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Rating Hybrids for Sclerotinia Resistance

Wednesday, April 1, 2009
filed under: Disease

If life in Sclerotinia-Land was only as simple as that in Rust-Land and Downy Mildew-Land, it would make everyone’s life much easier.

But it’s not. Whereas resistance to rust and downy mildew is a single-gene trait, with Sclerotinia it’s a multi-gene phenomenon. That reality has vexed sunflower breeders and plant pathologists for decades. It also makes the rating of hybrids for resistance to Sclerotinia stalk or head rot a very difficult, tricky proposition. How do you assign categories like Highly Resistant, Resistant, Susceptible or Highly Susceptible to specific hybrids when the differences among them appear as shades of gray rather than black and white?

You don’t. But USDA and university researchers in North Dakota and Minnesota have, in recent years, carried out coordinated trials to provide some quantitative definition to specific hybrids’ performance when infected with Sclerotinia. Ratings are based on the percentage of plants infected. The collaborative effort includes Fargo-based USDA-ARS plant pathologist Tom Gulya and scientists at North Dakota State University and the University of Minnesota-Crookston*. Also involved is Khalid Rashid, plant pathologist with Agriculture & Agri-Food Canada’s research station at Morden, Man.

The 2008 head rot resistance evaluations encompassed 75 hybrids submitted by commercial seed companies. These trials were conducted at Carrington, N.D., Crookston, Minn., and Morden, Man. “Retests” — in which the best 20 entries from the previous year’s trials were tested a second time to generate more data — were also planted at Carrington and two other North Dakota locations: Oakes and Langdon.

Stalk rot evaluations in 2008 were conducted at Crookston, Carrington, Davenport, N.D. (near Casselton), Breckenridge, Minn., and Ralls, Texas. (Data were not collected from the latter two locations due to insufficient disease infection.) Along with commercial hybrids, the stalk rot trials also included a broad range of USDA breeding materials.

The full listings of results from both 2008 and prior years are posted on the National Sunflower Association’s web site — Click on “Growers,” followed by “Yield Trials & Hybrid Disease Ratings.”

The hybrids evaluated for Sclerotinia resistance in 2008 were likewise rated for their resistance to rust and to downy mildew, both done in controlled greenhouse tests. Also, USDA-ARS entomologist Larry Charlet evaluated the hybrids in terms of damage levels from natural populations of sunflower midge and seed maggot at Mapleton, N.D., just west of Fargo. In addition to being on the NSA web site, these results also appear in North Dakota and South Dakota Hybrid Sunflower Performance Testing 2008, Bulletin A-652 (Revised), published by the North Dakota Agricultural Experiment Station and NDSU Extension Service. (Downy mildew tests are still in progress and will be posted within the next month)

Here’s a snapshot of the 2008 findings:

Sclerotinia Head Rot — At NDSU’s Carrington station, the percentage of plants infected ranged, by hybrid, from a low of 26% to a high of 97, with an average of 67%. At Crookston, where disease pressure was high, the range was from 23 to 100% infection, with an average of 75%. At Morden, infection percentages among hybrids ranged from 11 to 73, averaging 28.

Sclerotinia Stalk Rot — Two locations, Davenport and Crookston, provided “statistically sound” data. At Davenport, the 95 evaluated hybrids averaged 45% of plants infected, with 8% on the low end and 88% on the high side. The average percentage of plants infected, by hybrid, at Crookston was 34%. The best hybrid came in at just 5%, while the most susceptible one had 63% of plants infected.

Of what use are these resistance ratings to growers — and how should they be used when making hybrid selections?

First, the ratings’ main use is for comparison — i.e., how Hybrid A compares with other hybrids in its level of Sclerotinia resistance. A hybrid’s rating percentage does not, Gulya emphasizes, mean a grower should expect to see that same level of infection in his field. The reason? The hybrid entries in these trials are deliberately exposed to high levels of disease inoculum.

In the case of stalk rot evaluations, millet inoculated with Sclerotinia is sown in the plant rows. In the head rot plots, Sclerotinia spores are sprayed onto each head and an automated misting system helps ensure the environment is conducive for head rot development. So a hybrid that displays 100% infection in these trials may, in a grower’s field, show only 20%. “You’re not going to see the same level that we’re creating here,” Gulya stresses.

“But the take-home message is, ‘This’ hybrid will always perform better than ‘that’ hybrid.”

Second, as alluded to earlier, hybrid resistance to Sclerotinia takes on the form of a continuum, where resistance exists in degrees. It is not a strict “resistant” or “susceptible” scenario, as is the case with rust or downy mildew.

Finally, growers obviously must take into account yield, oil content and other parameters when deciding whether to go with a particular hybrid — even one that compares favorably against other hybrids in terms of its resistance to Sclerotinia stalk and/or head rot. If a hybrid does exceptionally well in its Sclerotinia ranking but is subpar in yield potential, it may not be a good fit — unless one expects to have serious Sclerotinia pressure.

The good news, Gulya says, is that progress continues to be made in the development of commercial hybrids with good resistance to Sclerotinia head rot and stalk rot, sometimes occurring in the same hybrid. The range of resistance is broad, with some hybrids still showing moderate or high susceptibility to this disease.

But within the past several years, “we have shown that many commercial hybrids now have significant levels of stalk rot resistance — and a few hybrids have resistance to both the stalk and head rot versions of Sclerotinia,” Gulya relates. “We could not have said that 10 or even five years ago.”

— Don Lilleboe

* Key cooperators include Blaine Schatz and Ezra Aberle, NDSU-Carrington; Walt Albus, NDSU-Oakes; Scott Halley, NDSU-Langdon; and Char Hollingsworth, Chris Motteberg and Justin McMechan UM-Crookston. Along with Tom Gulya, the Fargo USDA-ARS group includes Nikolay Balbyshev and Megan Ramsett. Gulya also credits the following seed companies for supplying plot land: CHS, Croplan, Dekalb/Interstate, Mycogen and Texas Triumph.
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