Producers, universities, industry and government have created a practical, applied defense to stop soybean rust from marching across North America.

SOYBEAN RUST

Soybean rust is a foliar disease caused by the fungus Phakopsora pachyrhizi. It can be an aggressive disease capable of causing defoliation and significant yield loss. Soybean rust was found for the first time in North America in 2004.

Key points to know about soybean rust

A national system of real-time soybean rust monitoring and forecasting is in place to help growers with spray decisions. This information is posted at the USDA Pest Information for Extension and Education (PIPE) website. Click on the state map for your local advisory.

Soybean rust has a wide host range

Phakopsora pachyrhizi has a wide host range, which is unusual for a rust pathogen. A “host” is a plant on which the rust fungus can survive. At least 89 different plants can host the fungus. The common weed kudzu is the host of greatest concern because it is so widespread in the southern United States. Other common hosts are yellow sweet clover, vetch, lupine, green and kidney bean, lima bean, and butter bean.

  • Agronomic impact

    The rust pathogen reduces both the amount and the efficiency of the green leaf area of the soybean plant. This reduces photosynthesis, which has a direct impact on yield.

    Soybean rust is an aggressive foliar disease that destroys photosynthetic tissue, causing premature defoliation and reduced yields. Rust infection affects pod filling, the number of seeds, and seed weight.

    Yield losses where rust occurs range from 10-100%, depending on how early in the season the plant is infected. Plants are most susceptible in the early reproductive growth stages.

    Yield Loss Prediction Tool
    Soybean physiologists have determined that the "effective leaf area index" — a measure of the photosynthetic capacity of leaf tissue — is an accurate and measurable tool to predict yield loss due to rust infection. This physiological basis of yield loss is now being used to develop a model to help growers predict potential yield loss in areas where rust epidemics occur.

    The interactive model will ask growers to supply the following information: yield potential (without rust), the growth stage of the crop, and an estimation of the severity of the epidemic (mild, moderate, or severe). Local Extension personnel can help growers determine the potential severity of the epidemic. The model will then calculate the yield potential (estimated yield loss with rust), the cost of treatment, and the economic benefit of treatment. Growers can access this interactive tool at Soybean Rust Yield Loss Prediction Tool.

  • Disease cycle

    Urediniospores of the soybean rust fungus are transported long distances by air currents.

    Spores of the soybean rust pathogen are transported readily by air currents and can be carried hundreds of miles in a few days. Weather conditions will determine when and where the spores travel from south to north.

    Rust spores, called urediniospores, are able to penetrate the plant cells directly, rather than through natural openings or through wounds in the leaf tissue. Thus infection is relatively quick: about 9 to 10 days from initial infection to the next cycle of spore production.

    Rust is a multi-cyclic disease. After the initial infection is established, the infection site can produce spores for 10 to 14 days. Abundant spore production occurs during wet leaf periods (in the form of rain or dew) of at least 8 hours and moderate temperatures of 60 to 80° F.

  • Scouting

    Soybean rust will develop in the lower canopy first. Photo: Iowa State University

    When to scout

    • The most critical time to scout is the R1 to R5 growth stages.
    • Scout if soybean rust has been reported in your area. You can follow the movement of rust from the southern states northward on the USDA ipmPIPE web maps. If rust development in the southern U.S. is slow, scouting in the North Central region can be delayed.
    • Scout if a large storm from a southernly direction has moved through your area.
    • If you think you may have soybean rust of your farm, work with your Certified Crop Advisor, your local extension agent, or certified professional agronomist to collect a sample for confirmation.

