Gerald J. Seiler

Sunflower (Helianthus L.) production ranked third in global oilseed production in 2021. Downy mildew (DM), caused by the oomycete pathogen Plasmopara halstedii (Farl.) Berl. et de Toni, is a major constraint for worldwide sunflower production, causing significant yield losses in susceptible hybrids. The search for new sources of resistance to DM is the primary objective of all sunflower breeding programs. Germplasms HA‐DM15 (Reg. no. GP‐385, PI 702359) and HA‐DM16 (Reg. no. GP‐386, PI 702360) were developed by introgressing two new DM resistance genes, Pl₃₇ and Pl₃₈, from the wild sunflower species H. annuus and H. praecox into oilseed sunflower, respectively. The BC₁F₃‐derived HA‐DM15 harboring the Pl₃₇ gene was selected from the cross of HA 89//NMS HA 89/wild H. annuus PI 435417. The germplasm line HA‐DM16 carrying the Pl₃₈ gene is an improved oilseed derivative of Rf PRA‐417 with DM resistance originally derived from the wild H. praecox PRA‐417 from the cross of cytoplasmic male sterile (CMS) HA89*2/wild H. praecox PRA‐417. HA‐DM15/Pl₃₇ and HA‐DM16/Pl₃₈ are immue to the most predominant and virulent DM races idenfied in North America and Europe. Pl₃₇ and Pl₃₈ have been genetically mapped to sunflower chromosomes 4 and 2, respectively, with single nucleotide polymorphism markers linked to the both genes facilitating their transfer to elite sunflower lines by marker‐assisted selection, providing new and diverse sources for sunflower DM‐resistance breeding.

Rust and downy mildew (DM) are detrimental diseases in global sunflower (Helianthus annuus L.) production. Disease often results in yield loss and reduced seed quality. Host resistance to DM and rust is mediated by single dominant genes in sunflower. To better utilize genetic resistance, gene pyramiding is a common practice in crop breeding to extend the durability and longevity of resistance. Two confection sunflower germplasm lines, HA‐R20 (Reg. no. GP‐387, PI 702361) and HA‐R21 (Reg. no. GP‐388, PI 702362), were developed using the pedigree breeding method and DNA marker‐assisted selection and released by the USDA‐ARS Sunflower and Plant Biology Research Unit in collaboration with the North Dakota Agricultural Experiment Station in September 2022. HA‐R20 is an F₃‐derived F₄ confection restorer selection from the cross of HA‐DM2 and HA‐R8, carrying rust resistance genes R₁₂ and R₁₅ and DM resistance gene Pl_Arg. HA‐R21 is an F₃‐derived F₄ confection maintainer selection from the cross of HA‐DM3 and HA‐R8, carrying rust resistance genes R_13a and R₁₅ and DM resistance gene Pl₁₇. Disease reaction and marker testing for DM and rust confirmed that both germplasms harbor DM and rust R genes in the homozygous condition and are resistant to all DM and rust races identified in North America to date. These multiple disease‐resistant lines will provide the sunflower industry with durable disease‐resistant sources for the continued improvement of the crop.

Downy mildew (DM) and rust are two major global sunflower ( Helianthin annuus L.) diseases causing significant yield losses and reducing seed quality. Host plant resistance mediated by dominant race‐specific genes has been extensively used in sunflower production to control these diseases. However, the considerable variability of the DM and rust pathogens caused by mutation or recombination has changed the dynamics of the diseases, significantly increasing their incidence in recent years. This necessitates the development and release of sunflower germplasm with enhanced levels of disease resistance. Germplasm lines HA‐DM12 (Reg. no. GP‐382, PI 700006), HA‐DM13 (Reg. no. GP‐383, PI 700007), and HA‐DM14 (Reg. no. GP‐384, PI 700008), all with multiple DM and rust resistance, were developed using phenotypic evaluation and marker‐assisted selection. The three lines share a common DM gene ( Pl _Arg ) and a rust gene ( R ₁₂ ) and a combination of different DM genes ( Pl ₈ , Pl ₁₇ , and Pl ₁₈ , respectively). The triple‐gene pyramids—HA‐DM12 ( Pl ₈ Pl ₈ / Pl _Arg Pl _Arg / R ₁₂ R ₁₂ ), HA‐DM13 ( Pl ₁₇ Pl ₁₇ / Pl _Arg Pl _Arg / R ₁₂ R ₁₂ ), and HA‐DM14 ( Pl ₁₈ Pl ₁₈ / Pl _Arg Pl _Arg / R ₁₂ R ₁₂ )—confer comprehensive resistance to all races of DM and rust identified so far and can be used in sunflower breeding programs to develop hybrids with durable resistance to DM and rust.

