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According to the USDA’s survey report on planting intentions, about 89% of soybean producers have heard about soybean rust. Minnesota’s efforts at getting the word out about a potentially devastating disease has been ongoing since November of 2004 when the disease was identified in southern states. Many producers are not only aware of the disease, but are also preparing to manage for soybean rust if it moves into this region.

The Minnesota Department of Agriculture (MDA) and the University of Minnesota (U of M) are committed to ensuring the agricultural community is prepared for soybean rust. The MDA has applied to the Environment Protection Agency (EPA) for use clearances on additional fungicides that weren’t previously labeled on soybean. The U of M is playing a crucial role in educating and informing producers about the biology and lifecycle of the fungal pathogen, Phakopsora pachyrhizi. Efforts are ongoing to develop successful disease management strategies.

While regulatory and land grant organizations in the US are involved in numerous efforts aimed at achieving a status of readiness, this article will summarize only a few of the most notable - from a producer’s perspective.

A number of efforts are being made to track soybean rust progress from one state, or region, to another.

1) Sentinel plots

Early disease detection efforts will be conducted across soybean producing regions during the 2005 growing season. Field locations at greater risk for disease will be monitored closely. The sites will be scouted weekly and a number of lower-canopy leaves will be removed and examined during each visit. The sentinel plot concept will be interpreted and modified by states as needed, but the reason for the scouting (e.g., early disease detection) will remain a universal constant across sites.

Some sentinel plots will likely be located on public land, such as university experiment stations, while other ‘plots’ will be localized areas in much larger commercially-planted soybean fields. In-field areas where plant leaves remain wet for extended periods (e.g., near tree rows, low spots or other protected areas) will be scouted extensively. On the other hand, plots established and maintained by university researchers will be planted in narrow rows and contain early-maturing varieties. The reasoning behind this is straightforward. In other countries, soybean rust severity generally worsens when two events occur: (1) soybean plant growth shifts from a vegetative habit (e.g., the production of stems and leaves) to a reproductive habit (e.g., the production of flowers, seeds, and pods); and (2) crop canopy closure is completed. Scientists haven’t determined which factor is the most important in soybean rust development (plant growth stage vs. canopy structure), so many states are planting sentinel plots to ensure that both criteria are present.

Minnesota will have a sentinel plot detection effort in place during the 2005 growing season. However, because of our northern growing climate, it’s unlikely that planting soybean plots earlier than commercial fields will result in a good plant stand. For this reason, soybean rust detection in the Red River Valley (RRV) will focus on earlier-planted, commercial soybean fields. Fields with advanced growth soybeans will be identified early in the growing season by U of M regional and local extension educators. Working closely with producer-cooperators, extension personnel will monitor plants on a weekly basis. This level of monitoring will continue until either the disease is confirmed in our region or is identified in a state which serves as a source of spores for wheat leaf rust. Each year leaf rust spores are routinely transported from states such as Texas, Oklahoma, and Kansas by wind and deposited in area fields.

2) North Carolina disease risk forecasting system website

http://www.ces.ncsu.edu/depts/pp/soybeanrust/forecasts/s050401.php

Researchers at North Carolina State University are currently tracking weather system movement from areas identified to have spores of P. pachyrhizi in the US. The website provides information on weather systems and wind directions. It estimates the risks for high and low level air currents that might deposit rust spores in fields from infested locations up-wind. These efforts provide information on the risk for disease by identifying potential spore trajectories.

Currently, Florida is the only state known to have live spores of the rust pathogen. On March 21, the disease was identified in the mid-section of that state ( Hernando County) on new spring growth of kudzu, an aggressive weedy plant species that’s known to be widespread. While rust spores were identified in Florida from kudzu leaves earlier this spring, spore viability (e.g. whether a spore is alive and is capable of infecting a susceptible plant host) tests were inconclusive. Since evidence of soybean rust has been discovered on early season plant growth in Florida, it appears likely that the fungus survived the winter in the US.

3) USDA soybean rust mapping website

http://www.sbrusa.net/

Plant pathologists and others from land grant universities, regulatory agencies, and private companies will submit data from disease detection and monitoring activities to the USDA. The information will be used to track disease progress of soybean rust in the US. Detection and monitoring efforts are already underway in Texas, Louisiana, and Florida and are illustrated on the website’s map. A small area within Florida has been colored red to indicate the location where diseased plant leaves were found. Results from other states are colored green to indicate that those locations were monitored, but the disease wasn’t detected.

