By Scout Nelson
Corn growers in South Dakota are beginning to face challenges from two important diseases: southern rust and tar spot. Both diseases have been reported in several counties and are being carefully monitored to protect crop yields.
Southern rust is now showing moderate levels across many corn fields in the state. In some areas, infection levels have reached fungicide spray thresholds, which are typically between 20 and 30 percent infection.
Experts recommend that farmers consider fungicide application if rust is seen at the top of the canopy, as this is where yield loss can occur. Farmers are encouraged to follow guidelines from the Crop Protection Network and SDSU Extension’s Pest Management Guide for specific fungicide options.
However, it is important to correctly identify the disease since common corn rust is also present in high amounts but does not cause yield loss. Counties with confirmed southern rust include Bon Homme, Brookings, Clay, Grant, Lake, McCook, Minnehaha, Moody, Roberts, Turner, Union, and Yankton, with Deuel reporting suspected cases.
Tar spot has also been confirmed for the first time in South Dakota on July 25, 2025. Since then, several reports have come in from southeastern counties with disease severity ranging from 1 to 25 percent. In most fields, fungicides are not needed because infection levels are still low.
Farmers are advised to continue scouting their fields weekly until corn reaches the R3, or milk stage. After this stage, most of the yield potential is set and chemical control is no longer effective. Confirmed tar spot cases are found in Bon Homme, Clay, Deuel, Grant, Minnehaha, Union, and Yankton counties, with Lincoln and Turner having suspected cases.
Photo Credit: gettyimages-dszc
Categories: South Dakota, Crops, Corn