Dr. Paul's Research Perspectives
“Old” Asian Soybean Rust research from many places holds valuable natural defense clues
South Asian experience and classical plant pathology findings hold promise for non-chemical ASR protection.

By Dr. Paul Hepperly

Soybean rust has many alternate hosts

In Puerto Rico, I helped Nader Vakili, Ph.D., collect the New World soybean rust in elevated areas, mainly on Lablab purpurescens bean. Host range for rust can be very important for rust epidemics. In Australia, perennial wild soybeans are the initial source of a yearly epidemic of Asian Soybean Rust (ASR) on soybeans. In North America, kudzu is considered as the yearly initial inoculum for ASR, starting in southern Florida. ASR does not overwinter on soybeans currently, but soybeans could potentially survive through the winter in true-tropic areas such as extreme southern Florida and Mexico.

The ASR, like the neo-tropical rust described above, infects a broad array of legume Genera including: Cajanus pigeon peas, Canavalia sword beans, Crotolaria shatter box, Desmodium, Glycine soybeans, Lablab beans, Lupinus lupines, Mucuna velvet beans, Pachyrizus yam beans, Phaseolus dry beans, Pueraria kudzu, Vigna cowpeas and others (Bromfield, K. R. 1976). ASR epidemic treatment needs to consider the importance of alternative hosts as well as determine if the pathogen is Asiatic or New World soybean rust.

editors' NOTE:

As New Farm Research and Training Manager at The Rodale Institute®, Dr. Paul Hepperly has been a regular contributor to NewFarm.org for some time, providing research updates, op-ed pieces, and white papers on topics like carbon sequestration in organic farming systems.

None of those venues do full justice to the range of Paul's experience, however. Paul grew up on a family farm in Illinois and holds a Ph.D. in plant pathology, an M.S. in agronomy and a B.S. in psychology from the University of Illinois at Champaign-Urbana. He has worked for the USDA Agricultural Research Service, in academia, and for a number of private seed companies, including Asgrow, Pioneer, and DeKalb. He has overseen research in Hawaii, Iowa, Puerto Rico, and Chile, and investigated such diverse crops as soybeans, corn, sorghum, sunflowers, ginger, and papaya. He has witnessed the move toward biotech among the traditional plant breeding community and the move toward organics among new wave of upcoming young farmers. Before coming to the Rodale Institute Paul worked with hill farmers in India to help them overcome problems with ginger root rot in collaboration with Winrock International.

Now we've decided to give Paul his own column, in which he can report on agricultural research from around the world and reflect on its relevance to The Rodale Institute's research program and to the progress of sustainable agriculture more generally in light of his own broad perspective. Enjoy.

How to contact Paul

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611 Siegfriedale Rd.
Kutztown, PA 19530

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.