Soybean Rust

Soybean Disease Information Note 8

Dr. Steve Koenning, Extension Plant Pathologist, College of Agriculture and Life Sciences, Department of Plant Pathology, North Carolina State University, Campus Box 7616, Raleigh, NC 27695-7616

Dr. John Mueller, Plant Pathologist, Clemson University, Edisto Research & Education Center, P.O. Box 247, Blackville, SC 29817

Dr. Robert Kemerait, Extension Plant Pathologist, College of Agriculture and Environmental Sciences, University of Georgia, P.O. Box 1209, 15 RDC Road, Tifton, Georgia 31794

Dr. Pat Phipps, Plant Pathologist, Tidewater Agricultural Research & Extension Center, 6321 Holland Rd., Suffolk, VA 23437, and Dr. Erik Stromberg, Plant Pathologist, Department of Plant Pathology, Physiology & Weed Science (0331), 401 Price Hall, Virginia Tech, Blacksburg, VA 24061

General Information

An important new disease threat to soybean is currently the cause of much concern in the agricultural community.  Asiatic soybean rust, caused by Phakospora pachyrizi, has emerged as a major constraint to soybean production in South America since 2001.  Another species of rust Phakospora  meibomiae has been endemic to portions of South America for many years but is considered less of a threat because it is not as aggressive as the Asiatic soybean rust.  During the 2003-2004 growing season in Brazil, Asiatic rust was severe in many areas and required sprays of fungicides in order to control this disease (Figure. 1). Many industry leaders and some scientists predict that its introduction to North America is eminent and that it will impact US soybean production.  Asiatic soybean rust is present in Hawaii, but has not yet been reported in the Continental US. Some plant pathologists, however, suspect that it would be a minor problem if and when it does arrive in the US.  Predictive models suggest that conditions in Georgia, South Carolina, Virginia, and North Carolina are favorable for development of an epidemic of soybean rust.  The soybean rust pathogen is primarily tropical in distribution and would be able to survive over winter in only the most southern portions of the US (Southern Florida and Texas). Alternate hosts for Asiatic soybean rust in the US include kudzu, winter vetch, and lupines. Should this disease be introduced to the US an emergency registration of additional fungicides for use on soybean will take effect for three years.

Symptoms:

Asiatic soybean rust causes superficial, tan to reddish brown, lesions that will be observed on plant tissues. Lesions will contain one to three rust pustules which are raised on the leaf surface (Figures. 2-5).  The lesions may have an angular appearance and be limited by leaf veins. Rust pustules may appear on cotyledons, leaves, petioles, stems, or pods, but are most likely to be observed as raised pustules on the under side of the leaf.  The pustules are quite small (about the size of a pin head) and can be confused with another disease, bacterial pustule.  Bacterial pustule, however, is rare in commercial soybean varieties, since most if not all are resistant to this disease.  A hand lens may aid in seeing the raised nature of the pustule. Also, placing leaves in a plastic bag with a moist paper towel for twenty four hours may cause the pustules to erupt, thus making identification easier. Each pustule contains hundreds of spores.  Spores are elliptical to obovoid in shape, colorless to yellowish or yellowish brown and minutely and densely spiny (Figure 6).  Infected plants will senesce early and have smaller seed with reduced yield.

Disease Cycle and Epidemiology:

Windblown spores infect susceptible tissue at temperatures of 45 to 83 degrees.  Leaves must be wet for infection to occur.  Pustules will develop within a week to ten days after infection, and spores are produced after about three weeks.  The spores are the only survival structures for this fungus and they will not survive freezing weather.  The role other hosts (kudzu, winter vetch, lima beans, dry beans, and lupines) might play is not known, but fields with kudzu along borders might be a good place to start scouting.  Soybean rust may be able to survive in southern Florida and Texas, but the soybean acreage in these states is limited at this time.  Several species of common bean are also hosts for this fungus, but fungicide sprays to control other diseases on these vegetable crops may limit rust development.   For states on the Atlantic Coast, windblown spores will have to move northward from southern Florida.  A second possibility is for spores to come from the Mississippi Valley.  In 2003, Soybean acreage in Florida was less than 10,000 and only about 150,000 in Georgia.  Thus the potential for windblown inoculum coming from these sources is not terribly high.  For rust to be damaging first infections will probably have to occur before the R3 stage of soybean development.

Management:

There are no commercial soybean varieties with resistance to soybean rust at this time.  Management of soybean rust will be with fungicides.  Currently, the only fungicides labeled for soybean that are effective in managing soybean rust are chlorothalonil (various brands) and azoxystrobin (Quadris).  Although Topsin M is also labeled for soybean, it is not effective against rusts.  If soybean rust is identified in the Continental US an emergency registration of eight additional fungicides will go into effect for three years (Table 1).  In general the recommendation will be to make one application at the R3 stage of development and a second one 10-14 days later. Late season sprays (after R5 to R6) have not been effective in South America unless they follow an earlier spray.  Because soybean rust may develop resistance to the triazole and strobilurin class of fungicides, it may be best to rotate the use of materials.  Preliminary reports indicate that the triazole type fungicides have more curative activity than the strobilurin class fungicides, the first spray should be a triazole fungicide and the second spray should be made with a strobilurin.  If a third spray is required chlorothalonil might be the best option since it may control some other late season diseases that the other materials do not.

Table 1: Fungicides for management of soybean rust

Figure 1. Soybean infected with soybean rust in Parana State BS near Londrina, Brazil; from left to right unsprayed, sprayed once with a fungicide and sprayed twice with a fungicide (Photo by Steve Koenning).

Figure 2.  Soybean Rust on soybean leaf (Photo by Steve Koenning).

Figure 3. Soybean rust on a mature and a senescing leaf (Photo by Steve Koenning).

Figure 4. Lesions containing several rust pustules of soybean rust (from Compendium  of  Soybean Diseases, APS Press, St. Paul, MN).

Figure 5. Enlarged view of leaf lesion of soybean rust containing pustules.  Note the angular appearance of the lesion (Photo from Compendium of Soybean Diseases APS Press, St. Paul, MN).

Figure 6.  Spores of soybean rust (Photo from Compendium of Soybean Diseases APS Press, St. Paul, MN).

If you suspect soybean rust the following actions should be taken:

1. Contact your local Cooperative Extension Agent for collection, packaging and submission of samples to one on the labs listed below.
2. Work with your local Agent in completing the extension form to provide the lab with field location, disease distribution, symptoms, etc.

A report of the results of diagnostic tests will be sent to the Extension Agent and person named on the extension form submitted with each sample.

Diagnostic Labs:

The following web sites contain additional information on and pictures of symptoms and signs of soybean rust.

http://www.unitedsoybean.org/soybeanrustguide.pdf

http://www.ncpmc.org/soybeanrust/

http://www.plantmanagementnetwork.org/pub/search/topsearch.asp?page=http://www.planthealth.info/rust/rust.htm

http://www.plantmanagementnetwork.org/pub/search/topsearch.asp?page=http://www.apsnet.org/online/feature/rust/

National Plant Diagnostic Network: http://npdn.ppath.cornell.edu/default.htm

Southern Plant Diagnostic Network: http://spdn.ifas.ufl.edu/

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Published by North Carolina, Georgia, South Carolina, and Virginia Cooperative Extension Service
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