Wild Helianthus species have been undeniably beneficial in sustaining the sunflower crop by providing plant breeders with a diverse genetic pool of potentially useful traits. Exploration to collect populations of wild sunflowers is one of the more difficult and challenging activities in the conservation and utilization of these valuable genetic resources. The logistics of collecting requires careful planning, locating the target species, obtaining permission to access and collect, and timing the exploration to ensure the availability of mature seed. The US Department of Agriculture, Agricultural Research Service (USDA-ARS) established the wild Helianthus seed collection in 1976 at Bushland, Texas with the goal of collecting and conserving the broadest representative genetic diversity possible and serving as a central repository of germplasm and related information. In 1985 this collection was transferred to UDSA-ARS, North Central Regional Plant Introduction Station, Ames, Iowa. Over the last half century, 37 explorations were undertaken covering 175,000 km to collect the 53 Helianthus species from their distributional ranges in the forty-eight conterminous states in the US, three Canadian Providences (Manitoba, Saskatchewan, and Alberta), Argentina and Australia. The many explorations have created a global crop wild relatives (CWR) genebank collection. The current wild CWR sunflower genebank contains 2562 accessions of 53 species with 1065 wild Helianthus annuus accessions (42 %), 617 accessions representing populations of the 13 other wild annual species (24 %), and 880 accessions representing 39 perennial species (34 %). This collection is the largest and most genetically diverse ex situ sunflower collection in the world and is vital to the conservation of wild sunflower species for the global sunflower community.

Key message Two new downy mildew resistance genes, Pl₃₇ and Pl₃₈, were introgressed from wild sunflower species into cultivated sunflower and mapped to sunflower chromosomes 4 and 2, respectively Abstract Downy mildew (DM), caused by the oomycete pathogen Plasmopara halstedii (Farl.) Berl. & de Toni, is known as the most prevalent disease occurring in global sunflower production areas, especially in North America and Europe. In this study, we report the introgression and molecular mapping of two new DM resistance genes from wild sunflower species, Helianthus annuus and H. praecox, into cultivated sunflower. Two mapping populations were developed from the crosses of HA 89/H. annuus PI 435417 (Pop1) and CMS HA 89/H. praecox PRA-417 (Pop2). The phenotypic evaluation of DM resistance/susceptibility was conducted in the BC₁F₂-derived BC₁F₃ populations using P. halstedii race 734. The BC₁F₂ segregating Pop1 was genotyped using an Optimal GBS AgriSeq™ Panel consisting of 768 mapped SNP markers, while the BC₁F₂ segregating Pop2 was genotyped using a genotyping-by-sequencing approach. Linkage analysis and subsequent saturation mapping placed the DM resistance gene, designated Pl₃₇, derived from H. annuus PI 435417 in a 1.6 cM genetic interval on sunflower chromosome 4. Pl₃₇ co-segregated with SNP markers SPB0003 and C4_5738736. Similarly, linkage analysis and subsequent saturation mapping placed the DM resistance gene, designated Pl₃₈, derived from H. praecox PRA-417 in a 0.8 cM genetic interval on sunflower chromosome 2. Pl₃₈ co-segregated with seven SNP markers. Multi-pathotype tests revealed that lines with Pl₃₇ or Pl₃₈ are immune to the most prevalent and virulent P. halstedii races tested. Two germplasm lines, HA-DM15 with Pl₃₇ and HA-DM16 with Pl₃₈, were developed for use in sunflower DM-resistance breeding.

