| Posted
April 12, 2007: For many decades, Asian Soybean Rust
(ASR) has been known as a serious pathogen, both on soybean
and on other legumes. This disease has been a cause of major
reductions in soybean seed yield in Taiwan, Thailand, the
Philippines and Australia—especially in humid subtropical
and tropical regions. Concern about this pathogen has increased
in the past 25 years in particular as virulent Asiatic races
were introduced and became significant in Africa, then South
America and finally North America.
Within this panorama, producers and agricultural scientists
are wondering out loud how they will be able to manage this
disease and how significant it will be in new areas such as
North American.
The entrance of ASR into Florida with Hurricane Ivan in 2004
has had domestic soybean researchers, extension specialists,
academics, producers and the entire industry scurrying to
confront this virulent fungus. In my
introduction to our role in ASR management research, I
reviewed how the biofungicide Ballad represents a biologically
based control option for managing ASR as demonstrated by research
results domestically, in South America and in Africa.
While Ballad shows potential to give short-term relief to
organic soybean farmers, it is also more environmentally friendly
than chemically based fungicides.
For decades, people in Asia have been managing soybean rust,
particularly those farming in humid tropical and subtropical
areas. It is important to recognize and confirm the rich information
base of international experience as a prime resource for managing
this fungal threat to soybeans.
I’ve found many young professionals are approaching
soybean rust with little historical context or awareness of
the international work done on ASR. For their benefit, I want
to address some of the developing information gaps.
Giants of soybean disease research
I had the good fortune to work with James Sinclair, Ph.D.—considered
the “Kissinger of Soybean Pathology”—during
my doctoral study. One of my graduate school contemporaries
was C.C. Yeh from Taiwan. Now a Ph.D., Yeh was heavily involved
in soybean rust work, including the development of rust race
identification and confirmation of rust resistance.
Dr. Yeh and I were pursuing doctoral studies in the ‘70s
when we participated in seminars and lectures with K. R. Bromfield,
Ph.D., and C. Kingsolver, Ph.D., two leading soybean rust
experts of the time. Dr. Bromfield dedicated the later and
largest part of his scientific career toward increasing the
understanding of soybean rust. Both studied ASR in the isolation
of secured quarantine laboratories at the USDA-ARS in Frederick,
Maryland.
Alas, this Golden Age of plant pathology was not meant to
last. As I write in 2007, the plant pathology department where
I trained has long since dissolved. This key discipline’s
degradation also occurred in many other leading plant pathology
programs around the nation and world. As the study of plant
pathology fades in the collective memory, it joins a number
of other disciplines which did not fit the current technological
paradigms. Without some intentional and extensive effort,
many of the insights and findings of this era will be lost,
to our great disadvantage.
Much ASR work grew out of the Asian Vegetable Research and
Development Center (AVRDC) program in Taiwan and resided in
publications such as Soybean Rust Newsletter (which are not
readily available in public conventional scientific literature).
At the AVRDC, A.T. Tschanz, Ph.D., a plant pathologist, and
S. Shanmugasundaram, Ph.D., a soybean breeder and program
leader, comprised a dynamic scientific duo. This team contributed
much important data on real-life soybean rust epidemics and
soybean reactions under field conditions that most present
ASR researchers have not accessed.
Unfortunately, just as an Golden Age of plant pathology passed
at the University of Illinois, in Taiwan by the mid-80's the
AVRDC had de-emphasized soybean work altogether. By the ‘90s,
Dr. Tschanz had left the institution, and there is little
current work there in soybean breeding or disease resistance
and management for ASR.
Some young researchers seem to believe there is practically
nothing recorded on Asiatic Soybean Rust. I’ve often
heard it claimed that there is little or nothing known of
ASR resistance and that this would not be a useful area for
study or concern. This myth seems to be associated with a
deep faith that the transfer of single genes from unrelated
species can solve just about any and all problems.
As Hamlet said, “I could be bounded in a nutshell ...
were it not that I have bad dreams.” The myth that there
is nothing to be found of use for ASR work in classical plant
genetics and breeding fades away if one delves back just beyond
the usual 10-year literature search. It would be a mistake
to think that anything more than a decade old has nothing
to offer.
Two phakopsora rusts on soybeans
Pre-dating the movement of ASR, a similar pathogen known
as Phakopsora vignae attacked soybeans in Latin America,
the Caribbean and Africa. However, unlike the very virulent
ASR Phakopsora pachirhizi, it occurred mainly at
elevations above 300 meters in the neo-tropics. While both
of these pathogens attack soybean, P. vignae produced
mainly red-brown (RB type) lesions. Red-brown lesions are
usually described by researchers as evidence of a partially
resistant reaction.
On the other hand, P. pachyrhizi characteristically
produces tan lesions on soybeans that have high numbers of
highly sporulating fruiting bodies (uredia) in greater number.
This tan reaction would generally be described by rust experts
as a fully susceptible disease reaction.
Although researchers were initially confused by two morphologically
similar species on soybean, the neotropical rust P. vignae
now is not considered closely akin to ASR P. pachyrhizi
nor a threat to the worldwide soybean industry, as P.
pachyrhizi is. “Pachyrhizi” refers to the
original description of ASR on yam bean, and “vignae”
refers to the original neotropical rust description on cowpea.
ASR in the field
Finally, I want to review how ASR infections occur, what
environmental conditions stimulate them and how soybean varieties
differ in their reactions.
Adult plant susceptibility.
Asiatic Soybean Rust (ASR) is usually found attacking soybean
leaves at flowering and during soybean reproductive stages.
