Southern corn rust continues to spread in the southern part of the US and is now detected in four counties across central and eastern Kansas (Figure 1). Unlike some other corn diseases, such as tar spot and gray leaf spot, southern rust does not survive in Kansas during winter months and blows in annually from more tropical regions. The severity depends on the weather, and southern rust likes 90-degree days, warm nights, and high humidity.
Figure 1. Southern corn rust (Puccinia polyspora) in Kansas as of July 11, 2024. Source: https://corn.ipmpipe.org/southerncornrust/
Here are some frequent questions related to managing southern rust in Kansas.
Q1. Should I apply a fungicide prior to observing southern rust?
A1. Applying a fungicide to control southern rust is not recommended unless the disease has been observed in the canopy. Now that southern rust has been reported in Kansas, it is time to scout corn fields. Once pustules are observed, the pathogen can reproduce rapidly if temperatures and humidity are high.
Q2. What factors should I consider when making the decision to spray for southern rust?
A2. It is important to consider hybrid susceptibility, disease incidence (how many plants are affected), and the crop's growth stage. Infection early in the season on a susceptible hybrid, coupled with conducive weather conditions, poses the highest risk for yield loss.
Q3. If I apply a foliar fungicide at tasselling (VT) or silking (R1) to control tar spot and gray leaf spot, will this application have efficacy against southern rust?
A3. Yes. Most fungicides labeled for tar spot and gray leaf spot are also effective for southern rust. Depending on the product, they will have residual activity for approximately three weeks after application. Fields should be carefully monitored for disease development.
Q4. What fungicides are best to control southern rust?
A4. Efficacy ratings for corn fungicide management of southern rust have been compiled by a working group of corn researchers and can be found here:
https://cropprotectionnetwork.s3.amazonaws.com/corn-foliar-efficacy-2024-1709214762.pdf
Q5. How do I know if what I’m seeing is southern rust?
A5. Southern rust produces characteristic orange pustules of spores, primarily on the upper side of the leaf (Figure 2). If you run your finger across the pustules, the orange spores will be visible on your hand. The Kansas State Plant Disease Diagnostic Lab can also confirm southern rust by observing spores under the microscope. Additional information about sending in a sample can be found here: https://www.plantpath.k-state.edu/extension/diagnostic-lab/.
The K-State Plant Disease Diagnostic Lab will run a free testing program for Southern Rust in the 2024 growing season. If you have any questions, contact the lab at clinic@ksu.edu.
Figure 2. Southern rust on corn. Photo courtesy of Rodrigo Borba Onofre, K-State Plant Pathology.
For more information on identifying corn rusts, see K-State Research and Extension Bulletin MF3016, Corn Rust Identification and Management in Kansas.
Rodrigo Borba Onofre, Plant Pathology
onofre@ksu.edu
Chandler Day, Crop Diagnostician
clinic@ksu.edu
Tags: corn southern rust foliar diseases diseases crop scouting