Nebraska Extension has been actively tracking the incidence and severity of southern rust this past month in Nebraska cornfields with the assistance of farmers, local independent crop consultants, private industry agronomists (Figure 1), cropping system Extension educators, and others. As of July 30, southern rust was lab confirmed in Saline and Gage counties, reported in Jefferson County, and confirmed or reported in 7 other counties (Figure 2). Changes in the distribution of the disease is tracked through a southern rust tracking website at https://corn.ipmpipe.org/southerncornrust/. Currently, the disease incidence and severity are both low in the area.
Southern rust spores must be blown in from the south each year. Favorable condition for disease development includes high relative humidity, rainfall, and irrigation along with warm temperatures in the upper 70s to 80s, which can occur during overnight hours too. Cool and dry conditions slow disease spread, which occurred in 2018 and 2019 when the disease was confirmed in early in the season, but failed to become widespread in most counties.
Southern rust pustules are often numerous and tightly clustered in patches. They may appear tan to orange in color. Most spores are produced in raised rust pustules on the upper leaf surface. In contrast, common rust produces brick-red to brown spores on both the top and bottom of the leaves. Yellow haloes may appear around common or southern rust pustules, depending upon the hybrid, complicating disease identification. Microscopic examination in the diagnostic laboratory can quickly determine whether rust spores are those of common rust or southern rust (Figure 3). You can submit samples to the UNL Plant and Pest Diagnostic Clinic for help identifying this and other diseases if you are unsure. Information on how to submit a sample is available on the clinic website (https://cropwatch.unl.edu/plantdisease/unl-diagnostic-clinic-lincoln) or by calling 402-472-2559.
Southern rust does not always require treatment, making scouting and disease monitoring critical. Southern rust can become severe in susceptible hybrids, but it may take two or more weeks under favorable weather conditions. Review company websites for hybrid southern rust susceptibility ratings or give your local representative a call. I suggest scouting for southern rust in your most susceptible hybrids and under center-pivot irrigation first, and then scout other hybrids and fields if the disease increases in incidence and severity.
When southern rust affects a large percent of the leaf area it can reduce yield and stalk quality. Foliar fungicides can effectively manage the disease. Most fungicides can provide protection of leaves from future infections for 21-28 days. Treating too early or automatically at the start of pollination in early to mid-July can result in the need for reapplication of fungicide later if southern rust spreads and worsens after the time when the fungicide residual has worn off. I encourage you to learn more about southern rust at our Crop Protection Network at https://cropprotectionnetwork.org/resources/publications/southern-rust
For more information about southern rust and other agronomic resources from Nebraska Extension, contact me at or 402-821-1722. Know your crop, know your tech, know your bottom line.
