6/29/07 - Steps to Detect and Respond to Soybean Rust in Minnesota Minnesota Crop E-News
6/29/07 - Soybean Foliar and Stem Disease Management Update Minnesota Crop E-News
New! National Soybean Rust Commentary - USDA
Soybean Association Rust Information Center
Maps and Forecast models
Figure 1. Early soybean rust infection showing flecking areas. (Figure 1. M. Bonde,USDA-ARS-FDWSRU).
Figure 2. Developing soybean rust infection showing chlorotic areas. (Figure 2. M. Bonde,USDA-ARS-FDWSRU).
Host plants infected with soybean rust first exhibit small lesions that gradually increase in size and turn from gray to tan or brown. Infection begins on the lower first leaves of plants and appears as chlorotic or mosaic-like areas with uredinia observed usually at or after the plant flowering stage (Fig. 1 and 2). Lesions may appear on most above-ground plant parts, but are most common on the underside of the leaves and in the lower part of the plant. As the plant matures and sets pods, infection progresses rapidly under the right environmental conditions (i.e., moisture, high humidity and heat) to cause high rates of infection in the middle and upper leaves of the plant. Clouds of spores have been observed within and above canopies of highly infected plant stands.
Figure 3. Magnified image of tan lesion of soybean rust showing cone-shaped lesion with pore before release of uredospores. (Figure 3. Jim Kurle. U of MN courtesy FDWSRU)
Figure 4. Magnified image of tan lesions of soybean rust on a soybean leaf showing masses of uredospores on uredinia. (Figure 4. Jim Kurle. UofMN courtesy FDWSRU)
Plants show two different lesion reactions to infection by soybean rust. Tan lesions consist of small uredinia surrounded by slightly discolored necrotic areas on leaf surfaces. Early stages show an ostiole, or small hole, where urediniospores emerge. As uredinia become larger, they release masses of tan colored urediniospores that appear as light brown or white raised areas (Fig. 3. Uredinial pustules become more numerous with advancing infection and often will coalesce forming larger pustules that break open releasing masses of urediniospores (Fig. 4).
Figure 5. Magnified image of soybean rust reddish brown lesions. (Joe Hennen, Botanical Research Institute, Ft. Worth, TX.
Figure 6. Close up of reddish-brown lesion. (AVRDC, courtesy of Arnold Tschanz, USDA, APHIS, PPQ, Riverdale, MD.)
The other type of lesion that occurs with soybean rust infection is the reddish-brown lesion (Fig. 5 and 6). These lesions have larger areas of necrosis that are reddish brown surrounding a limited number of uredinia. A few urediniospores are usually visible on the surface.
Early symptoms of soybean rust are easily confused with bacterial pustule (caused by Xanthomonas campestris pv. phaseoli (Smith) Dye), or bacterial blight (caused by Psuedomonas glycinea Coerper), and brown spot (caused by the fungus Septoria glycines). The diseases also occur often on the underside of soybean leaves causing a raised light brown blister within a lesion. These leaf lesions vary from small specks to large irregular brown areas that form when small lesions coalesce. A hand lens or dissecting microscope are usually used to distinguish these disease symptoms from ASBR, but early the stages of disease are difficult to distinguish if no spores, conidia, or bacteria are evident. Later in disease development the two bacterial diseases can be distinguished from soybean rust by looking at the pustules under magnification, using a hand lens or dissecting microscope. Look for the soybean rust mature pustule with a pore at the top of the cone and masses of urediniospores both within the uredinia and possibly on top of the cone, and usually no yellow halo around the lesion.
Plants suspected of soybean rust infection should be sent to the University of Minnesota Plant Disease Clinic where initial SBR identification will be confirmed. Suspect positives will be forwarded by the U of MN - Plant Disease Clinic to Dr. Mary Palm at USDA APHIS. USDA APHIS will provide final confirmation to MDA.
UM Extension will provide SBR identification training to MDA summer survey
staff, crop consultants, Extension staff, Regional Extension Educators, Extension
IPM Specialists, and soybean growers in order to increase the pool of knowledgeable
individuals in a position to provide early detection of the disease.
Upon confirmation that SBR has been identified in Minnesota, The Commissioner of the Minnesota Department of Agriculture shall issue a statement to that effect. The SBR Communication team consisting of representatives from MDA, UM, USDA-APHIS, MN Soybean Growers shall provide media with appropriate information to assist farmers and others to understand the situation and how to best respond.
For cooperative extension agents, field scouts, crop consultants, or anyone conducting surveys of soybean rust on legume hosts, for sample submission to university diagnostic laboratories.
Place leaf, stem, or pod samples in a self locking plastic bag and store under cool conditions. Alternatively, samples that must be kept under ambient conditions should be sealed in a paper bag to prevent mold growth. Once they can be refrigerated the paper bag can be placed in a self locking plastic bag. It would be helpful if leaves can be placed between paper towels or pieces of paper to keep them flat. Care should be taken to ensure the outside of the bags are not contaminated by the sample.
Record the collection information (date, exact location of the field and sample location within the field, county in which collected, host plant and collector’s name and phone number) on a piece of paper included with the sample. If the collector has a copy of the PPQ form 391, the pertinent sections of that form should be completed and submitted with the specimen to the state or university diagnostic laboratory. Form 391 --- http://www.aphis.usda.gov/ppq/ep/soybean_rust/PPQ391.pdf
Submit the sample to the University of Minnesota Plant Disease Clinic. Do not send suspect samples directly to the USDA Beltsville laboratory. It is essential that samples be screened by the U of MN Plant Disease Clinic to determine whether the disease is soybean rust or a disease that is similar to but not soybean rust. The Plant Disease Clinic mailing address is:
Plant Disease Clinic
Department of Plant Pathology
495 Borlaug Hall
1991 Upper Buford Circle
University of Minnesota
St. Paul, MN 55108
Phone: (612) 625-1275
Fax: (612) 625-9728
Contact: Amy Holm
University of Minnesota
Minnesota Department of Agriculture
Minnesota Soybean Office – Mankato, MN
Minnesota Soybean Research and Promotion Council - Providing information and education to producers.
Minnesota Soybean Growers Association
United Soybean Board - Improved technologies for soybean marketing.