The wait is over. Asian soybean rust (SBR) has been detected in the southern United States. At this time SBR infestations have been observed in ten states with the furthest north being Tennessee and Missouri. Minnesota soybean growers are justifiably concerned with the potential for damage to their crop and livelihood. This is a new soybean disease for North America. While some information can be obtained from experiences in managing the disease in other parts of the world, Asia, Africa and South America, it is expected that management strategies appropriate for the United States will be developed over time. While plant pathologists are developing a comprehensive set of management guidelines for Minnesota, some common questions can be partially answered.

When will SBR get to Minnesota?

This disease should not be able to overwinter in Minnesota. Rather, SBR spores will have to move north every spring with wind currents. This commonly occurs with many insect species and is typical of wheat stem and wheat leaf rust. If SBR successfully makes it through the winter in the south, it is likely to make it to Minnesota sometime during the 2005-growing season. This does not mean that spores will arrive in large enough numbers, early enough and with favorable weather to cause yield loss or even be detected. SBR infection is favored by moderate temperatures, high relative humidity, and prolonged periods (six hours or more) of leaf wetness. Frequent rainfall can increase the chances of infection.

Fortunately, severe problems from migratory pests do not occur every year. Our position at the northwest portion of US soybean production will be an advantage to Minnesota producers. It is expected that soybean rust will be a much less frequent and severe problem in Minnesota than in states to our south and east.

How far and how fast can we expect this disease to spread?

Soybean rust dispersal is highly dependent on environmental conditions. Once the pathogen is present, abundant spore production occurs during wet leaf periods of at least 8 hours (including extended dew periods) and moderate temperatures of 60 to 80°F. The spread of the disease within a field can occur quickly. Long-distance dispersal is dependent on wind patterns and weather conditions and is the subject of current research.

How does Asian Soybean Rust rob yield?

The rust fungus starts as a spore germinating in water on a leaf. The resulting fungal germ tube can enter through stomates or directly penetrate a leaf cell where it continues to grow through leaf tissue. Initially, infection appears as a light area on the leaf (usually the underside). Eventually leaf tissue is killed resulting in lesion. In as little as a week, the fungus produces uerdinia which erupt through epidermis and produce tremendous numbers of windborne spores which can infect soybean tissue in the same, neighboring or distant fields. The lesions reduce yield by destroying photosynthetic area. The fungus also diverts water and nutrients from the soybean plant compounding photosynthetic losses.

How much yield will I lose from soybean rust?

Like any other soybean management problem, yield losses from SBR are quite variable. Yield losses up to 80 percent have been reported in other parts of the world. This level of yield loss is rare but 50 % loss is not uncommon during severe outbreaks. It is also possible that under less sever disease outbreaks little or no detectable yield loss may occur.

Assuming that no control efforts (foliar fungicide applications) are made, yield loss will depend on the following factors.

• The virulence of the SBR strain. Soybean rust has considerable genetic variability similar to strains of human influenza. For example, some strains are more aggressive pathogens and others are able to infect a wider range of hosts.
• The amount of spores available to infect the field. This will depend on the number of spores produced in the south that move into Minnesota. How bad the disease will be in any given season and location will also very much depend on continued infection and spore production within local soybean fields.
• Favorable environmental conditions for infestation and continued disease epidemiology. Adverse conditions for the disease (high temperatures, low humidity and rainfall) can slow or stop SBR infections.
• The stage of soybean growth at the time of infection. The potential for yield loss deceases as soybeans mature.

What can I do if soybean rust is threatening my soybean yield?

Properly timed and applied fungicides can reduce yield losses from SBR. A selection of strobilurin and triazole fungicides has full or Section 18 labels for soybean rust control. It is prudent to include a fungicide application in crop budgets.

Can I cure soybean rust infections by applying fungicide?

The strobilurin fungicides prevent new infections and are most effective as a preventative application before infection has occured. The triazole fungicides can cure early stage rust infections and are preferred as a first application when rust symptoms are visible. None of these fungicides can cure advanced lesions in a leaf. Both strobilurin and triazole fungicides can be viewed as preventative. However, these products have a limited period of control and more than one application of fungicide maybe required if SBR occurs early in the growing season

Can I plant a soybean rust resistant variety?

No. Although differences in susceptibility have been found among varieties and several genes have been recently been identified that have a role in SBR resistance, rust resistant varieties are not currently available. As with most soybean diseases, the development of SBR resistant varieties is a long-term goal.

Will tillage or crop rotation help prevent Asian Soybean Rust?

