NCERA208: Response to Emerging Threat: Soybean Rust
Statement of Issues and Justification
Soybean rust (SBR), caused by the fungus Phakopsora pachyrhizi, was first discovered in the continental United States in November 2004. The identification of the disease caused great concern and alarm throughout the soybean industry because of the documented yield losses attributed to the disease in Africa and Brazil. Yield losses of up to 80% were reported from nations where the disease had previously been detected. Excessive yield losses were the result of extended, favorable environmental conditions for disease development. While several soybean diseases may be equally damaging in the absence of resistant cultivars, SBR is unique because commercially available resistant cultivars remain unavailable as of June 2011. The lack of resistant cultivars means the soybean industry remains vulnerable and completely dependent upon fungicides as the only means of yield loss prevention. Prior to 2005, fungicide evaluations had been conducted in Africa and South America and effective products were identified; however, none of the available chemistries were registered for use in the U.S. or Canada, and regionally specific management protocols had not been established. There was much to be learned and no time in which to conduct the research prior to the 2005 season. However, valiant efforts by members of NCDC202 resulted in Section 18 approvals of selected fungicides, which were subsequently made available to the soybean industry in 2005. Following the initial detection of SBR in 2004, in 2005 the disease spread throughout the Gulf South, and increased in incidence and severity each year until 2009 by which time symptoms had been detected as far north as Ontario, Canada. By 2009 the southern states, including Alabama, Arkansas, Florida, Georgia, Louisiana, Mississippi, and Tennessee had detected the disease annually, and some states, most notably Georgia, Louisiana, and Mississippi (in one particular area in 2009), experienced significant yield losses in isolated areas. Major soybean producing states, including Illinois, Iowa, and others in the Ohio Valley and Midwest, had experienced limited occurrences of SBR most notably during the 2007 season. A reprieve was provided during the winter of 2009/2010 when subfreezing temperatures killed kudzu, the primary known additional host, along the Gulf Coast and deep into the Florida peninsula. This resulted in sparse disease occurrences during the 2010 season. However, even though SBR was eradicated from the U.S. during the winter, Mexico and Caribbean islands likely continue to serve as sources of airborne inoculum especially since SBR was detected at Guantanamo Bay, Cuba in January 2011 and in Puerto Rico in February 2011 for the first time since the initiation of this program. In summary, the North American soybean industry is still at risk from SBR.Needs of stakeholders: During the past few years cooperative extension specialists and others have provided voluminous educational materials for producers, and have led workshops in their respective states where the latest results regarding SBR management were presented. Feedback from attendees clearly indicated that producers were concerned regarding the potential consequences of the disease if it was not detected at early developmental stages so that timely fungicide applications could be made. Producer sentiments also were expressed during annual workshops hosted by the North Florida Research and Education Center, a University of Florida research station in Quincy, FL. While this is not a major soybean producing region, the disease occurs there reliably, and the workshops provided attendees, including producers and other industry leaders from every major soybean producing state and Ontario, Canada, the opportunity to see the disease first hand and learn to competently diagnose the disease on kudzu and soybean plants. The more than 700 attendees at these workshops were a clear indication that stakeholders remain concerned about this disease. In addition, the American Phytopathological Society organized several national soybean rust symposia that were attended by consultants, industry leaders, researchers, extension specialists, media representatives, and others. Detailed surveys following each symposium clearly indicated that management of SBR is a crucial issue to the soybean industry. Finally, the ipmPIPE website, launched to provide daily updates on SBR, has received voluminous hits each season since 2004. Analyses of website traffic have been provided at each of the aforementioned symposia, which can be accessed via http://www.apsnet.org/.
Importance of the work: Objectives for this project are listed below. Briefly, we propose to continue the multi-state and international disease monitoring system that was initiated in 2005. The monitoring system comprises soybean plots (sentinel plots) that are planted at multiple locations in advance of the commercial crops at present in states that are funded to conduct this work, predominantly along the Gulf Coast (AL, AR, FL, GA, LA, MS, OK, SC, and TX). Additionally, mobile plots, comprised of early planted commercial fields, and locations with kudzu serve a similar purpose. Sentinel and mobile plots are monitored by highly trained professionals for the first signs of SBR. When SBR is detected, an alert is issued in that state, and the diagnosis is posted on the ipmPIPE website to inform the entire industry as to the potential spread of the disease, usually from south to north. Producers in a state where SBR has been detected may be advised to commence fungicide applications immediately, depending upon crop growth stage, and states within a particular region with positive finds can also be alerted. Early warnings are critical because it has been well documented that fungicide applications made after 5% incidence result in diminished efficacy. Five percent incidence is equivalent to one pustule, which may be 1 mm in diameter or less, on one leaflet on 20 plants; empirically, one plant with infected plant material out of 20 observed plants is equal to 5% incidence. Experience gained in southern states has determined that the disease can progress from 5% to 100% incidence at extremely severe levels within 2 to 3 weeks given prolonged conducive environmental conditions. The best method to detect such a low level of disease is by collecting leaves (usually 100) from each plot and carefully examining each leaflet with a dissecting microscope in the laboratory. Given this exceedingly low threshold level, the requirement for trained personnel, and the unprecedented explosive nature of SBR, it is clear that the sentinel plot program should be maintained.
