Hammering Away on Sclerotinia
Progress continues in the long, uphill battle to find and incorporate Sclerotinia resistance into commercial sunflower hybrids.
It’s a struggle that received minimal attention from researchers in the past due to the disease’s complexity. That changed in 2000, however, with the creation of the National Sclerotinia Initiative, which provides research dollars to key commodities like sunflower that can suffer large losses from this disease.
The complex multi-gene nature of Sclerotinia, coupled with its manifestation in two forms (stalk rot and head rot) and two dramatically different cultivars (oil and confection), has presented daunting challenges for scientists. But the relatively recent use of molecular genetics — as well as innovative field screening techniques (e.g., artificial inoculation for stalk rot tests, misting systems for head rot) — that are partially or fully underwritten by the Sclerotinia Initiative — has helped advance the effort.
First, a brief review of how current commercial hybrids stack up in terms of their resistance to Sclerotinia is in order. Evaluations of resistance to stalk rot and head rot have been conducted for several years at various sites in western Minnesota, eastern/central North Dakota and southern Manitoba. Tom Gulya, research plant pathologist with USDA-ARS, contacts all of the sunflower seed companies and asks them to submit up to seven new entries each year. “Our annual quota is 75 entries, and if we don’t get 75, I then contact the companies again to find out if some have more than seven they’d like tested,” Gulya notes. “Some of the entries are released hybrids, while others are experimentals — which may or may not ever be released, depending upon their performance on many ‘fronts.’ ”
The past two years’ findings can be summarized as follows:
• Sclerotinia Head Rot — A total of 75 hybrids (including a “resistant” check and a “susceptible” check) were evaluated in 2008. At North Dakota State University’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, Minn., where disease pressure was high that year, the range was from 23 to 100%, with an average of 75%. At the Morden, Man., location, infection percentages among hybrids ranged from 11 to 78 and averaged 28%.
The 2009 evaluations for head rot again encompassed 75 hybrids, including a new set of varieties that had not been tested the prior year. Fourteen commercial companies were represented. Averaged across the three sites (Carrington, Crookston and Morden), Sclerotinia head rot incidence ranged from 48% on the low end up to 84% on the high side. The 20 best-performing hybrids from the 2008 head rot trials were placed in a “repeat screening” test in 2009. There, when averaged across four sites (Oakes, N.D., was added for this round of testing), disease incidence ranged from 32% up to 78%. (Only one hybrid — Pannar PEX3426 — rannked among the top five at all four locations.)
• Sclerotinia Stalk Rot — A total of 95 entries — plus resistant and susceptible checks — were tested in 2008, with “statistically sound” data available from two locations: Crookston and Davenport, N.D. At Davenport, the 95 evaluated hybrids averaged 45% of plants infected, with 8% on the low side and 88% on the high end. At Crookston, the average percentage of plants infected, by hybrid, was 34%. The best hybrid had just 5% infection; the most susceptible one, 63%.
The 2009 stalk rot evaluations again encompassed 95 hybrids (plus checks), with data compiled from three sites: Crookston and Carrington and Grandin, N.D. Though most tested hybrids were oil-types, several confection varieties also were evaluated. The average percent of plants showing Sclerotinia wilt/stalk rot symptoms across locations (four replications at each) was 21%, ranging from 5% on the low side to 39% at the other extreme.
Along with evaluating commercial hybrids, scientists are exploring additional sources for resistance to Sclerotinia.
In 2008 and 2009, USDA-ARS-Fargo sunflower researchers, together with Laura Marek, curator of the sunflower collection at the USDA-ARS Regional Plant Introduction Station at Ames, Iowa, evaluated 250 different accessions from the Ames collection. This collection is comprised of about 3,300 accessions, of which half are cultivated sunflower lines. Of the roughly 1,600 cultivated accessions, about 80% had been evaluated for Sclerotinia stalk rot resistance (under natural infection conditions) during the 1990s. Through Marek’s work, most of the previously “unavailable” accessions (i.e., very limited seed quantity) were increased — which then allowed for their distribution for the first time for evaluation for Sclerotinia resistance and other traits.
Test plots (three locations per year) were inoculated with Sclerotinia mycelium in the furrow several weeks after planting. In looking at the stalk rot infection ratings (’08 and ’09 combined) for the 250 plant introductions and 12 USDA inbred lines, the best line — which has a USDA pedigree — showed just a 3.5% stalk rot infection rate. Second best was a Russian line at 6.9%.
“The newly available USDA sunflower PIs (plant introductions) exhibit a wide range of Sclerotinia stalk rot reaction,” summarizes Tom Gulya, “with a small number comparable to the best USDA inbreds [that have been] developed specifically for Sclerotinia resistance.” The PIs — some of which are confection types — may have different genes than those found in current USDA inbreds, he adds, which would diversify the genetic base of the USDA breeding material.
Gulya and his colleagues intend to test the most promising materials further — including evaluating them for head rot resistance in inoculated mist nurseries.
Another promising source of Sclero-tinia resistance is the wild sunflower species. Charles Block, plant pathologist at the ARS Regional Plant Introduction Station at Ames, is collaborating with Tom Gulya and Laura Marek on greenhouse and field tests to screen dozens of wild annuals. Work to date has confirmed that greenhouse screening can reliably identify susceptible germplasm. Thus, only those entries that look good in greenhouse tests need to be confirmed in much-more-laborious field tests. Field trials are planned in 2010 on several dozen accessions.
Testing of perennial sunflower species also has been going on for several years, with ARS cytogeneticist C.C. Jan spearheading that effort. Many perennials are immune to Sclerotinia diseases; but the challenge is to transfer all of the resistance genes to cultivated sunflower and reduce the chromosome number — which, in many perennial species, is either two or three times that of cultivated sunflower.
Lili Qi, a recently hired molecular geneticist in the USDA-ARS Sunflower Research Unit, is working to find molecular markers for resistance to Sclerotinia and several other sunflower diseases. “If these markers can pinpoint the exact genes governing disease resistance, the identification of resistant material will be much more precise,” Gulya says, “and the timeframe needed to develop resistant germplasm [will be] shortened considerably.”
Another new tool in the breeders’ arsenal is a technique referred to as “association mapping.” ARS sunflower geneticist Brent Hulke has obtained a grant from the National Sclerotinia Initiative to hire a post-doc for this project, and Dr. Zahirul Talukder be leading this effort.
The bottom line is this: While the “Promised Land” of Sclerotinia resistance has not yet been reached, it is much closer than it was a decade ago. It’s a step-by-step process, Gulya notes. “Since Sclerotinia resistance is controlled by multiple genes, our objective is to find them and progressively add them to the equation.”
“We now have a number of commercial hybrids with significant levels of stalk rot resistance — and a few with resistance to both stalk and head rot,” Gulya continues. “That was not the case several years ago. I expect continued progress, especially as we evaluate a broader genetic pool for resistance and gradually incorporate the best ones’ traits into commercial varieties.” — Don Lilleboe
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