Figure 1. Southern rust spores are typically orange to tan in color and produced in pustules predominantly on the upper leaf surface, although they also can be produced abundantly on or near the midrib on the underside of the leaves. (Photo courtesy of Jennifer Rees, UNL).

July 30, 2013

Also Common Rust, Goss’s Bacterial Wilt and Blight, and Physoderma Brown Spot

Southern Rust

Southern rust (Figure 1) has been confirmed in samples submitted to the UNL Plant and Pest Diagnostic Clinic from several counties in south central and southeast Nebraska. These samples were from fields that had low incidence of disease at this time. Warm temperatures and high humidity may promote development and spread of the disease.

Rust diseases produce large amounts of spores that can be easily moved by wind for long distances. Having a history of southern rust in corn does not have any impact on disease development now, because this pathogen does NOT overwinter in infected residue. The spores must be carried into the area from southern or western locations by winds from diseased areas. At this time, southern rust has not been confirmed in either Kansas or Missouri corn fields. If the disease continues to spread and worsen in Nebraska, those fields planted later are at higher risk for disease and potentially severe yield impacts. We recommend scouting fields, especially those at higher risk, such as later planted fields, in southern Nebraska counties.

A number of people have pointed out the lack of activity on the southern corn rust ipmPIPE monitoring website, a site that historically has been used to track observations of southern rust across the country. Federal funding for this website and its affiliated southern rust monitoring projects was eliminated and so activity there by state pathologists has greatly declined during the last two years. Thus, the maps on the website may not be complete or current and should not be strictly relied upon. Refer to reports from local university plant pathologists, diagnostic laboratories, and county Extension offices for the most recent information regarding southern rust distribution.

The characteristics used for differentiating between 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/orange in color.

 Figure 2. Common rust has also developed across Nebraska and can be confused with southern rust, a more aggressive and damaging disease problem. (Photo courtesy of Casey Schleicher, UNL)

The most reliable method for identifying corn rust diseases is based on examination of microscopic spore characteristics. This can be done with samples submitted to the UNL Plant & Pest Diagnostic Clinic.

Common Rust

Common rust has been evident across Nebraska for several weeks (Figure 2). Common rust spores are usually brick-red to brown in color; however, the color difference is not a reliable method for identification when both are not available for comparison and because the spore type can change and turn black later in the season for both diseases. Common rust spores can be produced abundantly on both the upper and lower leaf surfaces. Common rust is usually not a substantial threat to plant health and corn yield, as many contemporary hybrids have a natural level of resistance that slows spread of the disease. Prolonged periods of cooler weather have exacerbated common rust this year. Pustules have been more common in the lower canopy, sometimes appearing to mimic immature gray leaf spot lesions.

Fungicides

 

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)

Timely fungicide applications can be effective at controlling rust diseases in corn. Keep in mind that systemic fungicides can provide protection from disease spread for about 21 days, so application timing is very important to make the best use of the product’s protective and curative properties. Applications 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 is summarized in the Table on page 221 in the Plant Disease Section of the 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. See recent articles in Crop Watch for more information about Goss’s bacterial wilt and blight in Nebraska. Also see the UNL NebGuide Goss's Bacterial Wilt and Blight.

 

Figure 4. Physoderma brown spot has also developed in some Nebraska corn. Note the differences in lesion appearance in the midrib versus the leaf blade-both of which often occur in a banding pattern across the leaf. (Photos courtesy of Casey Schleicher (top) and Jennifer Rees)

Physoderma Brown Spot

Another disease that has begun to develop in Nebraska corn fields is Physoderma brown spot. This disease is normally not a concern, except in rare cases, such as on susceptible hybrids exposed to wet conditions.

The pathogen causing Physoderma brown spot requires standing water on plant parts, so lesions can develop in a banding pattern across the leaf after they emerge through the plant’s whorl during alternating wet/dry periods. Lesions may develop in both the midrib and on the leaf blade with very different appearances–a clue to the disease’s identity (Figure 4).

On the leaf blade, the small yellow/tan/brown lesions may be alarming and appear similar to southern rust pustules; however, they will lack the colored spores on the surface (common in rust pustules) that can be wiped away. Lesions that develop in the midrib are often larger than those on the leaf blade and are usually black. The pathogen overwinters in infected corn residue from previous seasons. Management of Physoderma brown spot is rarely necessary.

Tamra Jackson-Ziems
Extension Plant Pathologist
Kevin Korus
Plant and Pest Diagnostic Clinic
Jennifer Rees
UNL Extension Educator – Clay County