Southern Rust Confirmed in Southern and Northeast Nebraska - UNL CropWatch, July 12, 2012

Southern Rust Confirmed in Southern and Northeast Nebraska - UNL CropWatch, July 12, 2012

Southern rust in corn Souther rust in corn
Figure 1. Southern rust spores are typically orange to tan in color and produced in pustules predominantly on the upper leaf surface (Figure 1a, left), although they can also be produced abundantly on/near the midrib on the underside of the leaves (Figure 1b, right). (Photos courtesy of Casey Schleicher, UNL)

July 12, 2012

Corn Disease Update

Southern Rust

This content requires Flash Download the free Flash Player now!
Get The Code to Embed This Audio Clip

Southern rust (Figure 1) was confirmed on corn late last week (July 5) in samples submitted to the UNL 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.

 Common rust in corn

Figure 2. Common rust also has begun to develop at low incidence and severity in Nebraska and can be confused with southern rust, a more aggressive and damaging disease problem. (Photos courtesy of Casey Schleicher, UNL)

Goss's wilt in corn

Figure 3. Goss’s bacterial wilt and blight is an ongoing problem in some Nebraska corn fields. Be sure to look for the small dark “freckles” and glossy “exudate” on the leaf lesions (Photos courtesy of Casey Schleicher, UNL).

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

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:

Also see:

Tamra Jackson-Ziems
Extension Plant Pathologist
Kevin Korus
Plant and Pest Diagnostic Clinic
Jenny Rees
Extension Educator, Clay County