Southern rust (Figure 1) was confirmed on corn recently (July 5) in samples submitted to the University of Nebraska-Lincoln Plant and Pest Diagnostic Clinic and Pioneer Hi-Bred laboratory from Adams, Clay, Fillmore, Hall and Thayer counties in south central Nebraska and Burt County in northeast Nebraska. These samples were from pivot-irrigated fields that had very low incidence of disease at this time. Warm temperatures and high humidity may promote development and spread of disease.
Rust diseases produce large amounts of spores that can be easily moved by wind over long distances. Having a history of southern rust in corn does not affect current disease development because this pathogen does NOT overwinter in infected residue. The spores are carried into a field by wind from southern or western diseased areas. If the disease continues to spread and worsen, those fields planted later are at higher risk for disease and potentially severe yield impacts. We recommend scouting fields, especially those at higher risk, particularly later planted fields that are overhead irrigated and/or in south central Nebraska.
The characteristics used to differentiate common rust and southern rust are described and illustrated in the NebGuide, Rust Diseases of Corn in Nebraska. The simplest and most reliable way to differentiate the diseases without a microscope is to examine both leaf surfaces for spore production. Southern rust spore production is usually limited to the upper leaf surface and tends to be tan or orange in color. The most reliable way to identify corn rust diseases is through microscopic examination of spore characteristics. For this analysis, submit samples to the UNL Plant and Pest Diagnostic Clinic.
Common rust (Figure 2) has also begun to develop in some areas of the state, but at very low incidence and severity. Recent very high temperatures likely slowed the progress of this disease. (This is different than southern rust which can be exacerbated by warmer night temperatures and historically been the much more aggressive rust disease in Nebraska corn fields. Common rust spores are usually brick-red to brown in color, however, the color difference is not a reliable method for identification unless both diseases are available for comparison. Also, the spore type can change and, with both diseases, turns black later in the season.
Fungicides for Rust Diseases
Timely fungicide applications can be very effective at controlling rust diseases in corn. Keep in mind that systemic fungicides can provide protection from disease spread for about 21 days. Applications that were made several weeks ago likely are no longer providing protection from fungal foliar diseases. Pay close attention to the label restrictions on the most recent version of the product’s label as changes have been made for pre-harvest intervals and other use parameters.
A list of foliar fungicides labeled for use on corn in Nebraska and their characteristics are summarized in Foliar Fungicides for Corn Grown for Grain on page 215 in the 2012 Guide for Weed Management with Insecticides and Fungicides. Results from foliar fungicide trials conducted in Nebraska are available at the UNL Extension Plant Pathology team’s website, Plant Disease Central, under Management Trials for Corn. These results were gathered from trials with natural infestations of gray leaf spot and sometimes southern rust.
Goss’s Bacterial Wilt and Blight
Goss’s wilt (Figure 3) continues to be confirmed in samples submitted from across the state. Make sure that you know the identity of the disease(s) in your field before making a fungicide application, since Goss’s wilt and other diseases are also present right now and can’t be directly managed with foliar fungicide applications.
CropWatch has recently featured several articles on Goss’s bacterial wilt and blight, which may be helpful:
- Goss's Wilt of Corn Confirmed in Multiple Locations across Nebraska
- Goss's Wilt of Corn Continues to Develop across Nebraska