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Sugar Beet Science:
Many Crops Could Benefit
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Root-rotting fungi can weaken, stunt, or kill sugar beet plants. Here,
geneticist Leonard Panella evaluates sugar beet plants for resistance to the
fungal disease Rhizoctonia root rot.
(K8992-1) |
Hungry, microscopic worms called
nematodes are the nemesis of dozens of crops—from apricots to zucchini.
Nearly transparent and 10 times finer than an eyelash, these pests penetrate
roots and suck out plant juices, often reducing the quality and quantity of the
harvest. Their voracious feeding also opens the door for root-rotting fungi and
bacteria that can further weaken, stunt, or kill vulnerable plants.
Now, ARS plant geneticist Ming H. Yu at
Salinas, California, has discovered rare strains of wild beet that resist a
half dozen species of notorious nematodes. He says it may be
possible—through genetic engineering—to move the genes that convey
this natural resistance into crops such as peaches, beans, potatoes, carrots,
or tomatoes that are also plagued by the nasty worms. That's in addition to
sugar beets—a domesticated relative of the wild beets and the primary
focus of Yu's research.
Like Yu's studies, the investigations of other ARS sugar beet researchers
should benefit not only sugar beet farmers but also the growers of other
vegetable, fruit, and floral crops. "Many of these crops can be attacked
by the same species of nematodes that destroy sugar beets," says Yu. He is
in the ARS Crop Improvement and Protection Research Unit. |
Closeup of a sugar beet with severe galling from root-knot
nematodes.
(K8990-2) |
Searching for Resistance at
Salinas
A relative of Swiss chard, the versatile sugar beet does more than serve as a
reliable source of high-quality sugar to sweeten foods we eat. It provides a
nutritious addition to feed for beef and dairy cattle as well as sheep. Its
leafy greens are a favorite with some home gardeners. Sugar beet molasses, a
processing byproduct, is used for making yeast, chemicals, and pharmaceuticals.
Growers in 14 states produce America's sugar beet harvest, worth more than $1
billion annually.
Sugar beet growers helped fund Yu's research. In those tests, he showed that
his experimental M6-1 sugar beets, when exposed to very high numbers of
nematodes in the greenhouse, suffered little if any damage.
The six different species of root-knot, or Meloidogyne, nematodes that
Yu used in his studies are heavy hitters. They make up 98 percent of root-knot
nematodes in the world's agricultural soils. Yu's discovery is a first: So far,
no other researcher has reported any sugar beets—or any crop species, for
that matter—that resist all six species.
In addition to nematode enemies, sugar beets can also fall victim to diseases
like rhizomania, or crazy root, called that because of the odd-looking roots of
diseased plants; Erwinia root rot, caused by a bacterium; and virus yellows,
named for the sickly color of leaves of afflicted plants. |
Geneticist Ming Yu and technician Linda Pakish examine a sugar beet damaged by
root-knot nematodes.
(K8990-1) |
Plant geneticist Robert T. Lewellen at Salinas has bred lines of hardy sugar
beets that are resistant to these and other major diseases of sugar beets grown
in the western United States. Some of the new sugar beets have multiple
resistance. That means they can withstand attack by a combination of these
microbial villains.
Lewellen is also working with ARS plant pathologist John J. Weiland from Fargo,
North Dakota, and others at Salinas to find marker genes for natural resistance
to powdery mildew. When uncontrolled, this fungal opponent can reduce yields by
up to 30 percent. Somewhat like molecular signposts, markers may indicate the
presence of genes that confer valuable traits—in this case, disease
resistance.
Finding those genes, says Weiland, will speed development of sugar beets that
can shrug off attack by their worst enemies. In fact, molecular biologists
could rebuild these naturally occurring genes, then slip them back into plants
to "bulletproof" tomorrow's sugar beets. The work should help reduce
growers' reliance on fungicides and other farm chemicals.
Foiling Fungal Foes at Fort Collins
Meanwhile, scientists in the ARS Sugar Beet Research Unit at Fort Collins,
Colorado, have produced dozens of breeding lines with varying levels of
resistance to three other serious diseases: curly top virus, spread by insects
known as leafhoppers; Rhizoctonia root rot, a soilborne fungal disease; and
Cercospora leaf spot, another fungus.
"We've made a good deal of progress," says ARS geneticist Leonard W.
Panella. "We have lines that look good in a severe, artificially induced
field epidemic. They're not fully immune, of course, but they're highly
tolerant."
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In Salinas, California, geneticist Robert Lewellen and technician Jose Orozco
evaluate sugar beet breeding lines for disease resistance.
(K8991-1) |
Panella's team now aims to combine
resistance to multiple diseases. The scientists have already provided growers
with new sugar beets that offer excellent resistance to Rhizoctonia root rot
plus some tolerance to Cercospora leaf spot, for instance. And they're working
on new sugar beets that resist one or both of those fungal diseases—along
with curly top virus.