Another major issue to be addressed includes the development and deployment of a fungicide resistance monitoring protocol. Briefly, researchers in Brazil reported P. pachyrhizi developed resistance to triazole fungicides, rendering the products no longer effective within 5 years of the initial detection of SBR. Since fungicides remain the only means of yield loss prevention this is an alarming situation and must be prevented in the U.S. Moreover, specific recommendations regarding fungicide active ingredient rotations should be made to minimize the risk of fungicide resistance developing. The recent confirmation of frogeye leaf spot resistance to strobilurin fungicides in Illinois, Kentucky, and Tennessee supports this need.
Plant pathologists in the U.S. and Canada have had 6 seasons of experience with SBR. However, we are still elucidating some of the important epidemiological and etiological details that are necessary for the development of disease management strategies and tactics. For example, even though we have identified efficacious fungicides, we are still exploring optimal soybean growth stage application timings and rates. It would be economically imprudent to apply fungicides when conditions are not favorable for disease development even though the pathogen may be present. In addition, continued exposure of the pathogen to the fungicides could potentially hasten the development of fungicide-resistant strains.
The most important reason to renew this project are to formalize collaborations and coordination amongst participating scientists, which is best managed via the annual NCERA 208 meeting. These meetings have provided opportunities for focused and highly productive discussions and planning sessions regarding i) current research findings, ii) evaluations and refinements to the sentinel plot program and the ipmPIPE website, and iii) other educational materials. It is far too premature in our experience with SBR to relegate it to just another late-season disease .
Technical feasibility: Research and extension projects for all of the objectives listed below are currently in progress. Aside from procuring and maintaining a source of funding, there are no technical challenges to achieving objectives.
Advantages of a multistate effort: This aspect of the project is illustrated by the examples described above. In addition, the SBR pathogen is disseminated by airborne spores that can travel great distances and in some cases leap over and to certain areas as was evident during 2010 when SBR was detected in North Carolina and Georgia, but not South Carolina. It is estimated that 80 billion spores can be produced and released from each diseased acre every day under conducive environmental conditions. SBR knows no borders and has the ability to easily spread from field to field, state to state and country to country. Plant pathologists have been communicating via regularly scheduled conference calls (typically every other week until reproductive growth stages or high levels of the disease are detected when calls may be more frequent) and an email distribution list. Disease management protocols must be refined on an annual basis in each state, and it is important that researchers collaborate with one another to refine chemical control tactics. The potential for rapid spread of the pathogen dictates multi-state efforts in certain aspects of this project but most specifically in coordinating efforts to quickly respond to changes in the organismal population, methods to manage potential epidemics, educational opportunities, and other issues that would require the specific response from this committee. Moreover, the NCERA-208 between 2005 and 2010 was instrumental in building relationships not only across state lines, but with international collaborators and with national and state Soybean Associations, the United Soybean Board, state Soybean Promotion Boards, the North Central Soybean Research Program, and soybean producers throughout the production area.
Regarding item ii) in the previous section, sentinel plot strategies and tactics are evaluated following each season and changes are made for the upcoming season with regard to sample size (the number of leaves to be collected from each site), plot size and locations, the use of dedicated or mobile plots, budgets, and especially the designation of Tier I, Tier II, and Tier III states. The tier system is based upon risk assessments, specifically derived from our group experiences with SBR during each of the preceding seasons. In our previous meetings, we decided to modify the number of sentinel plots and sampling tactics as a function of risk assessment for each state, and this was the basis of tier designations. In addition to states along the Gulf Coast, Oklahoma (on the western edge) and South Carolina were added to the southern states since it was believed they would serve as early warning situations to the movement of SBR to the North Central soybean production area as well as the eastern production areas. The sentinel plot monitoring system is still fluid and should be evaluated annually. It is important to emphasize that we are still learning about SBR, and we must use an abundance of caution before scaling back monitoring efforts. Members of the Tier 1 states, comprised of AL, AR, FL, GA, LA, MS, OK, SC, and TX, as well as representatives from all states with soybean acreage and Ontario, Canada are essential participants in the annual discussions as plans are formulated for upcoming seasons since we all benefit by working as a single unit.
Likely impacts: These are described in greater detail below. Briefly, continuation of this project will allow us to 1) maintain and improve the multistate sentinel plot program, which is essential for providing early warnings to the possible occurrence of SBR; 2) develop and deploy a fungicide resistance monitoring program, which is crucial for the industrys continued dependence on fungicides as the sole means of yield loss prevention; 3) maintain and improve the ipmPIPE website, which has proven to be such a valuable conduit of timely information; and 4) serve as the main source of information as to the whereabouts of SBR throughout North America for concerned soybean producers.
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