Asian Soybean Rust in the US: 2007 Experiences and Future Management

• Asian soybean rust (ASR) (Phakopsora pachyrhizi) has now been present in the US for three complete growing seasons.
• The 2007 season saw a great expansion of ASR occurrences along the western and northern margins of the main soybean production area in North America compared to 2006.
• Only about 3% of the total soybean acres in the US were treated with a fungicide for ASR control in 2007. The greatest need for treatment occurred in close proximity to disease overwintering refuges along the Gulf Coast.
• Buildup of ASR inoculum earlier in the season in Texas and Louisiana resulted in extensive northward movement of the disease up the Great Plains corridor, similar to the historical pathway followed by wheat stem rust.
• Conditions favorable to ASR spore production and infection rates include temperatures between 65 to 75oF, relative humidities no lower than 75 to 80%, and continuous leaf wetness for 6 to 12 hours.
• Short exposure to temperatures above 90oF or ultraviolet-B radiation greatly reduces spore viability. Based on this, sixteen hours of clear-day exposure in Florida would result in a 90% loss in viability of ASR spores.
• More widespread incidence of ASR is likely in 2008 because overwintering refuges for the disease have remained stable in recent years and spore loads have remained at non-limiting levels since 2004.

Asian soybean rust (ASR) (Phakopsora pachyrhizi) has now been present in North America for three complete growing seasons. During that time, the disease has expanded its range west and north from annual overwintering sites. The number of states and provinces in Mexico, Canada and the US with confirmed occurrences has increased from nine in 2005 to 15 in 2006 and now 21 this past season.

As in previous years, producers, crop scouts, university scientists and input suppliers learned how to more effectively manage this disease. Timely scouting of commercial fields and early-planted "sentinel plots" combined with appropriate foliar fungicide applications have generally kept soybean yield losses to a minimum. This article will discuss experience with Asian soybean rust in the major soybean growing regions of the US in 2007, and its implications for management of this disease in 2008.

What was the biggest difference between the spread of ASR in 2007 compared to 2006?
As in 2006, ASR infections in 2007 were prevalent in the Mississippi Delta region and along the Atlantic and Gulf coasts. However, in 2007 a whole new "halo" of disease occurrences were identified along the western and northern margins of the main soybean growing area in North America. ASR detections were confirmed in 2007 for the first time ever for Veracruz, Mexico; Ontario, Canada; and the US states of Oklahoma, Kansas, Nebraska, and Iowa (Figure 1).

December 31, 2006 December 31, 2007 Scouted, confirmed Scouted, not found Figure 1. Asian soybean rust scouted and confirmed locations in the US and Canada following 2006 and 2007 seasons. Maps courtesy of USDA Soybean Rust Information site at www.sbrusa.net.

At what growth stage did ASR infection typically take place in 2007?
Across all states, over 75% of the infections occurred at or after the R6 growth stage (physiological maturity) (Scherm and Christiano, 2007). This is too late to impair seed filling and consequently, grain yield. Earlier infections were generally confined to areas in the gulf coastal states close to annual overwintering refuges for the disease. For example, about 85% of the soybean fields in Louisiana were infected early enough to warrant fungicide treatment. Overall, an estimated 1.34 M acres of soybean were sprayed for ASR control out of the nearly 64M acres in the US, or about 3.3% of the total (Giesler and Hershman, 2007).

Why did infections in the western Great Plains corridor occur so much later than in the Gulf coastal states?
Scientists at the University of Georgia (Scherm and Christiano, 2007) suggest that the pattern of later infection in non-overwintering states is caused by two main factors, the need for ASR inoculum buildup and conducive weather patterns.

Soybean is most susceptible to infection by ASR after early flowering (R1). This developmental stage is triggered by day length and occurs at about the same time in southern and midwestern soybean production areas. Thus, there is a time lag when inoculum present in the overwintering states must build up to sufficient levels before effective transport elsewhere can occur.

Once adequate inoculum buildup in Louisiana and Texas occurs, prevailing wind patterns can move spores northward into the Delta and Great Plains production areas (Figure 2). Inoculum produced in Mississippi and more eastern states tends to move toward the mid-Atlantic region and not due north.

