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The Road to Sclerotinia Resistance

Saturday, April 15, 2006
filed under: Research and Development

The most dreaded sunflower disease, Sclerotinia head rot, did not show its ugly head in the 2005 season. The annual National Sunflower Association field survey found only limited levels of infection. August to October was generally warm and dry, thereby virtually eliminating the disease. However, one good season has not lulled researchers into thinking the ‘storm’ is over. Sclerotinia head rot continues to be very high on everyone’s research radar screen.

Hybrid resistance is the one clear path of research. Breeding material and hybrids can now be screened for resistance thanks largely to misting sites that have been established in a number of locations. The misting sites duplicate the necessary weather requirements for the disease to infect and ultimately rot the head. With misting sites researchers can now get meaningful data each year as opposed to waiting for that one in ten year phenomenon when the disease is prolific, thanks to cool and wet conditions in the fall.

It takes an extended team of cooperating researchers to make progress on head rot resistance, and funding is a key component. Here is where the National Sunflower Association did its homework in helping to establish the federally funded Sclerotinia Initiative. Through the support of key Senate Appropriations Committee members, Senators Dorgan (N.D.) and Johnson (S.D.), the Sclerotinia Initiative has received consistent federal funding annually since 2001. These dollars have allowed key researchers to invest heavily in time, personnel, space, and equipment. In reality, more progress in sunflower Sclerotinia research has been made in the last five years than was made in the prior twenty years combined.

Who are the key researchers addressing Sclerotinia in sunflower, and what are the key research components?

One of the first parts to the puzzle is to identify possible resistant breeding material. That work is being done by Dr. Jerry Miller, research geneticist, and Dr. Tom Gulya, research pathologist, both members of the USDA ARS Sunflower Unit, Fargo. They have screened many sunflower lines under the misting systems. Their first release to the private sector of improved resistant oilseed germplasm was made in 2003. They continue to screen new material and have also incorporated confection germplasm screening and development into their efforts. Gulya has also been responsible for producing the fungal spores that are sprayed onto the flowering sunflower head at the misting site locations.

Dr. Robert Henson at the NDSU Carrington N.D. Research Extension Center has become the ‘guru of misting.’ His work is highly regarded and the Carrington location now has Sclerotinia misting sites for other broadleaf crops such as edible beans, canola, and field peas. The misting site is an absolutely necessary part of this sunflower research. In addition to Carrington, there is a misting location in Brookings, S.D. Two locations, Morden, Manitoba, and Langdon, N.D., were added in 2005. More misting sites are planned for Crookston, Minn., and Oakes, N.D, in 2006.

Henson says that the key to this misting research is to have consistency in disease results between the various locations.

“If we are having no disease on a hybrid in Carrington but lots of disease on the same hybrid at another location, then we have a problem with our methodology and the data is not reliable,” he explains. The 2005 results between the Carrington and Langdon sites were very acceptable, according to Henson. The Brookings location was lost to flooding and the Morden site did not have sufficient disease 2005. This illustrates the importance of conducting the research at multiple locations.

Henson notes that for the past few years, some confection lines have been the top performers for head rot ratings in his trial. This is especially positive since confection hybrids are noted to be the most vulnerable to the disease. The confection market was seriously impacted after the 2004 growing season with market shortages. Henson notes that “if hybrids can come through our intense disease pressure at the misting sites with less than 25% disease incidence, they’ll do quite well under most production conditions.”

It’s in the genes

It is well recognized that multiple genes are required for Sclerotinia head or stalk rot resistance. This is quite different from downy mildew or rust which require a single gene for resistance.

To complicate matters, the genes that provide resistance to head rot are not the same genes that provide resistance to stalk rot. This is where the wild sunflower species are likely to provide some of the answers. Dr. Gerald Seiler is the botanist at the USDA ARS Sunflower Unit and has made many wild species collections. He and others started to screen wild species for resistance several years ago.

Bringing the wild species into a cultivated background is the task of the Sunflower Unit’s cytogeneticist, Dr. C. C. Jan. The process is very complex, but Dr. Jan knows there is an abundance of resistance genes in populations of wild perennial species. “It is the gene bank for the next several years,” says Jan.

