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ARS Home » Plains Area » Fargo, North Dakota » Edward T. Schafer Agricultural Research Center » Sunflower and Plant Biology Research » Research » Research Project #424876

Research Project: Sunflower Genetic Improvement with Genes from Wild Crop Relatives and Domesticated Sunflower

Location: Sunflower and Plant Biology Research

2017 Annual Report


1a. Objectives (from AD-416):
Objective 1: Identify gaps and acquire new wild species to fill gaps or deficiencies in the sunflower germplasm collection. Objective 2: Identify insect pests and pathogens of sunflower, develop effective screening methods to optimize assessment of resistance to sunflower pathogens, determine mechanisms of plant resistance, phenotype germplasm for resistance to major insect pests and pathogens, and introgress insect and disease resistance genes from the wild species into diverse cultivated germplasm. Subobjective 2A: Identify and monitor pathogens. Subobjective 2B: Develop effective screening procedures for Phomopsis and insect damage. Subobjective 2C: Identify and assess mechanisms of insect resistance. Subobjective 2D: Transfer disease resistance, insect resistance, and other agronomic traits from wild Helianthus species into cultivated sunflower and improve methods for interspecific hybridization and release of pre-breeding germplasm. Subobjective 2E: Characterize and map resistance to pathogens and insect pests, and other agronomic traits. Objective 3: Develop sunflower germplasm with high yield, high oil content, and desirable fatty acid concentrations, as well as novel resistance genes for diseases and insects. Subobjective 3A: Develop new inbred lines with novel fatty acid compositions, such as low saturated fatty acids, and high oleic acid. Subobjective 3B: Pyramid disease and insect resistance genes with high yield and oil content in a common germplasm.


1b. Approach (from AD-416):
Currently, there are a number of factors that reduce sunflower yield including a host of insects and diseases that require careful and costly management practices, reducing profitability. Research is proposed to reduce the input costs by developing durable pest resistance, herbicide resistance, oil content and quality increasing the oil per acre yield of sunflower. Specifically, we will collect wild sunflower relatives to broaden the crop’s genetic base. This germplasm will be phenotyped for resistance to major insect pests and pathogens, cytoplasmic male sterility, and fertility restoration. Methods for improving interspecific hybridization will focus on techniques to detect introgressed alien chromosome segments in interspecific crosses using the genomic fluorescence in situ hybridization technique. Interspecific gene transfer will be evaluated using molecular markers for desirable agronomic traits such as resistance genes to rust and downy mildew. Interspecific germplasm with useful genes will be introgressed into cultivated sunflower and released as enhanced pre-breeding germplasm. Current diseases will be monitored for shifts in virulence and races, and for newly emerged diseases. A field test will be developed to reliably test for the newly emerged Phomopsis stem canker pathogen. An efficient non-destructive screening method will be developed for detecting damage of banded sunflower moth, sunflower moth, red sunflower seed weevil, and sunflower stem weevil. Insect resistance mechanisms will be identified and assessed for sunflower moth. Resistance genes for pathogens and insects, and other agronomic traits will be characterized and mapped. DNA markers for selected traits will be developed and used for marker-assisted breeding. Enhanced sunflower germplasm with high yield, high oil content, and desirable fatty acids concentration, as well as novel resistance genes for diseases and insects will be developed and released. Accomplishing these objectives will provide producers with improved sunflower that will provide a stable supply of high quality oil and confectionery sunflower, improving on-farm profitability and providing the consumer with a reliable domestic supply of a healthy oil, a staple in the American diet.


