MU Extension confirms first report of southern rust in US this season

University of Missouri Extension confirmed the growing season’s first case of southern rust corn disease in the U.S. on July 10.

The MU Plant Diagnostic Clinic confirmed that southern rust was present in a sample collected from west-central Missouri.

This common corn disease usually shows up in neighboring southern states before appearing in Missouri fields, said MU Extension plant pathologist Kaitlyn Bissonnette. “This year is an exception.”

Southern rust (Puccinia polysora) is a “tropical” disease that overwinters in warmer climates. Spores blow north on wind currents into corn-producing states each year.

Unusually warm and humid weather across much of Missouri creates the ideal environment for southern rust, which favors temperatures around 80 degrees F and high relative humidity. As the fungus multiplies in the host tissue, raised structures called pustules form. Masses of spores erupt through the leaf tissue.

Bissonnette said there are a few things to consider when scouting for and managing this disease.

• Orange to tan circular or oval pustules commonly form in dense clusters on the upper leaf surface. As the season progresses, the pustules can change to brown or black.

• Rust shows first in the mid-to-upper plant canopy. It appears initially along field borders or at the ends of rows where spores can easily land on the leaf surfaces. Bissonnette said the proportion of plants affected and severity of the disease are relatively low.

• Southern rust can be easily confused with other leaf diseases of corn such as common rust or Physoderma brown spot. Proper diagnosis helps to avoid unnecessary and costly fungicide applications, Bissonnette said.

Management of southern rust depends on the crop stage and environmental conditions, For up to R3 (milk stage), applying a preventative fungicide may be beneficial. “However, it is important not to apply a fungicide prematurely. Infection in the early R stages can result in more substantial yield losses than in the later R stages,” she said.

“Take note of the current distribution of southern rust in the area, the growth stage of the crop and the environmental conditions. These are the most important factors to consider when making decisions to apply fungicides,” Bissonnette said. A map of southern rust distribution is at

Because disease pressure is relatively low and there are no other confirmed reports to date of southern rust in Missouri or any other state, exercise caution when making costly management decisions, Bissonnette said.

For more information on southern rust, its management and look-alike diseases, visit the Crop Protection Network website at