The next step in the research process is to identify and establish DNA markers of the resistant genes. That is the work of the Sunflower Unit’s Dr. Jingo Hu, a molecular geneticist. That work has been in place for several years, and a post-doc has been added to the staff to accelerate the process.

A challenge with all of the backcrossing from wilds to cultivated types is the loss of one or more of the resistant genes. With marker technology, however, a researcher can quickly determine if the genes have been successfully transferred.

This research is not an overnight process – it takes a high commitment in funding, equipment, and personnel to carry out. Dr. Brady Vick, the Sunflower Unit research leader, says that the majority of the work at the Unit is now directed at Sclerotinia. The funding from the Sclerotinia Initiative has been vital as has the partnership with university people like Bob Henson and others.

“It really is a cooperative effort at many fronts and we are seeing some positive results,” says Vick. “Since Sclerotinia resistance involves multiple genes, we have to approach it like building a wall of blocks, one resistant gene on top of another. We have a very good start, thanks to a very coordinated effort,” says Vick.

The final leg of the partnership resides at the private hybrid seed industry. This group of breeders has also been testing breeding material domestically and overseas for Sclerotinia resistance. Getting public releases from the USDA ARS Sunflower Unit with additional genetic resistance is a bonus to their programs and allows them to make faster progress.

Private sector sunflower breeders need to ensure that what goes into the seed bag has all of the characteristics that farmers need – yield, standability, oil content, downy mildew resistance, herbicide tolerance and more. Head or stalk rot resistance is just one more component. But this industry has learned that it is an utmost important component. – Larry Kleingartner

Meeting Highlights Sclerotinia Research Efforts

Research achievements were presented recently during the sixth annual meeting of the National Sclerotinia Initiative, hosted by the USDA Agricultural Research Service in Bloomington, Minn.

Sclerotinia is a fungal disease, caused by Sclerotinia sclerotiorum, affecting more than 400 species of broadleaf plants. Since 2002, ARS has led a multi-state, multi-organization effort to counterattack the fungus on three fronts: epidemiology; development of resistance in germplasm; and chemical, biological or cultural control.

The initiative aims to protect seven crops that growers across the country are increasingly including in their rotation schemes: sunflower (oil and confection), soybeans, canola, dry edible beans, chickpeas, lentils and dry peas. Poor genetic resistance to Sclerotinia in these crops costs up to $280 million annually in degraded quality and reduced yields, notes Larry Chandler, associate director for the ARS Northern Plains Area headquarters in Fort Collins, Colo.

During the recent meeting, participants from more than 14 universities and 11 trade groups, including the National Sunflower Association, discussed progress to date, as well as identified future research plans and needs through 2009, according to Chandler, the Sclerotinia initiative's ARS coordinator.

Accomplishments to date include development of Sclerotinia risk-assessment maps that dry bean and canola producers can use to implement disease-management strategies; development of dry bean and lentil germplasm lines or cultivars that resist Sclerotinia; uses for the beneficial fungus Coniothyrium minitans as a biological pesticide product; genetically modified soybeans that produce an antifungal peptide against Sclerotinia; and the public release of the sequence for 14,552 of the fungus' genes. The database enables Sclerotinia researchers to search for genes by name, genomic location, their associated proteins and other information.

For more details, as well as recent research abstracts, visit the National Sclerotinia Initiative web site,

Head Rot Fungicides:

Still in the Research Pipeline

Controlling head rot with fungicides has not proven to be successful to date. Fungicide testing has been in place at all of the misting sites. One challenge is that the disease pressure at the misting sites may be too overwhelming for segregating possible fungicide differences. An attempt will be made to reduce the disease pressure in the fungicide trials. Key questions remain to be answered, however, such as timing of the spray and the impact of multiple applications.

One area that will be further explored in 2006 is placement of the fungicide on the plant. Since the infection is in the head, it is imperative that the fungicide gets placed on the face of the sunflower. That is a challenge with the maturing sunflower head facing down.

Work at the NDSU Langdon, N.D. Research and Extension Center by Scott Halley on surfactants shows some promise. There are new fungicides in the registration pipeline, two of which appear to provide better efficacy than some of the older chemistries. But at this point, fungicides to control head rot will require more research before becoming a commercial option.

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