    Areas to scout

    • Early-planted fields
    • Early-maturing varieties
    • Low-lying or protected fields with prolonged dew periods where leaves stay wet longer
    • Fields with early canopy closure

    How to check a field for rust

    • Walk through the entire field in a standard Z or W scouting pattern, checking plants as you go. Take a hand lens with you.
    • Look deep into the low to mid-canopy. Leaves from the lower canopy will show symptoms first. Select leaves from the main stem only. Leaves from the lateral branches are less mature, even if picked from the same height.
    • Look for small, gray spots, particularly on the undersides of leaves and along leaf veins. Backlighting may enhance the viewing of early symptoms
    • Older lesions are larger and change color from gray, to tan, reddish-brown or black. It is important to recognize that these symptoms are not exclusive to rust. Brown spot, bacterial pustule, bacterial blight, Cercospora, frog eye leaf spot, and particularly downy mildew can easily be confused with soybean rust.
    • Detecting low levels of soybean rust typically requires incubating samples for 24 to 48 hours and observing them under laboratory conditions by a trained diagnostician.
    • If soybean rust has become severe enough to be identified in the field, examine the underside of leaves for active pustules containing powdery tan spores of the rust fungus. The presence of sporulation is diagnostic for rust in the field.

    Adapted from: Scouting for Soybean Rust, University of Wisconsin and Common Soybean Leaf Diseases and Soybean Rust (pdf) Alison Robertson and Greg Tylka, Iowa State University

    What to do if you suspect soybean rust on your farm
    If rust is suspected, work with your Certified Crop Advisor, your local extension agent, or certified professional agronomist to collect a sample for confirmation. Check here for contact information for the university-based plant disease clinic in your state.

    Place leaf, stem or pod samples in a self-locking plastic bag and store under cool conditions. 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.

    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 place in a self-locking plastic bag.

    Include the following information with the sample:

    • date
    • county
    • exact location of the field
    • sample location within the field
    • host plant
    • collector's name and phone number

    Source: USDA-APHIS

  • Symptoms

    Figure 1. Early symptoms of rust appear on lower leaves deep in the canopy. Photo: University of Wisconsin. Click on image to view a larger version

    Rust infection begins in the low to mid-canopy and moves up the plant. The first symptoms are typically small, gray spots on the undersides of leaves and along leaf veins (see Figure 1) starting around the flowering stage. The spots increase in size over time and change color from gray, to tan, reddish-brown or black.

    It is important to recognize that these symptoms are not exclusive to rust. Other diseases of soybean including brown spot, bacterial blight, and particularly downy mildew all have similar symptoms and can easily be confused with soybean rust.

    Soybean plants are susceptible to soybean rust at any stage of development, but symptoms are most common during and after flowering.

    If symptoms are observed, look for signs (sporulation) of the rust pathogen
    Lesions caused by the rust fungus enlarge, becoming tan or reddish-brown with a raised appearance. These pimple-like structures, called pustules, are diagnostic for soybean rust. When rust pustules are mature, they burst and release masses of spores into the air. Pustules contain powdery spores, which are diagnostic for rust in the field (see Figure 2).

    As the plant matures and begins to set pods, rust symptoms can spread rapidly to the middle and upper parts of the plant. Lesions are found on petioles, pods, and stems but are most abundant on leaves.

    Figure 2. Rust spores as viewed through a hand lens. Click on image to view a larger version.

    Look-alike diseases
    In the early stages, it is easy to confuse the symptoms of soybean rust with symptoms of other soybean leaf diseases Detecting low levels of soybean rust typically requires incubating samples for 24 to 48 hours and observing them under laboratory conditions by a trained diagnostician.

    Familiarize yourself with the symptoms and signs of these diseases:

    Images of these foliar diseases can be found in Common Soybean Leaf Diseases and Soybean Rust (pdf) by Alison Robertson and Greg Tylka, Iowa State University

    View more images of rust in the Photo Gallery

  • Risk assessment

    The checkoff-supported sentinel plot program is an extensive system of monitored plots in U.S. soybean-producing states. The program is a cooperative effort of the NSCRP and USB (green) and the USDA (orange).


    The USDA PIPE soybean rust website is the most reliable source of ongoing information on soybean rust. Check the rust maps frequently. Click on your state to read current advisories.

    Rust spores are disseminated by winds over large geographical areas. The pathway shown here is similar for that of other rust diseases found on grains, and serves as a model for likely soybean rust movement from south to north.

