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Downy Mildew-Resistant Confection Germplasm

Wednesday, March 29, 2017
filed under: Research and Development

Lili Qi (left) and postdoctoral research associate Guojia Ma (right) in their lab. 

       Significant progress was made in 2016 with a USDA project to incorporate increased levels of downy mildew resistance into confection sunflower.  Germplasm resistant to downy mildew and rust will be released to the breeding community and public in 2017.
       Downy mildew (DM) is considered the most destructive foliar disease in sunflower.  It can cause up to 80% yield loss in cool, wet years, and it also adversely affects other aspects of seed quality.
       The best way to control downy mildew is to use sunflower hybrids with resistance to the disease.  To date, no DM-resistant germplasm or commercial hybrids have been available for confection sunflower. That situation has been aggressively addressed, however, by research within the USDA-ARS Sunflower and Plant Biology Research Unit at Fargo, N.D.  This research has been partially funded by the National Sunflower Association and by the Specialty Crop Block Grant Program administered by the North Dakota Dept. of Agriculture.
       The objectives of this project have been three-fold:
  1. incorporation of DM resistance identified in oil-type sunflower into confection sunflower;
  2. molecular mapping of DM resistance genes; and
  3. pyramiding DM and rust resistance genes into a single genetic background.
       The confection germplasm with DM resistance combined with rust resistance will be provided to the private seed industry for incorporation into finished hybrids.  Resistance to both DM and rust is an important agronomic factor in keeping U.S. confection sunflower competitive.
       Here is a brief recap of what has been achieved to date.
       Three breeding lines — RHA 464, HA 458 and a new line, HA-DM1, developed in Dr. Qi’s lab — were chosen as DM-resistant donors. RHA 464 carries a DM resistance (R) gene called PlArg and a rust resistance gene, R12, while HA 458 harbors the DM resistance gene Pl17, and HA-DM1 has a DM resistance gene Pl18.
       Two confection sunflower lines, CONFSCLR5 and HA-R6, were chosen as recurrent parents. HA-R6 possesses a rust resistance gene, R13a.  Three DM resistance genes — PlArg, Pl17 and Pl18 — together offer a broad spectrum of resistance against all DM pathogen races identified thus far in the United States. The two rust-resistant genes, R12 and R13a, likewise are effectively resistant to all known rust races.
       Initial crosses were made between CONFSCLR5 and RHA 464, HA-R6 and HA 458, and HA-R6 and HA-DM1. The process involved four rounds of backcrossing within seven seasons. Downy mildew and rust were evaluated in the first backcross generation (BC1) through the fourth backcross generation (BC4F1) in the cross of CONFSCLR5 and RHA 464, while only downy mildew tests were applied in the BC1 through the BC4F1 in the other two crosses. In the first selfed generation of the fourth backcross (BC4F2), DNA markers linked to three DM and two rust resistance genes previously developed were used to select homozygous double-resistant plants with the gene combinations of PlArg plus R12, Pl17 plus R13a, and Pl18 plus R13a, with the selected double-resistant plants being advanced to BC4F3.
       The three-year project began in January of 2014.  Progress made during 2016 can be summarized as follows:
  • We mapped the DM resistance gene from sunflower inbred line RHA 468 to the sunflower chromosome 1.  DNA markers linked to the DM resistance gene were developed for marker-assisted selection.
  • We introduced a novel DM R-gene — Pl20 — from the wild sunflower species Helianthus argophyllus into cultivated sunflower.  Pl20 was mapped to the sunflower chromosome 8 with closely linked DNA markers.
  • We successfully transferred three DM R-genes — PlArg, Pl17 and Pl18 — along with two rust R-genes (R12 and R13) from oil sunflower into confection sunflower.
  • We selected homozygous plants with the respective DM and rust gene combinations, using the DNA markers linked to the three downy mildew resistance genes and the two rust resistance genes, and advanced them to the BC4F3 generation.
  • The selected BC4F3 families were further tested for resistance to downy mildew and rust to confirm the results of marker-selection, and then grown in the field at Glyndon, Minn., in the summer 2016 for evaluation of agronomic performance and seed increase.
  • We will release the finished lines with the gene combinations of PlArg plus R12, Pl17 plus R13a, and Pl18 plus R13a, to the public in 2017.                                                                                   
* Lili Qi is research molecular biologist with the USDA-ARS Sunflower and Plant Biology Research Unit, Fargo, N.D.
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