Figure 2. In 2007 the northward movement of ASR followed a similar pathway to the historical route taken by wheat stem rust spores.

How accurate were disease dispersal models in predicting the major ASR outbreaks in 2007?
Simulation models that combine spore development, transport and infection modules have become increasingly accurate in predicting in-season trends in ASR development. This has even been demonstrated for individual, long-distance transport events. For example, research at Iowa State University has confirmed the movement of ASR spores into southwestern Iowa by Tropical Storm Erin on August 22 (Li and Yang, 2008). Following a 3 to 5 week lag for disease development, confirmed infections were made in Fremont County, Iowa just as predicted. This "bridging site" was then demonstrated to be the likely inoculum source for positive disease occurrences in up to an additional 37 Iowa counties located downwind from this source area.

What has been learned about the environmental conditions that favor or inhibit spore production and survival?
Teams of scientists at Louisiana State University and the University of Florida have determined that maximum ASR spore production and infection rates occurred between 65 to 75oF and at relative humidities of no lower than 75 to 80% (Mumma et al., 2007). Continuous leaf wetness for 6 to 12 hours was also quite conducive to disease development. Short exposure to temperatures above 90oF or ultraviolet-B radiation greatly reduces spore viability. In Florida, approximately 16 hours of clear day exposure would result in a 90% loss in viability of ASR spores (Schmitz et al., 2007)

What effect has the prolonged drought in the Southeast had on ASR severity and management?
One outcome of the two-year drought in much of the Southeast has been reduced incidence of ASR in non-coastal areas of the Gulf Coast states. This is largely attributed to higher-than-normal levels of solar radiation (as mentioned above) plus somewhat lower relative humidities. For example, Auburn University scientists documented much lower disease severity and fungicide use in northern and central Alabama compared to the southern portion of the state that was affected by the principle overwintering site in Baldwin County (Delaney et al., 2007). The same was observed in Georgia where the heaviest fungicide spraying took place in the southwest corner of the state (Sellers, 2007).

Are any cultural practices effective at reducing the progression of the disease within a soybean field?
Scouting, early detection and aggressive fungicide applications are the only management practices that can consistently slow the progression of ASR. Selecting a wider row spacing for soybean may facilitate better fungicide spray coverage. But a study by University of Florida scientists found no difference in the progression of the disease when a single infected plant was placed in the center of uninfected soybean plots with 7.5-, 15- and 30-inch row spacing (Narváez et al., 2007).

Will ASR be even more widespread in 2008?
Yes, there will probably be more detections of ASR, primarily in the western and northern portions of the major soybean production areas of North America in 2008. This is predicted because overwintering refuges for the disease have remained stable in recent years and spore loads have remained at non-limiting levels since 2004. Wetter than normal weather in August and September would greatly increase the northward migration of the disease.

What is the best strategy for anticipating when ASR will enter a certain geography?
Computer simulation models have proven quite accurate at predicting when ASR spores will be transported to a new location in sufficient numbers and under favorable conditions for infection to occur. These predictions should give growers a 3- to 5-week lead time to develop scouting and treatment plans for their specific fields.

What is the best strategy for discovering the presence of ASR in a field?
Begin scouting soybean fields at the R1 or early-flowering stage for small lesions or pustules on the underside of leaves in the lower canopy (Figures 3 and 4). Placing suspected leaves in a sealed plastic bag containing a moistened paper towel at room temperature for up to 24 hours will hasten pustule development. Use of 10 to 20X magnification will greatly aid in the identification of ASR foliar symptoms (Figure 3). Many practitioners urge caution in disease identification because many soybean foliar diseases express symptoms which mimic ASR.

Figure 3. ASR pustules on a soybean leaf at 14X magnification.

What fungicides will be available to control ASR in 2008?
Descriptions of 16 commonly used fungicides for ASR control can be reviewed at the following website: www.stopsoybeanrust.com/mc_Control.asp .

Several new ASR fungicide products are being considered for emergency Section 18 exemption and can be viewed at the following website: www.ipm.iastate.edu/ipm/icm/2007/3-26/fungicide_list.html . Be sure to check with the EPA for a current list of the states where these exemptions have been granted.

