Sclerotinia Initiative Underway
Crop scientists from 10 states and Canada reported on their first year's research. Close to two dozen papers were presented in three distinct research areas including epidemiology (how Sclerotinia develops and spreads); chemical and biological control; and germplasm enhancement/variety development.
"This research is in essence the first harvest of seed sown several years ago," said Larry Kleingartner, executive director of the National Sunflower Association, who played a key role in spearheading the Sclerotinia Initiative. In addressing the Initiative's first research reporting session, Kleingartner pointed out that research solutions are needed to keep sunflower a viable crop in the Northern Plains. "Scientists need (this funding) to attack this disease, and research results need to be translated back to growers."
At the Sclerotinia Initiative's first annual meeting, crop leaders prioritized Sclerotinia research needs, and gave an overview of the estimated economic impact of the disease in the U.S each year:
Tom Gulya, USDA-ARS plant pathologist, Fargo, pointed out that sunflower is the only crop where Sclerotinia in effect can cause three diseases:
Wilt (basal stalk rot) from infected roots caused by germinating Sclerotia.
Mid stalk rot, caused by ascospores infecting leaves and progressing to the stalk. This is the least widespread of the three, but does indeed result in lodging and yield loss.
Head rot, caused by ascospores. Very weather dependent, the incidence of head rot can fluctuate wildly from year to year, but it can have a significant impact. "It comes at the end of the season, and there's little a sunflower grower can do but watch the heads fall to the ground. It's a very helpless feeling."
Some sunflower hybrids are more tolerant to Sclerotinia than others, but none that can be called resistant. However, Gulya says crop scientists are analyzing promising sources of germplasm (initial plant material used by plant breeders to develop new crop varieties) from other parts of the world, including Europe, and sources from wild species of sunflower.
Marker-assisted selection will also help in the breeding process. Molecular geneticist Jinguo Hu, who joined the USDA-ARS sunflower research staff in Fargo last spring, will scan the DNA of promising germplasm, and identify genes responsible for Sclerotinia tolerance, using molecular markers to identify where the tolerant gene or genes is located within a plant?s genetic makeup.
The process of using molecular markers is not the same as biotechnology: Molecular markers are essentially a tremendously accurate means of "fingerprinting" germplasm, enabling Hu and his research colleagues to follow traits or genetic makeup in the course of natural sunflower breeding efforts. Hu is able to evaluate the presence or absence of tolerant genes in the lab, and distribute that information to the seed industry to use in their own breeding programs. Then, based on the pattern revealed by these molecular markers, crosses can be made for more Sclerotinia-tolerant hybrids through conventional breeding methods.
Sunflower Head Rot Screening
For the past three growing seasons, sunflower germplasm has been evaluated for susceptibility to head rot at the North Dakota State University Carrington Research Extension Center and South Dakota State University in Brookings.
Entries consist of production hybrids and experimental lines submitted by seed companies. Resistant and susceptible checks are included. Individual heads are inoculated with Sclerotinia ascospores, and field plots are misted every half hour to provide favorable conditions for disease development. After several weeks of misting, inoculated heads are evaluated for head rot symptoms.
To date, substantial progress has been made in developing the infrastructure (water delivery and misting systems) and methodology (inoculation and evaluation) for conducting the head rot screening nurseries, according to Bob Henson and Marty Draper, who coordinate the nurseries in Carrington and Brookings, respectively. Each year of testing, new lessons are learned and new questions arise, they say, and additional work on methodology is needed to increase the precision of the results.
Henson and Draper say that progress toward resistant commercial hybrids is difficult to assess from the results of the screening nursery. Entries vary from year to year, and by location. Also, while experimental lines may show promise as a source of disease resistance in a breeding program, they may lack other traits needed in a commercial hybrid.
Still, there are signs of progress. The best of the 82 entries in the first screening nursery in 2000 was used as the resistant check in 2001 and 2002. Eighty-five entries were evaluated in 2001 and 58 in 2002. Several were rated more resistant than the resistant check. "Promising germplasm does exist, in both oilseed and confection types," says Henson.
The chart illustrates screening results at Carrington last year. Of the 10 confection sunflower entries, two were among the best 10% of all entries for disease rating, and four were among the best 25%, indicating promising germplasm within this group.
Disease ratings were lower with later maturing hybrids in the screening nursery, which were inoculated with the disease to cause infection. "This may be an indication of higher resistance in germplasm with longer maturity, or may be an artifact of less favorable conditions for disease development later in the season," Henson speculates. ( Tracy Sayler)
Select Hybrids That Aren't Susceptible
Crop producers in areas vulnerable to Sclerotinia and white mold should look at growing hybrids that are more tolerant of the disease, no matter the broadleaf crop.
No seed company in the U.S. currently markets a sunflower hybrid that is considered resistant to Sclerotinia, although all of the major players have material in their research pipeline that is considered more tolerant. Even now, most seed companies offer hybrids that are considered less susceptible to the disease as others. For example, some hybrids have a stronger pith (the soft, sponge-like substance in the center of the plant stems) which helps increase disease tolerance. Producers should consult with seed companies to find which hybrids are recommended.
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