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 System Will Aid Testing for Head rot Resistance March 2000 You can call her what you wish; just don’t call her “predictable.” Despite big advances in meteorological science, Mother Nature still rebels at the insinuation that she can be typecast. And the regularity with which TV weathermen revise their forecasts is a testament to her defiance. While Mother Nature may scoff at the notion of “environmental consistency,” many agricultural researchers conducting multi-year studies do wish, at least for their own purposes, that she would relent. Certain types of research are often hamstrung by the simple fact that the growing season environment in regions like the Northern Plains changes significantly from year to year. Case in point is the search for hybrid tolerance or resistance to Sclerotinia head rot in sunflower. For years, breeders and plant pathologists have been screening sunflower hybrids and parental lines for tolerance to this troublesome form of this vexing disease. However, the degree to which head rot “rears its ugly head” in a given field within a given year is largely dependent upon the type of weather being experienced. Wet and warm conditions during flowering are much more likely to result in significant head rot, for instance, than would a cool and dry climate during the bloom period. So testing for tolerance to Sclerotinia head rot can become a real “hit or miss” proposition from year to year. Even in a disease-prone year like 1999, it’s difficult to determine whether a low incidence of head rot in a particular field is due to hybrid tolerance as opposed to planting or bloom date or some other weather-related factor that cannot be explained by a single year’s experience. Researchers need multiple-year results before they can attest, with confidence, to a hybrid’s inherent level of disease tolerance or resistance. If Mother Nature balks at providing a consistent environment, the logical response is to attempt to create one artifi-cially. In terms of effectively screening for Sclerotinia head rot, researchers are now hoping to use an irrigation misting system to create optimum disease conditions year after year. The National Sunflower Association (NSA) is actively supporting this strategy so that USDA and private sunflower breeders can test their genetic material. With financial assistance from NSA, efforts are moving forward to purchase the necessary misting system equipment. The system would be installed at the North Dakota State University Research & Extension Center near Carrington. The key word in this project is “mist.” For Sclerotinia head rot to develop, sunflower plants need to remain wet for at least seven to 10 days, 24 hours a day. To ensure that the disease is present, researchers would inoculate the plants with a laboratory-produced disease pathogen. Several such misting systems are already in use for other crops in the United States and foreign nations. A University of Illinois misting system, installed to test for soybean tolerance to Sclerotinia, will be in its fifth year of operation in 2000. At North Dakota State University, researchers working on scab resistance in small grains found their misting system to be extremely useful. Speaking to an audience of sunflower scientists during the January NSA Sunflower Research Forum, UI plant pathologist and North Dakota native Wayne Pedersen emphasized that a misting system is essential for this type of research. “Some researchers have tried sprinkler irrigation systems and have failed,” he reported. “They either washed the disease inoculum off the plants or ‘drowned’ the plants with water.” The Illinois group added sunflower plots into its misting nursery last year and successfully developed head rot. The UI scientists inoculated the sunflower plants with mycelia (disease fungi) produced in the laboratory and then sprinkled by hand onto the plant heads. Their success is based on the misting process. “The system is timed so that the plants are covered with a fine mist every 15 minutes, 24 hours a day,” for several consecutive days, according to Pedersen. While the misting system keeps plants wet, very little water actually reaches the soil surface. “We can walk in the plots without our shoes getting muddy,” he notes. Jose Bruniard, a researcher with an Argentine seed firm and current Ph.D candidate at North Dakota State University, has extensive experience in screening for resistance to Sclerotinia head rot in sunflower. Head rot is a more-consistent problem in Argentina than in the United States, and seed companies there have been breeding for resistance for quite some time. Bruniard concurs that in testing for head rot resistance, it’s essential to employ a system that keeps the plants sufficiently moist during the infection period. Artificial inoculation with the disease fungus also is crucial, Bruniard adds. His company infects the plants with the same type of ascospores that would cause infection in a farmer’s field. The ascospores have been produced in the laboratory from sclerotia bodies. (The sclerotia produce the mushroom-like apothecia, which in turn are the source of the infecting ascospores.) Tested hybrids are grouped according to maturity range in order to remove the bloom date factor when inoculating and determining resistance levels. Speaking this winter to a group of U.S. sunflower breeders and other researchers, Bruniard advocated testing finished hybrids first. “Start from the top and work down,” he suggested. “Some hybrids might show tolerance even though their parents do not display a similar degree.” Bruniard utilizes the term “resistance,” not “tolerance,” in his head rot testing program. (Though the word “resistant” implies total immunity to a disease or pest, “resistance” and “tolerance” are often used interchangeably.) He also uses a three-tiered rating system to compare tested hybrids against a known susceptible hybrid. If, for example, the susceptible “check” shows a 10-percent head rot infection level, those being tested would need zero head rot to be classified as having “resistance.” Should the check hybrid incur 50-percent infection, those being tested could be no more than five to 10 percent to meet the criterion. Finally, should the susceptible check suffer 100-percent infection, the tested hybrids can have no more than 50-60 percent. Assuming it becomes a reality — and the signs are very promising — the Carrington misting facility will provide space for any private seed company to submit hybrids for testing on a fee basis. “We will be learning as we go forward, testing a number of variables,” says Bob Henson, NDSU Carrington-based plant pathologist. “But we hope to minimize errors by learning from the experiences of others.” For private seed companies and public breeders alike, the Carrington system will offer the first U.S. opportunity to test their sunflower hybrids and parental lines on a consistent basis under a consistent environment. No company is eager to promote a hybrid’s apparent tolerance based on just one year’s data. At least three years of reliable test results usually are desired before there’s an adequate “comfort level.” Given the complicated and elusive nature of Sclerotinia, even a three-year testing period may leave questions in the minds of some scientists. But it’s a huge step in the right direction — a step whose ultimate beneficiaries will be those producers who farm in areas where Sclerotinia head rot can be a serious threat to the yield and quality of their sunflower crops. — Larry Kleingartner  Back to Disease Stories Back to Archive Categories
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