What resources will be available to track ASR development and guide in-field management of the disease in 2008?
Multi-agency cooperation at the national, state and local levels has successfully assisted growers in the effort to control ASR. This includes elucidation of disease biology, installation of a very effective early warning system (sentinel plots), development of robust scouting and treatment protocols and creation of an ever-increasing array of disease transport and development simulation models. The two biggest threats to continued information development on ASR are reduced federal funding and "rust fatigue."

Figure 4. Be on the alert for ASR pustules on the lower sides of leaves in the lower plant canopy.

Should producers make major changes in the way they grow soybean based on what was learned about ASR in 2007?
No. In fact, growers should be careful of making any major production changes. Producers should be sure to make use of early warnings of ASR presence in nearby sentinel plots and time initial fungicide applications accordingly. The USDA and other university and extension websites will also provide useful updates on the spread of ASR throughout the US in 2008.

• www.sbrusa.net/cgi-bin/sbr/public.cgi
• www.stopsoybeanrust.com

Soybean tissue samples can be submitted to the Pioneer Diagnostic Lab (7300 NW 62nd Ave Reid #33C Johnston, IA 50131) to determine the presence of ASR. Please coordinate your submission through your Pioneer sales representative, Account Manager or Area Agronomist. Follow these steps:

• Samples should be double-bagged in sealable plastic bags and placed into a sturdy box for shipping.
• Include a copy of the Diagnostic Lab's USDA PPQ Permit and Conditions with the shipment.
• Place a USDA shipping label with the correct permit number on the outside of the package.
• A Soybean Rust Diagnostic Sample Submittal Form also needs to be completed and included with the sample.
• Your local Pioneer contact can provide all the required documentation for sample submittal.

The widespread occurrence of ASR in 2007 from northwest Mexico to southern Canada reinforces previous warnings about the potential spread of this disease. It indicates that virtually all major soybean-producing areas in the US are at risk if favorable conditions for spore dispersal and infection occur. Despite this warning, growers must remember they can effectively manage ASR with a growing array of fungicides. However, the timing of the first spray is critical. Producers need to be ready to make the first fungicide spray between the early flowering (R1) and seed filling (R5) stages once ASR presence has been confirmed in nearby sentinel plots.

Much of the information presented here was obtained from the Proceedings of the 2007 National Asian Soybean Rust Symposium held in Louisville, KY, Dec. 12-14.
See: www.plantmanagementnetwork.org/infocenter/topic/soybeanrust/2007

• Delaney, M., E. Sikora and D. Delaney. 2007 The effect of drought on Asian soybean rust in Alabama. Proc. of the 2007 Nat. Soybean Rust Symp. Louisville, KY.
• Giesler, L. J. and D. E. Hershman. 2007. Overview and value of sentinel plots for 2007. Proc. of the 2007 Nat. Soybean Rust Symp. Louisville, KY.
• Li, X. and X.B. Yang. 2007. Analysis of the occurrence and the western pathway of soybean rust in 2007. Proc. of the 2007 Nat. Soybean Rust Symp. Louisville, KY.
• Mumma, E.P., R.W. Schneider and C.L. Clark. 2007. The effects of temperature on urediniospore production in Phakopsora pachyrhizi and development of national risk maps from the 2007 growing season. Proc. of the 2007 Nat. Soybean Rust Symp. Louisville, KY.
• Narváez, D., S. Isard, J. Marois, D. Wright and P. Eskert. 2007. Spread of soybean rust within a field - what happens after the spores arrive. Proc. of the 2007 Nat. Soybean Rust Symp. Louisville, KY.
• Scherm, H. and R. Christiano. 2007. Epidemiological models of spread of soybean rust in the Southeast. Proc. of the 2007 Nat. Soybean Rust Symp. Louisville, KY.
• Schmitz, H., W.M. Jurick, R.H. Grant, C.L. Harmon and P.F. Harmon. 2007. Ultraviolet radiation effects on soybean rust uridiospores. Proc. of the 2007 Nat. Soybean Rust Symp. Louisville, KY.
• Sellers, B. W. 2007. Management of Asian soybean rust: a grower's perspective from Georgia. Proc. of the 2007 Nat. Soybean Rust Symp. Louisville, KY.