Fungicides are licensed with the EPA and disease management recommendations are in place IF soybean rust becomes a production issue in Minnesota during 2005.

1) Section 18 fungicide registrations

During April of 2004, seven months before soybean rust was identified in Louisiana, Dr. Marty Draper, an extension plant pathologist at South Dakota State University, prepared a ‘section 18’ fungicide emergency exemption application. The application, submitted to the EPA by the Minnesota and South Dakota Departments of Agriculture, requested emergency registrations on a number of commercially-available fungicide products if/when soybean rust was identified in the US. Since that time, additional applications have been submitted to the EPA. Currently, a total of 13 fungicides (some with more than one formulation), have been evaluated and approved for use in the two states. Another section 18 request is expected to be submitted as I write this article. The EPA recently announced that two mixed active ingredient products, Quilt (Syngenta Crop Protection) and Headline SBR (BASF and Bayer CropScience), are also approved for soybean rust management. Please see the attached updated Soybean Rust Fungicide Table (pdf) for additional details (original author of the table is unknown).

In a March of 2005 letter addressed to the MDA, the EPA stated that “a maximum of three total applications using approved section 18 products collectively may be made under the soybean rust section 18”, but further directed “that no more than two applications may be made with any given active ingredient.” Producers and commercial applicators must be made aware that one application of a mixed active ingredient product (a product containing two different active ingredients) will be considered as one application use for two active ingredients under EPA’s section 18 exemption (e.g., Stratego contains propiconazole and trifloxystrobin; Quilt contains propiconazole and azoxystrobin; Headline SBR contains tebuconazole and pyraclostrobin). This regulation is expected to be more important for producers in southern states where the economics of soybean production will support more than one fungicide application for managing soybean rust.

2) Disease management recommendations

Most traditional tools for managing disease issues are neither effective nor available for use against soybean rust. For example, the disease isn’t residue-borne like many wheat diseases known in the Red River Valley (e.g., Fusarium head blight, septoria leaf blotch, tan spot, net blotch), so crop residue management isn’t expected to reduce soybean rust severity. Spores must be transported into the state from southern location. It remains unlikely that the fungus will survive a Minnesota winter.

Plant disease resistance can be a very effective disease management tool. But, soybean rust resistance isn’t available in commercially-available varieties. In the past, the fungus has overcome single gene resistance rapidly, so care must be taken to use as many different resistance genes as possible in varieties. This strategy will minimize the pathogen’s chances at overcoming plant resistance.

Short term disease management is the only option for protecting the US soybean crop. Timely and thorough applications of fungicides are known to adequately protect plants. As mentioned earlier, several products are registered for use against soybean rust in Minnesota. Good disease management depends on applying enough of the correct active ingredient at the right time. A simplified fungicide use guide  (pdf) based on product active ingredients has been developed by Kentucky, Ohio, and South Dakota extension plant pathologists. While the guide was developed to support multiple fungicide applications, Minnesota farming operations cannot remain economy viable if multiple fungicide applications are made routinely, especially if other production issues (white mold and soybean aphid) also reduce yield. The guide is useful for determining product use differences between chlorothalonil, strobilurin, triazole, and premix fungicides. Strengths and weaknesses of the different active ingredients are known, but US research has yet to verify if Brazilian reports on product efficacies are accurate in our environment.

• Chlorothalonil products are used as barriers against rust spore infection. The products must be applied thoroughly for complete plant coverage before spores are in the area and must be reapplied at regular intervals if rust spores are present. Adjuvants such as spreaders and stickers are recommended.
• Strobilurin products should be applied on the crop before rust spores are in the area and coverage on plants must be thorough. The products translocate into plant tissues, making products more resistant to weathering. Strobilurins do not stop disease development after plants are infected.
• Triazole products can be applied either before or immediately after soybean rust is detected, since fungal growth within plant tissues is disrupted. These products do not stop additional spores from being produced, so disease inoculum can increase rapidly if application is not made when the level of the disease is low.
• Triazole and strobilurin premix products are useful if soybean rust has already been identified in the region. The strobilurin active ingredient provides a barrier against additional fungal infections and the triazole active ingredient helps in controlling early infections.

Disease control efficacy varies among active ingredients and products. Contact a reputable agriculture professional for additional information concerning fungicide product selection, application timing, and plant coverage requirements.