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
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
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
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
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
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
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.
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
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.
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
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.
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
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
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.