On the leaves we see small spots about 1 mm in diameter with
a polygonal shape. Within the lesions are small volcano-like
fungal structures which exude the fungal spores through a
pore. Most fungal structures are found on the undersides of
leaves.
Nature of rust losses.
When many lesions are present, ASR can lead to a premature
defoliation of the plant and reduction in the size of soybean
seeds produced. With virulent ASR under favorable environments
for rust in areas such as Taiwan, Thailand, Philippines, south
China and Japan, losses can range from 50 to 90 percent.
Infection Cycle.
Uredospores (tiny reproductive particles) that are exuded
at the tip of the uredium (the volcano-like fungal fruiting
structure) are dispersed by splashing rain and wind. A liquid
water film must be present for six or more hours to give rise
to new infection. For most infections, 10 hours or more continuous
wet leaf moisture are required. In addition to wetness, temperatures
of 68° F to 86° F and high humidity with splashing
rains are conditions favoring epidemics.
Soybean plants do not become susceptible to soybean rust
until flowering occurs (Tschanz, A. T., and B. Y. Tsai. 1982).
Under optimum infection conditions, it takes five to seven
days from infection until new lesions become visible and nine
to 10 days for development of new spores.
Epidemiology.
Potential for soybean rust epidemics have been related to
temperature, humidity, rainfall, crop stage and genetics of
the host.
Qualifying rust host reactions
A susceptible host reaction for ASR is a tan-colored lesion
(T) that supports three to five uredia per lesion. An intermediate
susceptible reaction is known as red-brown lesion (RB) that
supports two or fewer uredia per lesion that sporulate only
sparsely. Finally, no visible lesions (0) is considered the
third rust reaction, which represents a functionally immune
response (Shanmugasundaram, S., A. T. Tschanz, and K. R. Bromfield.
1980).
Based on host cultivars and their host susceptibility responses,
several rust-resistance genes and pathogenic races have been
discovered. Asiatic soybean rust-resistant genes are identified
in cultivated soybean and wild soybean species. Resistant
gene combinations have been shown to have differences in their
long-term durability under field conditions.
Geography, outbreaks and response
In the 1950s, Japanese researchers in Shikoku documented
serious soybean rust epidemics. Shikoku, the southernmost
Japanese Island, has a humid subtropical climate ideal for
ASR development. After workers documented the severity and
seriousness of the disease, the movement of Japanese production
to the temperate northerly island of Hokkaido effectively
controlled the soybean rust losses in domestic Japanese production.
The crop was largely abandoned on Shikoku.
Besides Japan, Taiwan has been a site of considerable work
on soybean rust. At lowland study sites used by the Asian
Vegetable Research and Development Center, six to eight uredial
cycles are possible in a single soybean production cycle.
In the late 1970s and early 1980s, researchers at USDA ARS
suggested that the climate prevalent in the southeastern United
States would support serious epidemic potential for ASR (Bromfield,
K. R. 1980). The conditions examined to project this distribution
of potential damage included: high precipitation potential,
adequate temperature ranges and the presence of overwintering
potential on alternative host legumes such as kudzu and Sesbania.
In general, the further north and west in North America you
go, the lower is the prevalence for the likely combination
of favorable rust conditions. If we use China and Japan as
projections, we might conclude that only the southeast quadrant
of the US would support serious epidemics. In fact, ASR is
serious in the southern half of China and Japan and relatively
unimportant in the northern half of China, Japan and Korea.
Tolerance shows promise
Soybean crop tolerance to rust attack and slow rust development
has also been identified in Taiwan (Tschanz, A. T., and T.
C. Wang. 1980). Soybean released for Taiwan and Thailand generally
are bred for slow rust and rust tolerance. Soybean lines under
rust-favorable environments—while all looking susceptible—vary
in ASR yield reactions, showing from 21 to 90 percent yield
loss.
Different
yield loss between tolernat and non-tolerant soybean varieties
exposed to natural Asian Soybean Rust Epidemic in Taiwan
1982.
X axis = Soybean yield (1,000 kg/ha);
Light purple = Fungicide; Dark purple = No Fungicide
At Taiwan’s Asian Vegetable Research and Development
Center (AVRDC), rust-tolerant lines show 50- to 80 percent
more yield than non-improved soybean lines. These results
were gathered by using fungicide-protected and non-protected
split plots designs in Taiwan.
This experience shows tolerance can be an ideal strategy
for dealing with ASR as it implies none of the cost or environmental
damage associated with chemical-control strategies. Many sources
used by Taiwan breeding under S. Shanmugasundaram came from
Japanese programs and materials out of Shikoku, where the
1950 epidemic reactions were identified. Thailand soybean
breeders also worked on developing lines with tolerance to
soybean rust and might be very useful for developing programs.
International data, cooperation needed
here
US researchers would do well to review the full scope of
international experience with ASR. Being the most vulnerable
area, the southeast should host a site for this important
work. Specifically we should:
Seek genetic rust survival
traits. Concentrate efforts on rate-limiting,
slow-rust, and rust-tolerance mechanisms. Tolerance mechanisms
are known to have greater durability from past work compared
to single-gene approaches.
Work on most at-risk variety
groups. Target soybean maturity groups 4
and greater as they represent areas of most potential disease
damage.
Import Asian genetic material.
Focus on efforts to recover commercial germplasm from Taiwan,
south China, Thailand, Vietnam, south Japan and northeast
Australia, toward the goal of utilizing the tolerance and/or
resistance in these soybean strains into the North American
soybean breeding base.
While I’ve emphasized the ways that historical work
can advance ASR management, we have a unique opportunity to
map the genes associated with tolerance and resistance and
to understand them at a more precise level thanks to recent
technological advancements. 
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