As of July 19, southern rust has been confirmed in several counties in Nebraska, including the eastern and central part of the state. This year, the disease development occurred earlier than usual and has spread further north than it typically does. You can track the disease distribution at corn.ipmpipe.org/southerncornrust/. Favorable conditions for disease development include high relative humidity, rainfall, and average temperatures around 80°F. Recent frequent rainfall has created optimal conditions for the fungus causing southern rust.
Southern rust pustules are orange to tan in color and are mostly produced on the upper leaf surface. If you rub the rust spores off the leaves, you can see orange or tan streaks on your fingers or clothes. It can often be confused with common rust, which is not an economically important disease in corn. The color and location of the pustules are the best way to tell the difference between these types of rust. Common rust produces brick-red to brown spores on both the top and bottom of leaves. Microscopic examination in the diagnostic laboratory can quickly determine whether rust spores are common rust or southern rust. Samples can be submitted to the UNL Plant and Pest Diagnostic Clinic (plantpathology.unl.edu/plant-pest-diagnostic-clinic) for identification.
Fig. 1. Magnified pustules of the common rust fungus (a) and southern rust fungus (b). Photo Credit: Dr. Tamra Jackson-Ziems, UNL.
Fig. 2. Southern rust tracking map as of July 19, 2024 (corn.ipmpipe.org/southerncornrust/)
Fig. 3. Southern rust confirmed in Lancaster County July 17, 2024. Photo Credit: Dana Schleufer
As always, repeated scouting and monitoring for diseases is critical when managing southern rust. Nebraska Extension Plant Pathology Specialist Dr. Tamra Jackson-Ziems recommends especially scouting later planted fields or fields that have been replanted this year. Due to the wet spring conditions this year, a lot of corn fields were replanted, and these fields are likely at greater risk to southern rust infestations. Only a few commercially available corn hybrids are resistant to southern rust. So, review your individual hybrids and their disease resistance ratings and contact your local seed representative to find the correct information. Fields with susceptible hybrids and irrigated fields are especially important to scout and monitor for southern rust.
Severe southern rust can reduce yield and stalk quality, but many foliar fungicides can effectively manage the disease. It is important to review the application dates, active ingredients and residual protection of your fungicides to plan future scouting efforts. Most fungicides can provide protection from future infection for 21-28 days, so fields that were treated early in our area should be scouted again in the next couple of weeks, as southern rust will likely continue to develop and spread throughout the summer. Consult Fig. 4, Fungicide Efficacy for Control of Corn Foliar Diseases to review how your specific fungicides may protect your crop from southern rust. It is possible that fields that were treated in early July may require another fungicide application if southern rust continues to develop and spread after the first application’s residual has worn off.
Fig. 4. Fungicide efficacy chart for corn diseases
We encourage you to learn more about southern rust and its management at https://cropwatch.unl.edu/plantdisease/corn/southern-rust, and https://cropprotectionnetwork.org/publications/an-overview-of-southern-rust, or contact your local Nebraska Extension Water & Cropping Systems Educator with any questions.
Ritika Lamichhane
Extension Educator
Nemaha County
John Nelson
Extension Educator
Lancaster County
July 2024