EXTENSION PLANT PATHOLOGY North Dakota State University

ASIAN SOYBEAN RUST

Asian soybean rust, caused by the fungus Phakopsora pachyrhizi, was first found in the continental United States in November and December of 2004 in 9 states.  The introduction of soybean rust spores to the continental U.S. was most likely due to Hurricane Ivan. 

BIOLOGY OF SOYBEAN RUST

     Environment.  Phakopsora pachyrhizi prefers moderate temperatures and wet environments.  The temperature range for spore germination is 46 to 97°F; however, the optimum temperature range for spore germination is 61 to 75°F.  The temperature range for infection is 52 to 82°F; however, the optimum temperature range for infection is 66 to 75°F.  A minimum of 6 hours leaf wetness is required for infection to occur.

     Disease cycle.  Because Phakopsora pachyrhizi needs a green host to survive year-round, it will most likely only be able to overwinter in very southern locations in the U.S.  Kudzu, which is a prominent weed in the southeastern U.S. and a host for soybean rust, may serve as this overwintering host.  Only one type of spore (known as an urediniospore) is important in the disease cycle of soybean rust.  This spore is airborne and can travel many miles.  These urediniospores infect leaves, which will in-turn produce more spores.  Soybean rust will have multiple disease cycles under a favorable environment, making it a polycyclic disease.  With this type of disease, the amount of inoculum (spores) will increase exponentially, making it more difficult to manage.  For this disease to reach North Dakota, spores must be blown in.  This is similar to how North Dakota wheat growers can have wheat fields infected by wheat leaf rust (See Puccinia Pathway).

     Host range.  Phakopsora pachyrhizi can infect 31 legume species in 17 genera.  Hosts grown commercially in North Dakota include soybean, dry edible bean, and dry pea.  Some forage crops are also susceptible.  For a more complete list of hosts, click here (PDF file - Click here to download Adobe Acrobat Reader).      

MANAGEMENT OF SOYBEAN RUST

Fungicides are currently the only way to manage soybean rust.  For North Dakota, the presence and severity of North Dakota will most likely be different from year to year.  The severity of soybean rust will depend on when (or if) spores arrive in North Dakota, the local weather, and the stage of crop development.  Monitoring the presence of soybean rust in states to the south of North Dakota and scouting fields just prior to the R1 through the R6 development stages is important in determining the need to apply a fungicide for preventative or post-infection control in North Dakota.  To monitor the spread of soybean rust throughout the United States, check the USDA Public Soybean Rust Website on a regular basis. 

     Fungicides available.  Currently, the only way to manage soybean rust is with fungicides.  No resistant soybean varieties currently exist.  Four fungicides with efficacy against soybean rust have full section 3 labels on soybean in North Dakota.  North Dakota, along with other soybean-producing states, made a request to the United State Environmental Protection Agency (EPA) for quarantine section 18 exemptions of several fungicide products that currently do not have full section 3 labels on soybean.  A few of the requested products have already been granted section 18 exemptions by the EPA.  Below, are the fungicides that have full section 3 labels, and fungicides that have approved section 18 exemptions.

Labels for Section 3 products are available on-line at http://www.cdms.net/.  When the final use directions for the approved section 18 products are available for North Dakota, they will be posted on-line at http://www.ag.ndsu.nodak.edu/aginfo/pesticid/LabelPage.htm and www.agdepartment.com/Programs/Plant/Section18Exemptions.html.

     Fungicide efficacy.  The results of many soybean rust fungicide research trials are now available on-line at: 

http://www.ipmcenters.org/NewsAlerts/soybeanrust/efficacy.cfm and http://www.plantmanagementnetwork.com/pub/trial/fntests/.

     Fungicide timing.  Most of the research conducted on soybean rust control with fungicides has indicated that an application during the vegetative stages (V-stages) of growth is not necessary.  The reproductive stages (R-stages) of soybean development are critical in susceptibility to soybean rust and fungicide timing.  To minimize yield losses due to soybean rust, the soybean crop needs to be protected from the R1 to the R6 stages of development.

     Fungicide application.  For the best control, the best coverage possible is needed.  To obtain good canopy penetration and coverage, the proper nozzles and spray pressure must be used.  Flat-fan nozzles can be used for soybean rust fungicide application with higher spray pressure (60 to 70 psi).  Small to medium sized droplets (200 to 300µ) will provide the best coverage and canopy penetration.  For more information on fungicide application see, "Using Foliar Fungicides to Manage Soybean Rust". 

