Choosing the Right Relatives Is Possible
Editor’s Note: The research projects discussed in this article have been made possible by funding via the USDA National Sclerotinia Initiative.
Mark Twain once said, “I can choose my friends, but I can’t choose my relatives.” Like Twain, we all have to put up with a few relatives not of our choosing.
Cultivated sunflower has many relatives as well. Sunflower researchers have worked for many years on picking and choosing the best “wild relatives.” Now a greenhouse method has been developed that has streamlined the necessary “picking and choosing” of these generally ungainly wild relatives.
Dr. Gerald Seiler, USDA-ARS sunflower botanist, has been out looking for and collecting wild sunflower relatives from all over North America for much of his professional life. Wild sunflower (and there are 43 species of Helianthus) are native to North America, and these individual species have adapted to different climates, soil types and pests over the millennium.
Despite all the frustrations of working with these wild relatives, some of them are a gold mine for resistant genes to some of our most difficult diseases — such as Sclerotinia stalk rot, a disease common in the northern production region that can sharply impact yield.
Determining which wild relatives to work with requires a massive screening method that can be very costly and time consuming. Dr. Charles Block, USDA-ARS plant pathologist located at Iowa State University, has developed a greenhouse screening method for Sclerotinia stalk rot resistance. He has already screened well over 1,000 wild annual populations by using Sclerotinia-infested millet as the disease inoculum. He germinates the seed in small plastic cups and places the seedlings on top of the infested millet.
In the case of Sclerotinia stalk rot, the disease infects the roots of the plants. Block’s greenhouse method mimics what occurs in the field. He uses both a tolerant and a susceptible hybrid check for verification.
Block has found a number of wild annual species with good resistance in the greenhouse test. Since all annual wild sunflower species have the same number of chromosomes as cultivated sunflower, these wild “relatives” are fairly easy to cross with cultivated sunflower. He has also found that most of the perennial sunflower species have remarkable resistance. However, perennials are much more difficult for breeders since the perennials have two or three times the number of chromosomes compared to cultivated sunflower. All in all, Block’s greenhouse testing shows a marked improvement in resistance over the best cultivated check.
To verify his greenhouse results, Block takes the best-performing populations to the field for further screening. The field screening also uses infested millet which is placed near the roots of the developing plant in the vegetative stage.
The field screening method has been developed by Dr. Tom Gulya, USDA-ARS sunflower pathologist located in Fargo. The two scientists have been working in tandem on both projects for three years. The first several years of field testing were a bust. “Many wild annual sunflowers are native to sandy areas of the U.S., and don’t like the heavy clay soils at Fargo,” says Gulya. The scientists moved the testing site to Staples, Minn. (some 120 miles southeast of Fargo) where sandy soil has enabled all of the different species to thrive. Even then Mother Nature plays tricks. In 2010, with 300% of normal rainfall at Staples, only minimal stalk rot developed. However, in 2011 the planets were aligned correctly, and a high level of stalk rot was produced, which enabled Block and Gulya to demonstrate that seedlings rated as resistant in greenhouse trials were also resistant as adult plants under field conditions.
Field trials for wild species are very challenging and expensive. The plants need to be started in the greenhouse and then transplanted in the field when they are several weeks old. This past season, 3,000 plants were produced in Ames, Iowa, and trucked 350 miles northward to Staples for hand planting. Later in the season, the inoculum (infested millet) was placed in the soil near the root zone. This was also done by hand.
Given the 2011 field experience, both Gulya and Block feel confident that the greenhouse seedling test can accurately identify resistance or susceptibility. Thus, several cycles of disease evaluations can be done annually in the greenhouse and save a great deal of time and money
Dr. Lili Qi, USDA-ARS sunflower molecular geneticist, has been crossing Block’s best resistant annuals with USDA-ARS cultivated lines. She has completed greenhouse and field screening of these crosses with Sclerotinia-infested millet and is confident in Block’s greenhouse testing method. Qi continues to develop advanced generations for stalk rot resistance screening in the greenhouse and the field. The next step for her is to identify stalk rot resistance genes from the wild annual parent. These genes will be mapped in cultivated populations that will be distributed to private seed companies.
The “take home message” for Block and the other scientists is that the greenhouse testing method he developed is an excellent way to screen thousands of plants in a very short time. It is one of the first research shortcuts for Sclerotinia that seed companies and other researchers can implement in their own locations.
For these pathologists and geneticists, it goes back to what Mark Twain said when he warned about relatives. In the case of the wild sunflower relatives the strategy is to “take the best and toss the rest.” Charles Block’s greenhouse screening method makes that process a whole lot easier.
The Best Candidates
Some conclusions/observations from 2011 and prior years’ field trials:
• Wild Helianthus annuus, the same species as cultivated sunflower, has some moderate levels of resistance. But in general, it is much less resistant than other annual species and thus not the best donor for stalk rot genes. This is unfortunate, since the Ames, Iowa, collection has almost a thousand accessions of H. annuus.
• The cultivated susceptible check hybrid in 2011 had 54% infection, while infection was 18% on the resistant check. Thus, any wild sunflower with less than 18% would be a possible donor to improve the existing levels of genetic resistance.
• Helianthus debilis (the “beach sunflower”), which has five subspecies (all found either on Florida beaches or inland sandy soils of the southeastern states), has the highest level of stalk rot resistance, averaging only 3% infected plants of the three subspecies tested.
• The next most resistant wild annual sunflower is H. argophyllus (the “silverleaf sunflower,” native to Padre Island and the Gulf Coast of Texas). Two accessions of this species averaged 6% stalk rot. One challenge with this species is that some accessions may be eight to 10 feet tall. It generally does not flower in northern latitudes until September.
— Larry Kleingartner
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