3. Progress Report:
Objective 1-Identify gaps and acquire new wild species to fill gaps or deficiencies in the sunflower germplasm collection. The specific sampling strategy, collection route, necessary permits and contacts with local officials for an exploration planned in late September-early October to collect crop wild relatives of sunflower in Arkansas, Texas, Oklahoma, Tennesse, North Carolina, South Carolina, and Georgia has been completed. Species that will be collected are perennial H. hirsutus (hairy sunflower), as well as other species that may have seed to collect at the time to fill gaps in the USDA National Plant Germplasm System sunflower crop wild relatives genebank collection, making it available for research for increasing genetic diversity and improvement of cultivated sunflower, and preserving it for future generations. Subobjective 2B: Develop effective screening procedures for Phomopsis. The third year of field screening of a recombinant inbred line population for Phomopsis is underway at three locations during the 2017 growing season. Subobjective 2C: Identify and assess mechanisms of insect resistance. A first year of testing was completed to examine susceptibility of sunflower hybrids to banded sunflower moth (BSM). Hybrids were created by crossing each of 15 female lines (which varied in their susceptibility to BSM) with one male, RHA 266, which was the least-damaged male line in previous years of testing. Results suggest that female lines that are less damaged by BSM tend to produce hybrids with less damage from BSM. Though damage to an inbred parent cannot precisely predict hybrid damage, results suggest that if a goal is to develop hybrids with resistance to BSM, the most damaged inbred parents can be excluded from testcrosses. A second year of testing on the same lines is underway. Subobjective 2D: Transfer disease resistance, insect resistance, and other agronomic traits from wild Helianthus species into cultivated sunflower and improve methods for interspecific hybridization and release of pre-breeding germplasm. The characterization of a unique high frequency production of triploids observed when cultivated sunflower was pollinated by specific accessions of perennial diploid crop wild relatives progressed. Genomic in situ hybridization analyses indicated that the triploid F1s had two genomes from the wild pollen sources and one from the cultivated line. Mitotic chromosome analyses indicated that the frequency of triploid progenies was significantly higher than those of the polyploid progenies. Pollen stainability analysis suggested the existence of a low percentage of unreduced male gametes that preferred fertilization with the female gametes, due to the dosage factors related to recognition and rejection of foreign pollen during fertilization. Further studies of the genetic control of this trait will facilitate research on sunflower polyploidy speciation and evolution, and the utilization of this trait in sunflower breeding. Screening of South African oilseed germplasm for Sclerotinia basal stalk rot (BSR), a serious fungal disease of sunflower continued. A total of fifty-nine cultivated oilseed sunflower accessions from the Agricultural Research Council, Grain Crops Institute, Potchefstroom, South Africa sunflower collection were evaluated over four environments and two years for resistance to BSR in artificially inoculated field trials. Nine accessions from the South African sunflower collection were identified with a disease incidence of 18%, less than or equal to the moderately resistant sunflower hybrid, with one line with only 3% incidence. These lines represent genetic diversity from other than the traditionally used germplasm in breeding programs to introgress the genes for resistance to Sclerotinia BSR into other adapted lines, providing a more efficient, durable, and environmentally friendly host plant resistance. Research on somatic embryogenesis of sunflower, recalcitrant to manipulations in vitro, progressed in increasing the efficiency of plant multiplication. Interspecific amphiploids (2n=4x=68) G08/2280 (H. pumilus x P21) and G08/2260 (NMSHA89 x H. maximiliani) between cultivated sunflower and wild perennial Helianthus species were used as explant donors. Primary somatic embryos were induced directly from the surface of corolla tubes at the late uninucleate or binucleate microspore development stage. Secondary somatic embryos were rapidly produced from primary embryos when subcultured with an induction frequency of 100 %. Mature embryos were gradually converted into young shoots on hormone-free subculture media. Regenerated plants acclimated successfully and displayed similar morphology and chromosome number to the amphiploid donors. Continued validation and optimization of this