Panella is also studying strains of Rhizoctonia to ensure that newly
resistant sugar beets won't suffer a surprise attack from an unknown strain.
This research should help not only sugar beet growers, but tulip growers in The
Netherlands, as well. That's because Rhizoctonia can attack tulip bulbs.
Panella and Dutch researchers aim to develop a new test that will quickly
identify the most virulent Rhizoctonia strains. The test may play a key
role in strategies for controlling Rhizoctonia.
Says Panella, "Rhizoctonia has become very damaging to sugar beets
in Europe. We've tested our resistant sugar beets over there to make sure the
Rhizoctonia strains are the same as those we're facing here. So far, our
breeding lines hold up well on both continents."
Targeting a Maggot Menace at Fargo
The greyish-white sugar beet root maggot gives growers nightmares in Minnesota
and eastern North Dakota, a region with about half of the nation's sugar beet
acreage. The maggot larvae chew sugar beet roots, weakening or killing the
plants and increasing their susceptibility to root diseases.
To help growers fight back, ARS geneticist Larry G. Campbell at Fargo, along
with North Dakota State University colleagues, recently released some
impressive new breeding lines to the sugar beet industry. These plants produced
root yields 70 to 75 percent as high as those from commercial sugar beets
treated with insecticide to kill the maggots.
Recent experiments by Campbell and his colleagues showed that applying a
beneficial fungus, Metarhizium anisopliae, to the soil may also
safeguard plants against the feisty maggot.
"We're doing more tests," says Campbell, "to see whether the
fungus could become a commercially successful complement—or alternative
to—the few currently available soil insecticides. For example, we need to
know when to apply the fungus and how much to use for best results."
Besides researching helpful fungi that may become biological controls, the
Fargo scientists are looking for ways to identify and combat pathogenic fungi
that can cause multimillion-dollar crop losses.
"If growers could quickly identify offending microbes without having to
culture them for days in petri dishes," says John Weiland, "they
could start fighting back—with the right tactics—to prevent diseases
from getting the upper hand."
Using a technique of modern biotechnology known as polymerase chain reaction,
Weiland has developed just such a way to quickly and easily identify six genera
of destructive fungi. Now he's narrowing down DNA identification even further
to identify species within these genera, emphasizing Aphanomyces and
Cercospora. So far, he can analyze DNA to distinguish between
Aphanomyces cochlioides, which causes black root disease of sugar beet,
and A. euteiches, which causes root rot in peas and other legumes.
In other research, Weiland is teaming up with ARS colleagues Panella in Fort
Collins and J. Mitchell McGrath in East Lansing, Michigan, to research genes
that code for certain enzymes in the fungi. For example, they're interested in
fungal genes that enable these microbes to penetrate and infect sugar beet
leaves and roots. The studies, says Weiland, "could lead to new ways to
subvert fungal infections."
Markers Point the Way in Michigan
McGrath and co-researchers in the ARS Sugar Beet and Bean Research Unit at East
Lansing are seeking sugar beet marker genes for traits like higher sugar
content or bigger yields. The Michigan scientists have already used some
markers in scrutinizing experimental breeding lines of sugar beet.
The researchers are also using the markers as a guide in mapping the sugar beet
genome. Markers help them pinpoint the location and function of some of sugar
beet's 25,000 or so genes.
"Our team," says McGrath, "is one of only a handful of labs in
the world doing this type of work with sugar beets."
One target of their genome venture: genes for better germination and
establishment, to ensure a strong start for tender seedlings.
"Among this country's major crops," McGrath says, cultivated sugar
beets have the toughest time getting established. Growers plant high-quality
seed, but only half germinate and survive to harvest. Sugar beet growers tell
me this is a major problem that they need our help with.
"One reason for this dismal germination and survival rate," explains
McGrath, may be the low level of genetic diversity in today's commercial sugar
beets. It's one of the lowest of any major crop. Corn varieties, for example,
have genes representing about 60 percent of the diversity found in corn's wild
ancestors. Commercial sugar beets would be lucky to have even 10 or 20 percent.
"We're using marker genes as part of our breeding program to help us find
which "wild" sugar beet genes to reintroduce into domesticated
varieties," says McGrath. "That should broaden the crop's genetic
base and may give us truly superior sugar beets for tomorrow."—By
Marcia Wood,
Don Comis,
Ben Hardin, and
Kathryn Barry Stelljes, ARS.
This research is part of Crop Production, Product Value, and Safety, a group
of ARS National Programs described on the World Wide Web at
http://www.nps.ars.usda.gov/programs/cppvs.htm.
Scientists mentioned in this article can be contacted through
Marcia Wood, USDA-ARS
Information Staff, 800 Buchanan St.,
Albany, CA 94710; phone (510) 559-6070, fax (510) 559-5882. |
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"Sugar Beet Science: Many Crops Could
Benefit" was published in the
August 2000
issue of Agricultural Research magazine.
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