Date Published: March 1, 2019
Publisher: Public Library of Science
Author(s): Zahirul I. Talukder, Yunming Long, Gerald J. Seiler, William Underwood, Lili Qi, Sujan Mamidi.
Sclerotinia basal stalk rot (BSR) and downy mildew are major diseases of sunflowers worldwide. Breeding for BSR resistance traditionally relies upon cultivated sunflower germplasm that has only partial resistance thus lacking an effective resistance against the pathogen. In this study, we report the transfer of BSR resistance from sunflower wild species, Helianthus praecox, into cultivated sunflower and molecular assessment of the introgressed segments potentially associated with BSR resistance using the genotyping-by-sequencing (GBS) approach. Eight highly BSR-resistant H. praecox introgression lines (ILs), H.pra 1 to H.pra 8, were developed. The mean BSR disease incidence (DI) for H.pra 1 to H.pra 8 across environments for four years ranged from 1.2 to 11.1%, while DI of Cargill 270 (susceptible check), HA 89 (recurrent parent), HA 441 and Croplan 305 (resistant checks) was 36.1, 31.0, 19.5, and 11.6%, respectively. Molecular assessment using GBS detected the presence of H. praecox chromosome segments in chromosomes 1, 8, 10, 11, and 14 of the ILs. Both shared and unique polymorphic SNP loci were detected throughout the entire genomes of the ILs, suggesting the successful transfer of common and novel introgression regions that are potentially associated with BSR resistance. Downy mildew (DM) disease screening and molecular tests revealed that a DM resistance gene, Pl17, derived from one of the inbred parent HA 458 was present in four ILs. Introgression germplasms possessing resistance to both Sclerotinia BSR and DM will extend the useful diversity of the primary gene pool in the fight against two destructive sunflower diseases.
Cultivated sunflower (Helianthus annuus L.) is an important oilseed and confection crop worldwide. Fungal diseases caused by Sclerotinia sclerotiorum are of concern in sunflower production in the United States, as well as other parts of the world causing millions of dollars of crop losses annually . S. sclerotiorum causes infection of sunflower plants at any growth stage and results in so-called Sclerotinia wilt or basal stalk rot (BSR), mid-stalk rot (MSR), and head rot (HR) diseases . Sclerotinia BSR and HR are the most common sunflower diseases in the humid temperate production areas of Argentina, Europe, China, and the Northern Great Plains, where most of the U.S. sunflower crop is grown. The mode of infection for the two diseases differs. BSR is initiated by root infection from mycelia (unique to sunflower), while HR is caused by germination of airborne ascospores on sunflower capitula. Despite the common causal agent, the inheritance of resistance to Sclerotinia BSR and HR in sunflower is different based on the lack of a relationship between the two forms of the diseases [3,4]. Therefore, specialized screening nurseries and inoculation procedures are required for breeding of the two forms of Sclerotinia resistance, which effectively doubles the effort to combat the loss caused by the fungus.
In the present study, we used five highly BSR-resistant H. praecox accessions, one from subspecies praecox and two each from subspecies runyonii and hirtus to transfer BSR resistance into the cultivated sunflower. As predicted, the F1 hybrid seed set was very low for each cross (Table 1). In earlier studies, fewer than expected seed sets were reported in F1 interspecies hybrids between cultivated sunflower and the wild H. praecox subspecies due to the meiotic chromosomal aberrations [48,49]. Although our crossing program began with five highly BSR-resistant H. praecox accessions from three subspecies, we ended up with segregating progenies only from the cross involving PI 468853 H. praecox subsp. runyonii. Infertility of the segregating generations or reduced recombination between the chromosomes of the wild H. praecox subsp. praecox and H. praecox subsp. hirtus and cultivated sunflower might have eliminated progenies for BSR resistance evaluation.