system with a larger number of sunflower genotypes will be studied in the future. Subobjective 2E: Characterize and map resistance to pathogens and insect pests, and other agronomic traits. A second year of field testing of two BC2F3 populations for Sclerotinia stalk rot was conducted at two locations the past growing season. These populations were developed from annual sunflower crop wild relatives. These populations will continue to be tested next growing season. A wild Helianthus annuus accession, PI 435414, exhibits resistance to downy mildew. Resistance was introduced into a confection sunflower mapping population. Genetic analysis indicated that a single dominant gene Pl19, controls the most predominant and virulent races of downy mildew currently identified in North America. The Pl19 gene is located on linkage group 4 of the sunflower genome map and is flanked by two single nucleotide polymorphism (SNP) markers, which are well suited for marker-assisted selection in molecular breeding. A new source of resistance to downy mildew was discovered in a wild annual silver leaf sunflower (Helianthus argophyllus) from Texas. Resistance is controlled by a single dominant gene, Pl20, making it easier to transfer into cultivated sunflower. Pl20 controls the most predominant and virulent races of downy mildew currently identified in North America. It is located on linkage group 8 of the sunflower genome, with identified molecular markers for the novel gene that can be used in sunflower breeding programs. Mapping the glandular trichome trait was completed. Published evidence indicates that this trait is a factor for resistance to sunflower moth and banded sunflower moth. The trait is a product of two major genetic loci which determine near-zero density (providing susceptibility) to hundreds of glandular trichomes per floret (providing resistance). Association mapping for Phomopsis resistance was completed. However, plans are to use RNAseq technology to increase marker density and reliability of the resistance model this year. Previously released TX16R germplasm is resistant to all known U.S. sunflower downy mildew and rust races. Additional SNP markers have been identified for downy mildew and rust resistance genes in TX16R, increasing the number of markers and saturating the mapping of these genes. Molecular markers linked to these resistance genes will enhance the development of downy mildew and rust resistant lines for the sunflower industry. Research to unravel the interesting phenomena that causes vigor reduction in the absence of nuclear vigor restoration genes in cytoplasms of perennial Helianthus species continued. Progress was made in further mapping of the vigor restoration gene in the perennial cytoplasm. Following the mapping of a vigor restoration gene commonly existing in cultivated lines, a second vigor restoration gene existing in perennial Helianthus giganteus was mapped. This will facilitate sunflower line development when using cytoplasms of wild perennial Helianthus species for crop improvement. Subobjective 3A: Develop new inbred lines of sunflowers with novel fatty acid compositions, such as low saturated fatty acids, and high oleic acid. A half-diallel genetic population analyzed in previous years has shown plants with variation in linoleic, stearic, and palmitic acids in a high oleic background. A biparental population with a HA 466 genetic background has been developed from two disparate genotypes from the half-diallel and will undergo a quantitative trait loci analysis in future years to determine the genetic structure of these heritable differences and to develop markers for use in breeding. Meanwhile, we have analyzed and selected all of the 2016 breeding lines to optimize oleic acid and saturated fat content of the seed oil. Additional populations were made last year that segregate for high oleic and low saturated fat content. Subobjective 3B: Pyramid disease and insect resistance with high yield and oil content. Over 2000 nursery rows of high yield, high oil, disease, insect, and herbicide resistant sunflower experimental lines were grown in nurseries in Fargo, North Dakota, Puerto Rico, and Chile. Of these, lines from 16 pedigrees are candidates for release, including several Sclerotinia and Phomopsis resistant sunflower lines of both heterotic groups. Lines will be made publicly available. Three sunflower confection germplasms, HA-DM2, HA-DM3, and HA-DM4 were developed by the backcross and pedigree breeding methods, with selection in each generation for downy mildew and rust resistance. The released germplasms carry the gene combinations of PlArg (DM R gene) plus R12 (rust R gene) (HA-DM2), Pl17 plus R13a (HA-DM3), and Pl18 plus R13a (HA-DM4), providing diversity for resistance to downy mildew and rust.


