Agronomy Talk

Agronomy Talk: SOYBEAN RUST

Published on Wednesday, November 4, 2020

Soybean rust, (Phakopsora pachyrhizi), a serious disease of soybean and other legumes, was first discovered in the continental U.S. (Louisiana) in 2004. This disease has been reported to cause upwards of 80 percent yield loss when present under optimal, conducive conditions. Current soybean varieties grown have little to no resistance to soybean rust, thus making proper identification and management decisions even more crucial, as we begin/continue harvest in our area. Ultimately, treatment beyond the R6 growth stage may not be warranted but should still be evaluated on a case by case basis as there are many later planted soybean acres filling pods. Treatment measures should include a fungicide application with a stand-alone triazole if the soybeans are at the R5 to R5.5 growth stage and have not received a fungicide application previously.

BACKGROUND

Soybean rust is caused by the fungus Phakopsora pachyrhizi and was originally reported in Japan in 1902. The disease made its journey to South America and eventually to the US, arriving late in 2004, via winds from Hurricane Ivan. The host range of soybean rust is soybean, as well as many other legume plants with kudzu being the other preferential host in the area. Many plant pathologists and crop consultants have been using kudzu as a source to track the movement of this pest throughout the growing season.

SYMPTOMOLOGY

Soybean rust can be a pest that is rather difficult to scout if it is not present in high levels in the mid to lower canopy of soybean. It 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 believed to be due to the amount of moisture present and the length of time moisture is present on this portion of the plant, allowing for a conducive environment for the establishment of the pest. Lesions will not increase in size but will however increase in number present. 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. The area to pay extra close attention to is the opposite side, or underside, of the leaf from the lesion on the upper leaf surface. On the underside of the leaf, the lesions will make a raised, pimple-like, structure, called a pustule. The pustules, when active, will mimic a volcano with spores coming from the structure on the underside of the leaf. The coloration of the pustules can give some information about the stage of the disease, for example, light tan in color is somewhat young in the disease cycle whereas the 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 (59-86 degrees), and high humidity (>75%).

MANAGEMENT

Current soybean varieties have minimal to no resistance to soybean rust, so it is key to take steps to manage this pest. Although most situations will be evaluated on a case by case basis, current recommendations out of Mississippi State University Extension from Dr. Tom Allen are as follows:

  • Soybeans that have reached the R6, or full pod, the growth stage is deemed to be safe from yield loss from soybean rust.
  • Later planted soybeans that are as young as R3/R4, it is recommended to make a fungicide application. It is recommended to attempt to make this application at the R4.5 growth stage if possible, to maximize the residual of the fungicide applications from later infestations of soybean rust.
    • At this timing, a dual-mode of action product (strobilurin+triazole) will likely be the most beneficial choice.
  • Soybeans at the R5/R5.5 growth stages that have not received a fungicide thus far in the season, then a stand-alone triazole fungicide application could be beneficial.
    • This application should provide 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.
    • It's very important to read the label as PHI and other application restrictions may apply. 

ADDITIONAL THOUGHTS

As we progress through harvest be sure to continue to scout those later planted acres for not only soybean rust but other pests, including stink bugs. I have seen many groups of green and southern green stink bug nymphs walking R7+ soybean fields over the past week, so be mindful as we continue to harvest and those ‘green islands’ become more sparse the numbers will continue to increase. Some of the fungicide applications mentioned above to protect against soybean rust may become more feasible if we can tag along with some late insect sprays. Remember, the threshold for stink bugs: (adults and nymphs count the same): 

  • R1 to R6 = 9 per 25 sweeps
  • R6 =18 per 25 sweeps
  • R6.5 = termination of stink bug sprays
    • Exception to redbanded stink bugs (R7)

Not sure if soybean rust is in your area? The most up to date information on the distribution of soybean rust in the US can be found here: soybean.ipmpipe.org/soybeanrust/

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Author: Shane Carver

Categories: Agronomy, Agronomy Talk

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