Holding Soy Fungi at Bay
In studies to verify the more than 20 races of Phytophthora sojae found
in Indiana, ARS plant pathologist Scott Abney (left) and Purdue University
graduate student Jose Melgar examine thousands of fungal cultures.
Phytophthora infestans, the fungus infamous for causing the Irish
potato famine, has a form called Phytophthora sojae that attacks
And this one is bad news, too. It causes one of the most serious diseases of
soybeans in the United States, Phytophthora root rot.
P. sojae actually attacks soybeans at all growth stages. In
seedlings, it causes rotting of the stem, called damping-off. Later, it causes
the stem and lower branches to turn brown, while leaves turn yellow and wilt.
During the growing cycle, it may also invade roots and lower yields without
showing any visible above ground symptoms.
After 30 years of investigations, Agricultural Research Service plant
pathologist T. Scott Abney has become an expert on combating
Phytophthora, as well as another soybean disease, sudden death syndrome
(SDS). Thanks to his research on the genetics and virulence of fungal
pathogens, the arsenal of weapons available to fight these enemies has
increased substantially over the past few years.
Says Abney, "Far more diverse types of fungal pathogens attack soybean
seedlings, roots, and shoots than were known previously. Some work together
regularly, infecting plants, reducing their growth, and lowering yields.
"Once damaged," he says, "weakened plants become susceptible
to more pests."
One of Abney's tactics is to find out how host plants and fungal parasites
interact and how much inherent disease resistance soybeans have for their
defense. Most have some form of natural resistance, so yield losses of about 25
percent or 10 bushels per acre are more common than total crop failure.
Abney has focused major attention on the Phytophthora fungus and the
soybean genes called Rps.
ARS plant pathologist Scott Abney (left) and research assistant Tom Richards
check the growth of soybeans inoculated with field isolates of Phytophthora
sojae. Disease reactions involving specific genes help identify the 45
races of P. sojae that have been reported in the United States.
"These genes trigger the production of an antifungal compound
phytoalexin that protects the plant from the fungus," he says. "Once
soybeans with specific Rps genes that resist the new Phytophthora
races are identified, public and private soybean breeders can use them to
develop commercial soybeans with improved resistance to Phytophthora root
Since 1955 when Phytophthora root rot was first documented in the United
States, 14 resistance genes and more than 40 races of the fungal pathogen have
been identified. Abney's research goals with Phytophthora are to
document the status of Phytophthora races in soybean production areas
and to develop more effective screening techniques to improve the efficiency of
identifying which race or races is infecting plants.
To describe the races of Phytophthora found in U.S. soils, Abney
collects diseased plants and soil samples from soybean fields. Back in the lab,
he cultures the fungi isolated from the plant and soil samples. Then he
inoculates them into soybean seed lings representing different Rps genes
to identify the race of P. sojae.
"Disease reactions of these soybean varieties are the only way we can
identify races of the fungus and predict what regions of the state different
races will occur in," he says.
Abney's work on determining the prevalence and severity of the fungus led to
his discovery and identification of several new races of P.
sojaesuggesting that genetic diversity among populations of this pathogen
is much greater than previously thought.
"Phytophthora root rot is more widespread in Indiana and other
midwestern and southern states than expected," says Abney. In the early
1990s, 13 of the 27 known races were in Indiana. Abney identified seven
additional races in 1993 and 1994 and is currently confirming the existence of
A cultured Phytophthora sojae sample (on white pad) is inserted into a
naturally resistant soybean seedling.
Working with Mississippi State University scientist Kenneth W. Roy, Abney
found that Fusarium solani and another form, F. oxysporum, were
the most frequently isolated pathogens of soybean seedlings and mature plants.
"We also found that the highly pathogenic form A of F. solani
that causes SDS did not differ between southern- and northern-grown
soybeans," Abney says.
When they examined plants from soybean fields in Arkansas, Illinois,
Indiana, Kentucky, and Mississippi to determine the incidence and geographic
distribution of F. solani in roots of older plants, Abney and Roy
discovered that form B, differing from form A in spore-producing
characteristics was found most often and was widely distributed geographically.
Additional SDS research involving selected soybean varieties and diverse
germplasm sources showed that
germplasm resistant to soybean cyst nematodes (SCN) the P.I. 437654
extensively used in breeding programs is super-susceptible to SDS.
"Awareness of this SDS reaction and selection of progeny that are not
highly susceptible to SDS will permit researchers to reduce the genetic
vulnerability of soybeans to both SCN and SDS," says Abney.
"Knowledge of pathogen variability and host vulnerability to SDS is
leading to more effective control of this new disease of soybeans."
As a member of the ARS-Purdue University soybean improvement team, Abney has
collaborated with colleague James R. Wilcox, an ARS geneticist at the West
Lafayette, Indiana, laboratory, to develop and release 14 soybean varieties
with improved disease resistance.
For all his pioneering work on soybean diseases, Abney was recently
commended by the Soybean Development Council's research committee.
Charles Noble, a member of the committee and a soybean farmer in Daviess
County, Indiana, says, "I'm pleased with Abney's work and his commitment
to Indiana soybean farmers. His research, which is funded in part by the
council, is used by a lot of seed companies, and that's helping us producers
get more from our seed investment."
For his outstanding research contribution to U.S. soybean producers, Abney
was awarded the production research award by the American Soybean Association
in 1995. By Hank Becker.
Scott Abney is in the USDA-ARS Crop Production and Pest Control Research
Unit, Purdue University, West Lafayette, IN 47907; phone (765) 494-9859