By Blake Jackson
Southern rust, a fungal disease affecting corn, can reduce nutrient content and lower digestibility in feed, warns University of Missouri Extension field agronomist Nick Wesslak.
Infected corn often experiences early dry down, which complicates silage packing and increases the risk of poor fermentation along with higher mold and yeast populations.
“The dairy industry relies heavily on corn silage, and milk production is highly sensitive to subtle changes in nutritive value,” says Eric Bailey, MU Extension state beef nutrition specialist.
Corn affected by southern rust is generally more concerning for dairy operations than beef, due to this sensitivity.
While southern rust (Puccinia polysora) does not produce mycotoxins itself, it can create openings for stalk and ear rot diseases that do.
“Producers should run a mycotoxin panel screening before feeding forage or grain infected with southern rust,” advises MU Extension veterinary toxicologist Tim Evans. Diluting infected feed with clean feed and using mitigation strategies can help reduce risk.
Southern rust was confirmed in 25 Missouri counties in 2025, according to the Corn IPM Pipe. Warm, humid conditions early in the season contributed to its early appearance. It appears as orange to brown raised lesions on the top of leaves, with yellow flecking on the underside, though it can be confused with common rust.
“When symptoms first arise, Wesslak recommends sending samples to a diagnostic lab, such as the MU Plant Diagnostic Clinic, to determine which species are present,” he says.
The disease often causes premature plant death and early dry down, interfering with water regulation. Bailey notes that optimal corn silage moisture is 60%-70%, but southern rust can result in overly dry silage, preventing proper packing at under 15 pounds of dry matter per cubic foot.
Yield losses occur as the pathogen feeds on plant glucose and reduces leaf photosynthetic area, lowering starch and energy content while reducing digestibility by more than 10%.
Testing infected feed is critical. Bailey advises considering fermentation quality analysis, noting, “Lactic acid should be between 4% and 6%; butyric acid should be less than 0.25%; and ammonia should be less than 12%.” If results are unfavorable, contact a local MU Extension livestock specialist to develop a plan for safely using southern rust-affected silage.
Photo Credit: gettyimages-awakr10
Categories: Missouri, Crops, Corn