Figure 1. Southern corn rust has been confirmed in samples from Adams, Buffalo, Clay, Fillmore, Jefferson, Thayer,  Seward, and Sherman counties in southeast and south central Nebraska.

 
August 6, 2010

While some diseases, like gray leaf spot, have not developed to the extent that many people expected after the rainy conditions earlier in the season, other problems are plentiful in corn and require monitoring and careful identification. (Below the article: View the author discussing this topic on the Aug. 13 Market Journal.)

Southern Corn Rust

Southern corn rust (Figure 1) was confirmed in samples from Adams, Buffalo, Clay, Fillmore, Jefferson, Sherman and Thayer counties in southeast and south central Nebraska this week. (On Aug. 12 it was also confirmed in Seward County.) The disease was only sparsely observed in these locations and has not been confirmed in any other Nebraska locations, so it appears that the distribution is very limited with low severity.

Also this week, southern rust was confirmed in three counties in Illinois, and has been confirmed in a single county in southeast Kansas and in much of the mid- and deep South. To monitor the movement of southern rust, visit the Southern Corn Rust page on the IPM PIPE website.

Numerous other samples suspected of being southern corn rust have been submitted to the UNL Plant and Pest Diagnostic Clinic, but most have been either common rust or Physoderma brown spot, which may be difficult to differentiate. There is usually a distinct color difference between the spore-producing pustules of common and southern rust.

Common rust tends to appear more dark red to brown, while southern rust may range from orange to tan (Figure 1b). However, without side-by-side lesions to compare, it can be difficult to differentiate them based on color and both rusts will eventually produce black spores toward the end of the season making color an unreliable feature.

A more reliable method is to identify where the majority of spore production is occurring. Common rust sporulates well on both the upper and lower leaf surfaces and may have larger pustules than southern rust, which tends to develop smaller pustules mostly on the upper leaf surface. For more information on how to identify corn rust diseases, see the UNL NebGuide, Rust Diseases of Corn in Nebraska. In the laboratory, spore appearance is examined microscopically to make a definitive diagnosis.

Rust pathogens do not overwinter in Nebraska and their spores must blow into the state each year. So, having a history of the disease will not impact its development now or in the future. Humid and wet weather, as well as day or night temperatures in the upper 70s to lower 80s ºF will exacerbate the spread of this and other fungal foliar diseases. The southern rust and common rust pathogens are specific to corn, and will not affect other crops. Southern rust is more problematic than some other foliar diseases because of its aggressiveness and ability to reproduce quickly under conducive weather conditions with large numbers of spores.

Management

The common systemic foliar fungicides used for gray leaf spot management also can be used to control rust diseases. However, at this point in the season, much of the corn is likely mature enough to be less vulnerable to the effects of southern rust observed in 2006 and 2007 when the disease was first observed in mid-July. Many fields may not need a fungicide application to manage southern rust and fields should be scouted regularly to monitor for it.

Late season foliar fungicide application timing trials have been conducted at the UNL South Central Agricultural Laboratory. In 2008, two hybrids planted at two planting dates showed, applications made at dent (R5 ) for moderate to severe gr

ay leaf spot control were only economical in about 50% of the treatments at current prices. But, no applications made late season in 2009 under minor disease pressure were economical. These and other later season fungicide application timing trial results are available on Plant Disease Central under Management Trials.

Trials conducted by Iowa State University evaluating late-season fungicide applications in 2007-2009 also indicated that the potential for economic return of fungicide applications made at R5 (dent) was strongly related to disease severity. When disease severity on the ear leaf was less than 5%, yield increases averaged only 4.83 bushels/acre. However, when disease averaged more than 5% coverage of the ear leaf, the yield increase was almost 9.5 bushels/acre, making the application economical. For more results from these trials, see the ISU Integrated Crop Management News article published July 7, 2010.

