Idaho USDA-ARS Research Aids in Work to Rein in Rhizomania, Curly Top
By Ann Perry*
The whole point of growing sugarbeets is to produce sugar. But once the beets are harvested and stored for processing, they slowly start to decay, which lowers their sucrose levels.
Roots store sugar even more poorly if they originate from fields infested with the virus that causes rhizomania, a disease that also severely affects yield. Resistance genes in sugarbeet help protect the plant from rhizomania, but some strains of the virus have evolved to overcome one of the resistance genes, Rz1.
“The economic loss from damage to stored beets is quite large,” says Carl Strausbaugh, who works at the Agricultural Research Service’s Northwest Irrigation and Soils Research Laboratory located near Kimberly, Idaho. “For instance, if we could figure out how to save even 1% of the sucrose in beets during storage, it could save producers in the Pacific Northwest $4 million every year.”
The Best Beets
For years, Strausbaugh and ARS molecular biologist Imad Eujayl have studied sugarbeets from the field to the processing factory. Eujayl also works at the ARS Kimberly laboratory.
The two researchers have made several key findings about the pathology of rhizomania, which is caused by beet necrotic yellow vein virus (BNYVV). Some of their evidence suggests that the right genes can help keep beets from going bad and losing sugar during storage.
The team grew around 30 commercial sugarbeet varieties in 2006 and 2007 in fields that were naturally infested with BNYVV. Then they collected samples from each variety — all of which showed some evidence of typical rhizomania infection — and calculated the average sugar content of each variety after at least four months in storage.
The scientists found that roots from some varieties stored indoors had lost as much as 100% of their recoverable sugar content, and roots from some varieties stored outdoors had lost as much as 60%.
The scientists also observed that the beet varieties that exhibited the greatest rhizomania resistance and the best storability — indicated by the lowest levels of fungal growth and lowest levels of weight loss from root damage — also had the highest sugar levels. Breeders can use this information to develop new varieties that retain more sugar during storage, based on selecting for storability and improved resistance to rhizomania.
Appearances Are Deceiving
Strausbaugh’s studies also established a whole new model that explains how pathogens succeed in infecting healthy sugarbeets.
“The fungus Rhizoctonia solani was thought to be responsible for most of the root rot we see in Idaho sugarbeets, and it does have a certain amount of impact,” Strausbaugh says. “But we found that most root mass is lost to bacterial activity, not fungal activity.”
Along with plant geneticist Anne Gillen, who now works in the ARS Crop Genetics Research Unit at Stoneville, Miss., Strausbaugh confirmed that the gram-positive bacterium Leuconostoc mesenteroides subsp. dextranicum is responsible for around 70% of the loss in beet root mass. “We showed that L. mesenteroides starts the fermentation process in the root mass, which then creates a pathway for other organisms to come in and cause spoilage,” Strausbaugh says.
This might sound like business as usual between successful microbes, but results from this research helped to confirm that gram-positive bacteria like L. mesenteroides can be the first pathogen — and often the most damaging one — involved in the root rot process.
Curtailing Curly Top
Every year, western U.S. sugarbeet producers also battle beet curly top virus, which is transmitted by beet leafhoppers. Back in the lab, Eujayl set out to develop a set of genetic markers that plant breeders could use in developing curly top-resistant sugarbeet varieties.
Strausbaugh and Eujayl started by infecting 200 wild, commercial or other different sugarbeet varieties with curly top. Then they ranked each plant according to the severity of its physical responses to infection. When these visible physical responses are the result of the underlying genetics, they are called “phenotypic” traits.
Eujayl then analyzed the phenotypic data with 1,000 sugarbeet DNA genetic markers that had been identified by a process called “diversity array technology” (DArT). He analyzed these markers to identify which ones were associated with the disease-resistance genes. The analysis indicated that 11 of these genetic markers were significantly associated with resistance to curly top — and that five of the 11 markers were linked to the phenotypic resistance trait.
“The DArT markers are abundant compared to other marker systems, like simple sequence repeat markers or single nucleotide polymorphism markers,” Eujayl says. “Using DArT allowed us to identify many markers that we would not have found with the other techniques.”
Strausbaugh also conducted a two-year field study in southern Idaho to see whether curly top damage could be controlled by treating sugarbeet seeds with insecticides, which control the leafhopper that transmits the virus. Working with colleagues, he treated seeds from four sugarbeet cultivars with one of two commercial pesticides, Poncho Beta or Gaucho.
The researchers observed that both insecticides reduced the incidence of curly top in the fields. But as the growing season progressed, plants grown from seeds treated with Poncho Beta produced higher yields — especially in hybrids that were more vulnerable to the disease.
Averaged across all tested cultivars, the recoverable sugar content increased 21%. Because of the substantial increase, the U.S. Environmental Protection Agency used these data sets to issue an emergency exemption for the use of Poncho Beta. Since genetic resistance to curly top is not always available, Poncho treatment will allow for near-normal levels of sugarbeet production, and it also provides an excellent research tool for breeders to use in evaluating other plant diseases.
“The environmental footprint from using foliar insecticides to protect young sugarbeet plants is very large,” Strausbaugh says. “[T]reating the seed with Poncho leaves a much smaller environmental footprint and can protect young plants through the early season growth stages, when they’re highly susceptible to curly top.”
* Ann Perry is a staff writer for Agricultural Research, a publication of the USDA’s Agricultural Research Service. This article initially appeared in the January 2012 issue of the magazine.
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