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.
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%).
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:
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):
Visit soybean.ipmpipe. org/soybeanrust for the most up-to-date information on the distribution of soybean rust in the US.
Author: Shane Carver
Categories: Agronomy, Agronomy Talk