    Advance warning system
    The threat of soybean rust started an unprecedented sentinel system developed within five months of the initial confirmation of rust in North America. Because of this system, we have not lost track of where soybean rust is since February 24, 2005.

    Until rust-resistant soybean varieties are in place, the main decision that growers will face is if the potential for rust merits the application of a fungicide. The purpose of the national monitoring and forecasting network is to help producers assess the risk of a soybean rust epidemic in their area as the season progresses.

    The network is designed to give producers a seven-day notice of when rust could arrive. It consists of

    • a network of fields or plots monitored regularly for the presence of soybean rust (sentinel plots)
    • a program that monitors the hosts of the rust pathogen
    • a spore-tracking system that determines the presence of soybean rust spores
    • climate-based epidemiological models that help to forecast spore spread and deposition

    The advance warning network may be the soybean producers' best tool to manage rust. It is the most aggressive disease-monitoring program ever developed and is the collaborative effort of many universities, federal agencies, and checkoff organizations. The information is posted at the USDA Pest Information Platform for Extension and Education (ipmPIPE) website. Several hundred people are involved in the sentinel plot system each season, and over 20,000 observations are uploaded to the ipmPIPE website. This represents an enormous amount of cooperative work.


    Soybean rust in the North Central region depends on three key factors:

    • The occurence of soybean rust during the spring and early summer in the Gulf Coast area, which determines the amount of spores to blow northward. Soybean rust does not overwinter in the north, and must spread from south to north each growing season.
    • The weather conditions in July-August in the northern United States: if the weather is hot and dry, then rust will be less of a concern than if the weather is cool and wet. Rust requires wet leaf periods of at least 8 hours (in the form of rain or dew) and moderate temperatures of 60 to 80° F.
    • The northward movement of soybean rust spores in large tropical or mid-lateral storms.

    The USDA ipmPIPE website helps growers and advisors track the movement of soybean rust and, together with the weather conditions and long-range forecast, gauge the risk of soybean rust developing in northern states. Plant pathologists and epidemiologists in each state keep close track of this information throughout the growing season and make appropriate announcements when the risk of soybean rust is elevated.

  • Management

    Rust-resistant soybean varieties
    Planting resistant varieties has been very successful in managing cereal rusts on corn and wheat, and is expected to be an effective, long-term solution for soybean rust as well.

    Soybean breeders and plant pathologists have been field-testing resistant soybean germplasm in the U.S. for several years now. Several hundred soybean lines are evaluated annually in eight or more different locations. Several genes for resistance have been identified and named. The next step is to make the selections and to cross the resistant germplasm with "elite parents"—- soybean varieties with the desired agronomic characteristics.

    FOLIAR FUNGICIDES

    Vegetative Growth Stages
    Current data indicate that fungicide applications are not recommended in the early vegetative growth stages for soybean rust control.

    R1 through R5 Reproductive Stages
    Soybean rust develops most rapidly during soybean reproductive growth stages. There are several factors to consider in making spray decisions to manage soybean rust. Generally, fungicide should not be applied until the risk of infection is high and the crop is at a susceptible growth stage of R1 to R6.

    A nationwide, real-time monitoring and forecasting system is in place (USDA ipmPIPE). Use that system to guide spray decisions.

    Choosing the right fungicide, timing, and coverage of the low and mid-canopy are the important factors for success. Read the updated reference guide Using Foliar Fungicides to Manage Soybean Rust for more details.

  • Resources

  • Photo Gallery

    Early leaf symptoms. Reid Fredricks, USDA/ARS

    Rust lesions

    Rust pustules as seen with a hand lens on the underside of the leaf. USDA

    Urediniospores as seen with a hand lens, on the underside of the leaf. USDA

    Chlorotic spots on the top of leaves. USDA

    Rust lesions. USDA

    Rust lesions. USDA

    Rust lesions. USDA

    Rusty-colored urediniospores of the soybean rust fungus, Phakopsora pachyrhizi. USDA

    Leaf tissue dying from rust infection

    Soybean field with plants infected with Asian soybean rust. USDA