4. Accomplishments
1. Durable sunflower downy mildew and rust resistance. Downy mildew and rust are two devastating diseases that seriously reduce yields for sunflower producers. Very few suitable confection (eatable) inbred sunflower lines with high levels of resistance to downy mildew and rust resistance are available for commercial confection sunflower breeders. ARS scientists in Fargo, North Dakota, developed and released three germplasms resistant to both downy mildew and rust. The germplasms carry a stacking of one downy mildew and one rust gene, with different genes in each germplasm providing resistance to all known races of North American rust and downy mildew, representing the first confection germplasm with combined resistance to both downy mildew and rust. Molecular markers related to both disease genes have been provided to the sunflower industry, enabling breeders to develop additional hybrids with resistance to multiple pathogens, thus assuring sustainable sunflower production in the presence of these two devastating diseases.

2. Novel sources of high yielding Phomopsis and Sclerotinia resistant sunflower. Commercial sunflower hybrids are needed that combine high yield and agronomic quality, with disease resistance and end market traits like high oleic acid. ARS scientists in Fargo, North Dakota, developed one restorer line with imidazolinone herbicide tolerance, high oleic fatty acid oil content, and Sclerotinia head and stalk rot resistance, two restorer lines with imidazolinone herbicide tolerance and Phomopsis stem canker resistance, and a maintainer line with imidazolinone herbicide tolerance and high oleic fatty acid oil content for the sunflower breeding industry and ultimately the producers. The germplasms have been released to commercial seed companies, public researchers, and others to develop improved sunflower hybrids for sunflower producers. The discovery of new sources of resistance and tolerance will provide a more efficient, durable, and environmentally friendly host plant resistance to sustain sunflower as an economically viable crop.

3. Sclerotinia white mold resistant sunflower. Sclerotinia is the causal agent of a serious sunflower disease epidemic that causes three distinctly different diseases on sunflower: head, basal stalk or wilt, and mid-stalk rot with the former two accounting for over 80% of the disease damage. Since limited chemical and biological controls for Sclerotinia are available, and the present-day hybrids lack sufficient resistance, identification of new sources of genetic resistance becomes necessary to manage this disease. The crop wild relatives are native to North America where they are distributed over a large geographic area, which exposes them to a wide range of environmental conditions and disease organisms that coevolved with the crop, providing the opportunity to discover disease resistance genes in natural populations. The genetics of Sclerotinia basal stalk and head rot is quantitative, requiring several genes for control, which complicates the breeding process. Due to the lack of resistance genes in the cultivated sunflower, new resistance genes were discovered by the ARS scientists in Fargo, North Dakota in the perennial sunflower crop wild relatives. Two head and five basal stalk rot interspecific germplasms based on the perennial relatives have been publically released. These germplasms will provide sunflower breeders and producers additional sources of genes to control a major production limiting disease which will help sustain sunflower production in an environmentally friendly manner.

4. Sclerotinia basal stalk rot resistant germplasm. Sclerotinia basal stalk rot (BSR) is a serious fungal disease of sunflower causing significant yield reduction in the cool and humid areas of the world. Host resistance is the most effective method for controlling BSR disease caused by Sclerotinia white mold. The genetics of resistance to BSR is quantitative, requiring several genes for control. ARS scientists in Fargo, North Dakota, identified six significant quantitative trait loci (QTL) associated with BSR tolerance from a recombinant inbred line population. Two loci, each explaining 31.6 and 20.2% of the observed phenotypic variance, respectively contributed to the increased resistance. A highly tolerant Sclerotinia BSR oilseed germplasm was developed and released with 1.6% incidence compared to 20% for the parents. Genetic analysis of the germplasm revealed that it possessed three resistant loci from each parent. The successful transfer of Sclerotinia BSR resistance from crop wild relatives into adapted lines provides a more efficient, durable, and environmentally friendly host plant resistance.

5. Interspecific amphiploid sunflower. Crop wild relatives of sunflower have played an important role in establishing sunflower as a valuable global oilseed crop. Interspecific amphiploids derived from crop wild relatives crossed with cultivated sunflower have been developed to help mine potential genes from the very large gene pool of 53 different species, especially the 39 hard-to-cross perennial species. The value of these genetic stocks is that they can act as a bridge in interspecific gene transfer, allowing for easier backcrossing with the cultivated sunflower to further broaden the genetic diversity of the crop, as well as for the transfer of specific target genes, especially for disease resistance. ARS scientists in Fargo, North Dakota, developed and publicly released 12 amphiploid genetic stocks based on nine perennial species. The genetic stocks will help conventional breeders identify and transfer desirable genes from the perennial species with greater ease. The genetic stocks will also allow for the development of chromosome addition lines, with individual wild species chromosomes added to the background of cultivated sunflower lines for genetic studies of specific chromosomes, similar to marker assisted selection.

