Technical Science Working Group
Soybean Rust (SBR) – Teleconference #2
Held on January 8, 2003
A. Purpose of Working Group
B. Current Range of SBR
C. DNA Sequencing of Phakopsora spp.
D. Experiences with SBR
E. Field Testing of PCR Probe for Phakopsora spp.
Note: An excellent web resource on various aspects of soybean rust can be viewed at: http://www.aphis.usda.gov/ppq/ep/pestdetection/soybean_rust/soybeanrust.html.
A. Purpose of Working Group – presented by Kent Smith, OPMP, Washington, DC
The basic purpose of this working group is to enhance communication between state and federal scientists concerning soybean rust. Specifically, we hope to better prepare state extension specialists to deal with soybean rust given that they will make the recommendations to growers concerning this disease when it arrives in the continental United States.
B. Current Range of SBR – presented by Mo Bonde, ARS, Frederick, MD
Soybean rust (SBR) is caused by two species of fungi, Phakopsora pachyrhizi and P. meibomiae. Soybean rust caused by P. pachyrhizi was first discovered in Southeast Asia and the western Pacific and is now recognized as a virulent, yield-limiting disease of soybeans. Soybean rust caused by P. meibomiae, however, is limited to the Carribean, Central and South America, and is considered of little or no economic importance to soybean production.
P. meibomiae induced soybean rust is not of interest as a yield-limiting disease and therefore is not the inferred species of interest when soybean rust is discussed. However, it is easily confused with the yield-limiting species, P. pachyrhizi, and is consequently often responsible for misidentification of that species. For this reason, ARS has developed a molecular identification technique for both fungi that should yield rapid and accurate identifications.
Since its early discovery in Southeast Asia and the western Pacific, P. pachyrhizi has been discovered in most regions of the world where soybeans are grown. Recently it has been reported in Hawaii, Africa (Zimbabwe, South Africa, and Nigeria), and South America (Brazil, Paraguay, and Argentina). Thus far, it has not been identified in the Western Hemisphere north of Brazil. Entry into the continental United States appears probable creating the need to prepare for its arrival.
C. DNA Sequencing of Phakopsora spp. – presented by Reid Frederick, ARS, Ft. Detrick, Maryland
ARS has contracted with the U.S. Department of Energy’s Joint Genome Institute (JGI) in Walnut Creek, California to sequence the genomes of the Phakopsora species that cause soybean rust. This work began in November of 2002 and should conclude sometime near the end of 2003. Once the genomes of these fungi are completed, the sequences will be published or made available to all interested researchers through a Web-based database.
It is hoped that these gene libraries will prove useful in various molecular experiments. Full genomic sequence data from the two species of soybean rust will be indispensable in identifying polymorphic DNA sequences with high potential for strain identification, in addition to the obvious utility of comparative genomic studies between the two rust species to identify genes that make a species highly virulent. Also, this information could prove useful in developing resistant soybean varieties through non-traditional methods (i.e. genetic engineering).
Information about the JGI can be found at the following Web address: http://www.jgi.doe.gov/
D. Experiences with SBR
1. Taiwan and Southeast Asia I – presented by Arnie Tschanz, APHIS, Riverdale, MD
During the years of 1978 to1988, Arnie was Head of Pathology at the Asian Vegetable Research Disease Center (AVRDC) in Taiwan. Throughout his entire stay, he was involved with research on the epidemiology and control of SBR. They were able to add several new hosts to the host range of SBR, although the degree of likely field severity was often unresolved.
In the early 1970s and during Arnie’s tenure, the AVRDC screened virtually all the U.S. germplasm available at that time. Several lines showed some element of resistance, including a red-brown lesion type with reduced uredinia production. Unfortunately, all forms of identifiable resistance broke down over time. One complicating factor in this work was that resistance characters are heavily influenced by the host and the environment, making them difficult to use in the field. Arnie’s team was able to show the presence of tolerance, where certain soybean cultivars would have a high relative yielding ability while sustaining severe SBR symptoms. They also developed a selection technique to identify tolerance in segregating lines.
An attempt was also made to identify different races of the pathogen. This proved extremely complex because most races were capable at some point of causing a susceptible reaction on the ‘resistant’ differential cultivars. Other disease factors investigated were photoperiod effects, rate of rust development, rainfall effects, and fertilizer effects. It is interesting to note that SBR was usually more severe in the well fertilized and managed fields at the experiment station than in nearby local farmers’ fields managed under a low input system.