SYMPTOMS OF SOYBEAN RUST (verbatim from the Soybean Rust National Pest Alert):

Symptoms begin on the lower leaves of the plant as small lesions that increase in size and change gray to tan or reddish brown on the undersides of the leaves.  Lesions are most common on leaves, but may occur on petioles, stems, and pods.  Soybean rust produces two types of lesions, tan and reddish brown.  Tan lesions, when mature, consist of small pustules (uredinia) surrounded by slightly discolored necrotic area with masses of tan spores (urediniospores) on the lower leaf surface.  Reddish brown lesions have a larger reddish brown necrotic area, with a limited number of pustules (uredinia) and few visible spores (urediniospores) on the lower leaf surface.  Once pod set begins on soybean, infection can spread rapidly to the middle and upper leaves of the plant.

Above photographs courtesy USDA-ARS

IF YOU SUSPECT SOYBEAN RUST IS PRESENT IN YOUR FIELD

The USDA has established a protocol for verifying the introduction of soybean rust to a state.  If you suspect soybean rust has affected your soybean field in North Dakota, collect several affected leaves (at least 20 leaves, but more is welcome) in a sealed plastic bag, and send them immediately to the NDSU Plant Diagnostic Lab.  If soybean rust is suspected by the NDSU Plant Diagnostic Lab, then samples will be sent to a plant pathologist at the USDA-APHIS Beltsville, MD for confirmation of the first positive sample in North Dakota.  Contact information for regional Land Grant University Plant Diagnostic Labs are as follows:

For North Dakota Sample Submission:

North Dakota State University Plant Diagnostic Lab

For Walk-In Samples:  206 Waldron Hall

For Sending Samples:  306 Walster Hall, Box 5012

Fargo, ND 58105

Tel: 701-231-7854

Fax:  701-231-7851

E-mail:  diaglab@ndsuext.nodak.edu

Website:  http://www.ag.ndsu.nodak.edu/diaglab/

For Minnesota Sample Submission:

University of Minnesota Plant Disease Clinic

495 Borlaug Hall

1991 Upper Buford Circle

St. Paul, MN 55108

Tel:  612-625-1275

Fax:  612-625-9728

Website:  http://www.extension.umn.edu/distribution/cropsystems/DC3170.html

For South Dakota Sample Submission:

South Dakota State University Plant Disease Clinic

PSB 119, Box 2108

Brookings, SD 57007

Tel:  605-688-5157

Website:  http://plantsci.sdstate.edu/planthealth/subpage.cfm?ID=12

CROP INSURANCE ISSUES

Soybean growers with crop insurance need to know what to do to be covered for losses.  According to the USDA-Risk Management Agency, covered losses must be naturally occurring and farmers must follow good farming recommendations of agricultural experts such as extension agents and certified crop consultants.  Farmers need to document advice received and the actions taken.  Failure to purchase and apply recommended fungicides because of the costs is not an insurable cause of loss.  For more information regarding crop insurance issues for soybean rust coverage go to:  http://www.rma.usda.gov/news/soybeanrust/

USEFUL LINKS

General Information

USDA Soybean Rust Website = http://www.usda.gov/soybeanrust/

Plant Management Network Soybean Rust Site = http://www.plantmanagementnetwork.org/infocenter/topic/soybeanrust/

Plant Health Initiative Soybean Rust Site = http://www.planthealth.info/rust_basics.htm

Using Foliar Fungicides to Manage Soybean Rust = http://www.oardc.ohio-state.edu/soyrust/  

Soybean Fungicide Efficacy Trials

Regional IPM Centers Website = http://www.ipmcenters.org/NewsAlerts/soybeanrust/efficacy.cfm

Fungicide and Nematicide Tests = http://www.plantmanagementnetwork.org/pub/trial/fntests/

Fungicide Labels

CDMS Information Systems = http://www.cdms.net/

Greenbook = http://www.greenbook.net/

NDSU Extension Pesticide Program = http://www.ag.ndsu.nodak.edu/aginfo/pesticid/LabelPage.htm

ND Dept. of Agriculture = http://www.agdepartment.com/Programs/Plant/Section18Exemptions.html

Soybean Rust Forecasting & Tracking

USDA Soybean Rust Observations Maps = http://www.sbrusa.net/

DTN Soybean Rust Information Center = http://www.dtnsoybeanrustcenter.com/index.cfm

Prepared by:  Dr. Carl Bradley, Extension Plant Pathologist

Updated:  June 10, 2005