6. New downy mildew genes from crop wild relatives. Global sunflower production is plagued by many diseases, among them downy mildew (DM), which is one of the most destructive. New races of DM are continually emerging rendering the current resistant sunflower hybrids ineffective, necessitating the search for new resistance sources. ARS scientists in Fargo, North Dakota, discovered two new DM resistance genes derived from annual sunflower crop wild relatives and transferred the genes into cultivated confection and oilseed sunflower. The new genes are highly effective against the most predominant and virulent races of downy mildew currently identified in North America. Molecular markers for the novel genes were identified so they can be used in sunflower breeding programs. The discovery of new sources of resistance to DM will provide a more efficient, durable, and environmentally friendly host plant resistance to sustain sunflower as an economically viable crop.

7. New sources of Sclerotinia basal stalk rot. Sclerotinia basal stalk rot (BSR) is a serious fungal disease that reduces yield of global sunflower production. Since limited chemical and biological controls for BSR are available, and the present-day hybrids lack sufficient resistance, identification of new sources of resistance becomes a necessity to manage the disease now and in the future. A total of fifty-nine cultivated oilseed sunflower accessions from the Agricultural Research Council, Grain Crops Institute, Potchefstroom, South Africa sunflower collection were evaluated by ARS scientists in Fargo, North Dakota, in four environments over two years for resistance to BSR in artificially inoculated field trials. Nine accessions from the South African sunflower collection were identified with a disease incidence of 18%, less than or equal to the moderately resistant sunflower hybrid, with one line with only 3% incidence. These lines represent genetic diversity from other than the traditionally used cultivated germplasm used in most breeding programs to introgress resistance genes for Sclerotinia BSR into other adapted lines, providing a more efficient, durable, and environmentally friendly host plant resistance.

8. Relationship of floral traits to pollinator visitation and yield. Sunflower and bees have coexisted for years. Multiple years of field testing have shown consistent pollinator preference (mostly wild bees) for specific USDA sunflower inbred lines. ARS scientists in Fargo, North Dakota, examined floral and nectar traits of tested inbred lines shown to vary in the amount of sugar in floral nectar, composition of nectar (sucrose as % of total sugars), and accessibility of nectar (depth of corollas). Increased visitation to inbred lines was associated with both the amount of floral nectar and accessibility (i.e., shorter corollas make nectar more accessible), suggesting that these traits can increase profitability of hybrid seed production for seed companies. Follow-up studies also show that increased pollinator visits are positively related to increased yields for growers of commercial hybrids. The high sucrose line and corolla depth variation identified in public inbred lines are being used in ongoing research, with a goal of developing sunflowers with traits (and associated genetic markers) that will benefit sunflowers through increased pollination and yield, and likely benefit pollinators though increased availability of food in areas planted to sunflowers.

9. Genetic diversity from perennial crop wild relatives. Cultivated sunflower is still represented by a relatively narrow genetic base, which greatly limits its future success as a competitive major global oilseed crop. A significant number of the perennial crop wild relatives have been identified as highly resistant to diseases and parasites of global concern including broomrape, a parasitic weed, Sclerotinia white mold, Phomopsis stem canker, downy mildew, leaf rust, Verticillium wilt, and Rhizopus head rot. Of the 53 crop wild relatives, 39 are perennial and difficult to cross with cultivated sunflower. Thus, they have been rarely utilized for sunflower improvement and represent unexploited sources of genetic variation for sunflower improvement. ARS scientists in Fargo, North Dakota, developed 15 interspecific bulk populations based on 10 perennial crop wild relatives, providing breeders with the opportunity to incorporate previously unavailable genetic diversity into their programs. These bulk populations will help diversify the sources of disease resistance, helping to sustain sunflower production in an environmentally friendly manner and improving producers’ net returns.