2. Taiwan and Southeast Asia II – presented by Glen Hartman, ARS, Urbana, IL
Glen served in the same position as Arnie at the AVRDC from 1989 to 1993 in Taiwan. His experiences with soybean rust involved a continuation of many of the same epidemiology and host resistance studies. He noted that SBR was more of a problem in the Spring and Fall in Taiwan and would be expected to act the same in the Midwest of the United States.
Kudzu, another host of P. pachyrhizi, is expected to be a significant source of inoculum in the United States as it has proven to be in many areas of the world. This and several other hosts of the SBR pathogen are expected to render P. pachyrhizi endemic wherever it is introduced and capable of overwintering.
3. China and Thailand – presented by X.B. Yang, Iowa State University, Ames
XB carried out a Foreign Agricultural Service project for USDA in the mid 1990s dealing with disease forecast modeling. He became familiar with the devastating effect of SBR on Chinese soybean acreage and has shared with us a map of disease occurrence of SBR published by Tan et al. (PowerPoint map attached)
Areas in China with similar latitude lines to the U.S. may serve as an indicator of rust severity to be expected in the United States. In China, the latitude line of 35 degrees north is the limit of frequent SBR disease occurrence. Areas south of this line are frequently affected by SBR, while areas north of the line may be affected by SBR but only occasionally. In the U.S., this latitude line passes through North and South Carolina, the southern border of Tennessee, Arkansas, and Oklahoma. While other epidemiological considerations may moderate this observation, it may give us some idea of what to expect.
Chinese farmers in disease prone areas have found that one or two fungicide applications are sufficient to manage SBR. It must be remembered that varieties of soybeans grown in China are different from the United States as are cultivation practices. Also, they are dealing with about 40 identified races of the pathogen, which may vary considerably from what arrives in the United States.
4. Africa and South America – presented by Reid Frederick, ARS, Ft. Detrick, MD
Reid has been working with several cooperators in Africa and South America as this pathogen has spread from continent to continent. These cooperators have offered many insights into the advance and management of this disease, as well as acting as co-investigators on comparative fungicide efficacy experiments.
The African experience with SBR is seen as a good parallel to what may happen in the United States. In Africa, soybean rust has been reported on several legumes since the 1960's (see the APS Soybean Rust Monograph by Bromfield). In 1997, it was first reported on soybeans in Uganda and Kenya during the previous “long rains” (October/November 1996) by P. Tukamuhabwa at the All-Africa Crop Science Congress in Pretoria, South Africa (Clive Levy, personal communication). In February 1998, soybean rust was found for the first time in Zimbabwe, and it has reoccurred there every year since. Dr. Clive Levy, Plant Pathologist with the Commercial Farmers Union of Zimbabwe, has conducted fungicide trials that show that three applications of fungicide are required to control soybean rust, and this is the current recommendation for farmers in Zimbabwe. Dr. Levy is currently conducting fungicide field trials for six compounds that might be used in the U.S. to control SBR. This research is part of a United Soybean Board sponsored research project on SBR being monitored by Reid Frederick of ARS.
In the U.S., entrance of the pathogen of SBR is expected somewhere around the gulf coast, an environment closer to Africa and South America and more suitable to the pathogen than other regions of the U.S. In Africa, several years passed before SBR reached it full expected range, arriving in South Africa in 2001. Opinion varies on the time required for SBR to dominate its expected range in the U.S., but several years would likely be required for SBR to attain its full intensity and range.
South America is relatively new to yield-limiting SBR. Although P. meibomiae has been around for several decades, P. pachyrhizi was only identified in 2001 in Paraguay. Its present range includes Brazil, Paraguay, and the northeast portion of Argentina that boarders Paraguay and Brazil where it is estimated to infest only 1% of the available soybean acreage. Given favorable environmental conditions, the range and severity of SBR is expected to increase quickly.
E. Field Testing of PCR Probe for Phakopsora spp. – presented by Doug Luster, ARS, Ft. Detrick, MD
Doug indicated that the PCR probes developed by Reid Frederick in his ARS Foreign Disease and Weed Laboratory need field testing. He has already transferred the probes to Mary Palm, the APHIS identifier, but he would like to see them validated by an appropriate field unit. He is willing to work with anyone who can help by providing this important service.
Once validated, the PCR probes will be shared with the Regional Diagnostic Centers to enhance our capability to respond quickly to suspected samples of soybean rust. Under consideration are plans to offer this technology in a kit form in order to reduce cost and increase ease of utilization.
Following the teleconference, Laurene Levy, USDA-APHIS-PPQ, indicated that her laboratory would assist in this effort. They are currently testing molecular probes for Sudden Oak Death.