Agronomy Talk

Agronomy Talk: SOYBEAN RUST

Published on Wednesday, November 4, 2020

Soybean Rust, a severe disease of soybeans and other legumes, was first discovered in the continental U.S. (Louisiana) in 2004. This disease has been reported to cause upwards of 80% yield loss when present under optimal, conducive conditions. Soybean Rust is caused by the pathogen Phakopsora pachyrhizi.

Symptomology

Soybean Rust typically begins in the lower canopy of the soybean plant at flowering until later reproductive stages. The first lesions will occur toward the base of the leaflet near the petiole and leaf veins. This is likely due to the length of time moisture is present on this portion of the plant, which allows for a conducive environment for the establishment of the pest. The lesions on the upper leaf surface will look like small pinpoint lesions, ranging from brown to maroon in color, with a yellow halo around the cluster of lesions. On the underside of the leaf, the lesions will make a raised, pimple-like structure called a pustule. The coloration of the pustules can give some information about the stage of the disease. For example, if light tan in color, the pustule is somewhat young in the disease cycle, whereas dark brown or maroon means that the pustule is more mature. Conditions that are considered favorable for the spread of Soybean Rust are extended periods of wet weather, moderate temperatures (59 to 86°F), and high humidity (>75%). 

Management

Current soybean varieties have minimal to no resistance to Soybean Rust, so it is critical to take steps to manage this pest. Although most situations should be evaluated on a case-by-case basis, below are the current recommendations from Dr. Tom Allen of Mississippi State University Extension:

  • Soybeans that have reached the R6 (full pod) growth stage are deemed safe from yield loss from Soybean Rust.
  • A fungicide application is recommended for later planted soybeans at the R3 to R4 growth stages. The best timing to apply fungicides is at the R4.5 growth stage to maximize the residual of the fungicide applications from later infestations of Soybean Rust.
    • At this stage, a dual-mode of action product (strobilurin + triazole) will likely be the most beneficial choice.
  • A stand-alone triazole fungicide application could be beneficial for soybeans at the R5 to R5.5 growth stages that have not received a fungicide.
    • This application should provide approximately two weeks of protection and get the crop to the R6 growth stage when it would be considered safe from yield loss from Soybean Rust. 

Additional Scouting Considerations

Continue scouting later planted acres for other pests, including stink bugs. Some of the fungicide applications mentioned above that protect against Soybean Rust may become more feasible if we can include an insecticide when needed. Remember, the threshold for stink bugs (adults and nymphs count the same):

  • R1 to R6 = 9 stink bugs per 25 sweeps
  • R6 =18 per 25 sweeps
  • R6.5 = Termination of stink bug sprays
    • Exception to red-banded stink bugs (R7) 

NOT SURE IF SOYBEAN RUST IS IN YOUR AREA?

Visit soybean.ipmpipe. org/soybeanrust for the most up-to-date information on the distribution of soybean rust in the US. 

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Author: Shane Carver

Categories: Agronomy, Agronomy Talk

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