10. Diversification of parental lines for hybrid sunflower. Globally, sunflower is the fifth largest hybrid crop. It is currently based on a single female parent, a French sterile cytoplasm developed in 1969, derived from the wild prairie sunflower, and a few male parental lines resulting in a crop with a very narrow genetic base. This leaves sunflower very vulnerable to attack by pests similar to the disaster seen in corn with the outbreak of southern corn leaf fungal blight in the 1970s. Crop wild relatives of sunflower provide a readily available resource for broadening the genetic base of sunflower. ARS scientists in Fargo, North Dakota, discovered a new female cytoplasm derived from perennial Jerusalem artichoke and complementary male parental lines. The new cytoplasm and male parental lines can be used by breeders to diversify the currently used single cytoplasm as an alternative for the development of hybrid sunflower, making it better able to withstand an ever-changing environment.


Review Publications
Zahirul, T.I., Seiler, G.J., Song, Q., Ma, G., Qi, L. 2016. SNP discovery and QTL mapping of Sclerotinia basal stalk rot resistance in sunflower using genotyping-by-sequencing (GBS). The Plant Genome. 9(3). doi:10.3835/plantgenome 2016.03.0035.

Prasifka, J.R., Hulke, B.S. 2016. Relative susceptibility of sunflower maintainer lines and resistance sources to natural infestations of the banded sunflower moth (Lepidoptera: Tortricidae). The Canadian Entomologist. 148:736-741.

Zhang, Z.W., Ma, G.J., Zhao, J., Markell, S.G., Qi, L.L. 2017. Discovery and introgression of the wild sunflower-derived novel downy mildew resistance gene Pl19 in confection sunflower (Helianthus annuus L.). Theoretical and Applied Genetics. 130:29-39.

Feehan, J.M., Schneibel, K.E., Bouras, S., Underwood, W., Keller, B., Somerville, S.C. 2017. Purification of high molecular weight genomic DNA from powdery mildew for long-read sequencing. Journal of Visualized Experiments. doi:10.3791/55463.

Seiler, G.J., Qi, L.L., Marek, L.F. 2017. Utilization of sunflower crop wild relatives for cultivated sunflower improvement. Crop Science. 57:1-19. https://doi.org/10.2135/cropsci2016.10.0856.

Seiler, G.J., Marek, L.F. 2016. Collection of wild Helianthus anomalus and deserticola sunflower from the desert southwest USA. Helia. 39(65):139-155.

Qi, L., Long, Y., Talukder, Z., Block, C., Gulya, T.J. 2016. Genotyping-by-sequencing uncovers the introgression alien segments associated with Sclerotinia basal stalk rot resistance from wild species—I. Helianthus argophyllus and H. petiolaris. Frontiers in Genetics. doi:10.3389/gene.2016.00219.

Prasifka, J.R., Marek, L., Lee, D., Thapa, S., Hahn, V., Bradshaw, J. 2016. Effects from early planting of late-maturing sunflowers on damage from primary insect pests in the United States. Helia. 39(63):45-56.

Underwood, W. 2016. Contributions of host cellular trafficking and organization to the outcomes of plant-pathogen interactions. Seminars in Cell and Developmental Biology. 56:163-173.

Long, Y.M., Chao, W.S., Ma, G.J., Xu, S.S., Qi, L.L. 2017. An innovative SNP genotyping method adapting to multiple platforms and throughputs. Theoretical and Applied Genetics. 130(3):597-607.

Prasifka, J.R., Prasifka, P.L., Lee, D.K. 2016. Efficacy of insecticides to limit caterpillar damage to prairie cordgrass seed. Arthropod Management Tests. 41(1):tsw030. doi:10.1093/amt/tsw030.