No. Rust spores are not long lived and this fungus needs live host tissue to survive, Historically, the green hosts for soybean rust are very scarce in Minnesota from November - March. Since the fungus cannot overwinter and each season’s initial infection results from wind borne spores, crop rotation within a field and tillage are irrelevant in the control of this disease.

How do I know if my field needs to be treated?

Unfortunately, SBR management requires applying control before symptoms are easily observed. Additionally, in the critical early stages soybean rust resembles other diseases.

For management decisions, this disease does not lend itself well to scouting. Several methods to monitor rust progress from the south, model and track wind and rainfall events bringing spores into Minnesota and detect and monitor the progression of the disease once it arrives are planned. Unlike soybean aphid, management of SBR is best handled with a preventative approach and on a regional rather than individual field basis. Stage of soybeans and weather forecasts may reduce the need to apply fungicides.

Do I need a new sprayer?

It depends on your current equipment’s ability to deliver sufficient water volumes (20 gallons) at adequate pressures. Minimally, you may need to switch to different nozzles. Controlling SBR with fungicides requires good coverage of the soybean canopy. This requires small to medium droplets with sufficient water volume and pressure to reach the lower part of the canopy. Droplet size, water volume and pressure for effective fungicide applications will be much different than the low volume, low pressure nozzles that create coarser, non-drifting droplets desired for application of glyphosate. Think more along the types of equipment needed with contact herbicides.

The two most common classes of fungicides that are most effective (triazoles and strobulurins) move very little within the plant. All movement is upward and usually confined within a leaf. One droplet containing fungicide will not protect an entire leaf. The combination of water volume, pressure and nozzle type will need to create droplets that are small and numerous enough to provide good coverage of foliage and with enough size and speed to penetrate to the lower canopy. Although some nozzles currently being tested may allow lower water volumes and maintain control you may need to apply up to 20 GPA by ground to obtain optimal control.

Proper sprayer setup becomes increasingly important as the density of the soybean canopy increases.

How come we can’t get absolute answers?

SBR is a relatively new disease outside of Asia and just recently reported in North America. Additional information is becoming rapidly available. Secondly, it is expected that this will not behave the same in North and South America due to differences in weather patterns, soybean development, soybean acreage and alternative host distribution and differences in soybean genetics. Initial SBR management strategies will be adopted from other areas of the world but over time can be geared toward the interaction between soybean rust and its new North American home. A large amount of work has already been completed. Section 18 fungicides labels have been pre-approved. Public and private soybean pathologists have been communicating and working with colleagues in areas where rust occurs. Under USDA quarantine initial variety screening has occurred. Finally, soybean checkoff dollars have been largely responsible for having prompted much of the current accomplishments.

Where can I get additional information as it becomes available?

US SBR updates from USDA-APHIS
www.aphis.usda.gov/lpa/issues/sbr/sbr.html

Minnesota updates and approved fungicide lists from MDA
www.mda.state.mn.us/invasives/soybeanrust/default.htm

SBR management recommendations for Minnesota farmers – coming soon
www.soybeans.umn.edu

Soybean Rust Contact List

Authors of this fact sheet

*Seth Naeve – 612-625-4298 -- naeve002@umn.edu

*Bruce Potter – 507-752-5066 -- bpotter@umn.edu

*James Kurle – 612-625-3167 -- kurle001@umn.edu

*Geir Friisoe – 651-297-7174--Geir.Friisoe@state.mn.us

Minnesota Department of Agriculture

* Primary SBR Contact Person – Geir Friisoe 651-297-7174

* Pesticide Registrations – John Sierk 651-296-4292

* Soybean Pest Surveys – Mark Abrahamson 651-296-6509

University of Minnesota

* Soybean Research – Dr. Jim Kurle 612-625-3167

* Plant Disease Diagnostician – Sandee Gould 612-625-1275

* Soybean Extension – Dr. Seth Naeve 612-625-4298

USDA – APHIS, PPQ

* Kevin Connors ( Minnesota contact) 612-725-1722

USDA - Agricultural Research Service

* Soybean/Maize germplasm, pathology and genetics research unit. Dr. Glen Hartman - Urbana, IL 217-333-1117

Minnesota Soybean Research and Promotion Council
Minnesota Soybean Growers Association

* Rob Hanks, Research Committee Chairman 507-561-3667

* Mike Youngerberg or Jim Palmer 888-896-9678 (staff)

United Soybean Board

* Ed Ready, Production Committee 888-579-1580 (staff)