Downy mildew (DM), caused by the obligate pathogen Plasmopara halstedii (Farl.) Berl. & de Toni, is a worldwide yield‐limiting disease of sunflower (Helianthus annuus L.). To obtain durable DM resistance in sunflower hybrids, there is a need to stack new resistance genes into the best available germplasm. The DM resistance genes Pl₈, Pl₁₇, and Pl₁₈ were recently fine mapped to sunflower chromosomes 13, 4, and 2, respectively. The current study pyramided these genes into a single genotype by marker‐assisted selection. Three oilseed sunflower germplasms, HA‐DM9 (Reg. no. GP‐381, PI 700009), HA‐DM10 (Reg. no. GP‐379, PI 700004), and HA‐DM11 (Reg. no. GP‐380, PI 700005), are dual DM‐resistant germplasm lines developed and have been released by the USDA‐ARS, Sunflower and Plant Biology Research Unit in collaboration with the North Dakota Agricultural Experiment Station in 2021. The three lines, HA‐DM9 (Pl₈Pl₈/Pl₁₇Pl₁₇), HA‐DM10 (Pl₈Pl₈/Pl₁₈Pl₁₈), and HA‐DM11 (Pl₁₇Pl₁₇/Pl₁₈Pl₁₈), each stacked two different DM genes, providing broad resistance to all P. halstedii races identified in North America and Europe so far, and can be used in sunflower breeding programs to enhance DM resistance.

The effects of prolonged storage on the germination and viability of seeds of sunflower crop wild relatives stored under less-than-optimal conditions in working genebanks is not known. Seeds of two wild annual species, Helianthus annuus (common wild sunflower) and annual H. petiolaris (prairie sunflower), were stored at room temperature (20 to 22°C) and low humidity of ~ 22% in the laboratory for 38 years. Germination of stored annual sunflower seeds was 5.5%, while that of prairie sunflower seeds was 0.75%, compared with the germination of freshly harvested seeds of 34.7 and 18.5%, respectively. Tetrazolium, a vital stain, indicated that the seeds were alive. The viability equations were used to predict final viability of the stored seeds. Using the three tests of viability (germination with and without gibberellic acid, and tetrazolium staining), all appeared to underestimate the actual condition of the stored seeds of annual sunflower, but provided better predictions for prairie sunflower. This study indicated that wild sunflower seeds can be stored at less than ideal conditions in working genebanks where viability decreases over time reducing germination, but gibberellic acid treatment can sustainably increase germination to produce a number of plants to maintain the genetic integrity of the original stored seeds often used in long-term prebreeding programmes.

Sunflower rust, incited by the fungus Puccinia helianthi Schwein., is a serious fungal disease in the sunflower (Helianthus annuus L.) growing areas worldwide, with an increasing importance in North America in recent years due to the frequent evolution of new pathogen races. HA‐R18 (Reg. no. GP‐377, PI 698197) and HA‐R19 (Reg. no. GP‐378, PI 698198) are rust‐resistant germplasm lines that were developed and released cooperatively in 2020 by the USDA‐ARS, Sunflower and Plant Biology Research Unit and the Agricultural Experiment Station of North Dakota State University. The rust resistance in HA‐R18 and HA‐R19 originated from KP193 and KP199, respectively, which were identified segregating for rust resistance after screening of 58 sunflower lines introduced from South Africa. Rust evaluation of HA‐R18 and HA‐R19 with 11 P. helianthi races indicated that both lines exhibited resistance to the most rust races tested including the most predominant and the most virulent races currently identified in the United States. HA‐R18 and HA‐R19 possess the new rust resistance genes R₁₇ and R₁₈, respectively, both of which have been mapped to sunflower chromosome 13. The two new lines will diversify the pool of rust resistance genes and should serve as useful sources of rust resistance in sunflower.

Sclerotinia head rot (HR), caused by Sclerotinia sclerotiorum, is an economically important disease of sunflower with known detrimental effects on yield and quality in humid climates worldwide. The objective of this study was to gain insight into the genetic architecture of HR resistance from a sunflower line HR21 harboring HR resistance introgressed from the wild perennial Helianthus maximiliani. An F₂ population derived from the cross of HA 234 (susceptible-line)/HR21 (resistant-line) was evaluated for HR resistance at two locations during 2019–2020. Highly significant genetic variations (p < 0.001) were observed for HR disease incidence (DI) and disease severity (DS) in both individual and combined analyses. Broad sense heritability (H²) estimates across environments for DI and DS were 0.51 and 0.62, respectively. A high-density genetic map of 1420.287 cM was constructed with 6315 SNP/InDel markers developed using genotype-by-sequencing technology. A total of 16 genomic regions on eight sunflower chromosomes, 1, 2, 10, 12, 13, 14, 16 and 17 were associated with HR resistance, each explaining between 3.97 to 16.67% of the phenotypic variance for HR resistance. Eleven of these QTL had resistance alleles from the HR21 parent. Molecular markers flanking the QTL will facilitate marker-assisted selection breeding for HR resistance in sunflower.