 53 Helianthus species (39 perennial and 14 annual) of sunflower wild crop relatives that represent a considerable amount of genetic diversity available for improvement of cultivated sunflower which has a very narrow genetic base. The objective of the study was to evaluate interspecific germplasm in various stages of breeding in artificially inoculated field trials for BSR incidence at three locations, Carrington, ND, Grandin, ND, and Crookston, MN. Sclerotinia BSR resistance was successfully transferred from three wild annual Helianthus species into cultivated sunflower, with two H. petiolaris, six H. argophyllus, and five H. praecox introgression lines. Whole genome scans using genotyping-by-sequencing was used to detect the presence of the wild introgressions segments in the selected lines. Single nucleotide polymorphism markers revealed the significant presence of wild segments in the cultivated sunflower background located on linkage groups 1, 3, 8, 9, 10, and 11. Additionally, 411 progeny families from crosses of amphiploids, hexaploid, and diploid perennials with cultivated lines were tested for BSR. More than 150 new early generation families of perennial H. hirsutus, H. salicifolius, and H. occidentalis tested in replicated BSR field trials suggested excellent BSR resistance, further confirming successful gene introgression. Six BSR germplasms are scheduled for release based on their higher levels of BSR resistance. Accessions of sunflower crop wild relatives from the USDA-ARS-NPGS genebank continue to contribute specific traits to combat emerging pests and environmental challenges and at the same time preserving them for future generations.