Hulke, B.S., Gao, Q.M., Foley, M.E. 2017. Registration of the sunflower oilseed maintainer genetic stocks HOLS1, HOLS2, HOLS3, and HOLS4, possessing genes for high oleic and low saturated fatty acids, and tolerance to imidazolinone herbicides. Journal of Plant Registrations. 11:200-203.

Underwood, W., Ryan, A., Somerville, S.C. 2017. An Arabidopsis lipid flippase is required for timely recruitment of defenses to the host-pathogen interface at the plant cell surface. Molecular Plant. 10(6):805-820.

L.Qi, L., Talukder, Z.I., Hulke, B.S., Foley, M.E. 2017. Development and dissection of diagnostic SNP markers for the downy mildew resistance genes PlArg and Pl8 and maker-assisted gene pyramiding in sunflower (Helianthus annuus L.). Molecular Genetics and Genomics. 292(3):551-563.

Ma, G.J., Markell, S.G., Song, Q.J., Qi, L.L. 2017. Genotyping-by-sequencing targeting of a novel downy mildew resistance gene Pl20 from wild Helianthus argophyllus for sunflower (Helianthus annuus L.). Theoretical and Applied Genetics. 130(7):1519-1529.

Kurt, A., Torun, H., Nesrin, C., Seiler, G., Hayirlioglu-Ayaz, S., Ayaz, F.A. 2017. Nutrient profiles of the hybrid grape cultivar 'Isabel' during berry maturation and ripening. Journal of the Science of Food and Agriculture. 97:2468-2479.

Seiler, G.J., Gulya Jr, T.J. 2016. Sunflower: Overview. In: Wrigley, C.W., Corke, H., Seetharaman, K., and Faubion, J., editors. Encyclopedia of Food Grains. 2nd edition. Oxford, UK: Elsevier. p. 247-253.

Seiler, G.J. 2016. Botany of the Sunflower Plant. In: Harveson, R.M., Markell, S.G., Block, C.C., Gulya, T.J. editors. Compendium of Sunflower Diseases and Pests. St. Paul, MN: APS Press. pp.4-11.

Foley, M.E., Dogramaci, M., West, M.S., Underwood, W.R. 2016. Environmental factors for germination of Sclerotinia sclerotiorum sclerotia. Journal of Plant Pathology & Microbiology. doi:10.4172/2157-7471.1000379.

Van Tassel, D.L., Albrecht, K.A., Bever, J.D., Boe, A.A., Brandvain, Y., Crews, T.E., Gansberger, M., Gerstberg, P., Gonzalez-Paleo, L., Hulke, B.S., Kane, N.C., Johnson, P.J., Pestsova, E.G., Picasso Risso, V.D., Prasifka, J.R., Ravetta, D.A., Schlautman, B., Sheaffer, C.C., Smith, K.P., Speranza, P.R., Turner, M.K., Vilela, A.E., von Gehren, P., Weaver, C. 2017. Accelerating Silphium domestication: an opportunity to develop new crop ideotypes and breeding strategies informed by multiple disciplines. Crop Science. 57(3):1274-1284.

Liu, Z., Seiler, G.J., Gulya, T.J., Feng, J., Rashid, K.Y., Cai, X., Jan, C. 2017. Triploid production from interspecific crosses of two diploid perennial Helianthus with cultivated sunflower. Genes, Genomes, Genetics. 7(4):1097-1108.

Mallinger, R.E., Prasifka, J.R. 2017. Bee visitation rates to cultivated sunflowers increase with the amount and accessibility of nectar sugars. Journal of Applied Entomology. 141(7):561-573.

Xu, S.S., Liu, Z., Zhang, Q., Niu, Z., Jan, C., Cai, X. 2016. Chromosome Painting by GISH and Multicolor FISH. In: Kianian, S.F., Kianian, P.M.A., Editors. Plant Cytogenetics: Methods and Protocols. Methods in Molecular Biology. New York: Springer. p. 7-21.