Some fields are at higher risk for damage and yield loss than others. Hybrids may vary in their genetic resistance/susceptibility to the disease. Also, fields that were planted late, such as replanted fields following hail and flood damage, will be more vulnerable than others and should be monitored closely to determine if a fungicide will be needed. Leaf diseases that develop during grain fill and cause substantial loss of leaf area, can cause yield loss and lead to stalk rots and standability problems later in the season. Fields should continue to be scouted regularly to monitor for disease development and spread.

Goss’s Bacterial Wilt and Blight

Counties in the eastern half of Nebraska have submitted numerous samples to the UNL Plant and Pest Diagnostic Clinic that tested positive for the leaf blight phase of Goss’s bacterial wilt and blight. The initial infections likely occurred a few weeks ago during severe weather outbreaks. Disease severity has increased rapidly in some fields over the last two weeks. Since the re-emergence of the disease in 2006, it has been confirmed in 49 Nebraska counties (Figure 2) and is likely in many more counties. (As people have become increasingly familiar with the symptoms, fewer samples have been submitted to the UNL Plant and Pest Diagnostic Clinic for confirmation.)

Remember to look for two key features of the disease when trying to make a diagnosis:

• Freckles — Also known as discontinuous water-soaked spots, these “freckles” appear as small dark green to black spots on the edges of expanding lesions. While some lesions may lack this symptom, those expressing it most likely have Goss’s bacterial wilt and blight, as it is the only known pathogen to cause the symptom. But, be careful to avoid confusing these “freckles” with the development of secondary fungal growth in the centers of lesions that give the surface a dusty appearance as they grow and produce spores on dead leaf tissue.
   
• Ooze — Also known as bacterial exudate, ooze occurs when bacteria are secreted on the surface of lesions. When fresh, the “ooze” may appear sticky and brown, but once dried, it gives the leaf a glistening or sparkling appearance. Be sure to check the bottom side of the leaf carefully, as well, as the exudate may be washed away by rainfall or overhead irrigation.
   

Identifying both of these characteristics is a likely indication that the corn plants have Goss’s bacterial wilt and blight. For rapid confirmation, you can submit samples to the UNL Plant and Pest Diagnostic Clinic and for more information, see the UNL NebGuide, Goss’s Bacterial Wilt and Leaf Blight of Corn.

Disease Lesion Mimics

The most common symptom (Figure 3) on corn samples submitted to the UNL Plant and Pest Diagnostic Clinic recently is likely not caused by a disease pathogen, but instead by a genetic mutation that can cause lesion development under stressful conditions. This can also be indicated by small yellow flecks on the leaves (Figure 4). This symptom may be easily confused with early fungal infections, but the flecks will not eventually develop into sporulating pustules as they would with a disease such as a rust or gray leaf spot.

These symptoms have been frequently reported in Nebraska since 2005 across multiple hybrids from several companies and brands and in many counties across the state. The symptoms behave and appear similar to a residue-borne foliar diseases because they often begin on the lower leaves and spread up the plant, but no pathogen has been confirmed in samples. Often these symptoms won’t develop in neighboring hybrids in the same field.

The likely cause of these symptoms is a genetic mutation called a disease lesion mimic. There are many types of disease lesion mimic mutations. More detailed information on mimic mutations in corn is available in Disease Lesion Mimic Mutants of Maize, a detailed article by Dr. Gurmukh S. Johal of Purdue University and published on the American Phytopathological Society’s website.

There is no known treatment for this disorder and fungicides are not expected to mitigate its effects. Since it initially begins as yellow flecking on the leaves, similar to early rust or other disease infections, it is important to make an accurate diagnosis to avoid unnecessary pesticide use in attempts to control it.

Information Resources

For information about these and other diseases, their identification, and management, see the UNL Extension Plant Pathology team’s website, Plant Disease Central, the Crop Watch website, including the Corn Disease Management section, or contact your local UNL  Extension Educator.

Tamra Jackson
Extension Plant Pathologist, Lincoln
Amy (Ziems) Timmerman
Plant & Pest Diagnostic Clinic Coordinator, Lincoln
Jenny Rees
Extension Educator, Clay County
 

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UNL Extension Plant Pathologist Tamra Jackson discusses current
corn diseases on the Aug. 13 Market Journal Program.