Corn Disease Update: Southern Rust Confirmed and Gossâ€™s Wilt Continues - UNL CropWatch, Aug. 5, 2011
August 5, 2011
Several diseases of Nebraska corn require careful identification and monitoring at this time.
Southern Corn Rust
Figure 1. Southern corn rust was confirmed in a single field in Clay County this week in south central Nebraska. Producers and consultants should scout fields, especially in southern Nebraska counties, to monitor for the disease and its potential spread.
Figure 2. Common rust spores tend to be reddish brown in color in contrast to those of southern rust, which tend to be tan to orange.
Southern corn rust (Figure 1) was confirmed this week in a single field in Clay County in a closely monitored research trial at the South Central Agricultural Lab. The disease was only sparsely observed in this location and has not been confirmed in any other Nebraska locations, so it appears that the distribution is very limited with low severity at this time. Southern rust distribution in the southern states has been sparse this year as well, likely due to the severe drought conditions there. To monitor the movement of southern rust, visit the Southern Corn Rust
page on the Pest Information Platform for Extension and Education (IPM PIPE) website.
Rust Diseases of Corn. Two rust diseases are possible in corn. Common rust has been observed at low incidence and severity across the state. 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 (Figure 2), while southern rust may range from orange to tan. However, without side-by-side lesions to compare, it can be difficult to differentiate them based on color. Both rusts eventually will produce black spores toward the end of the season, making color an unreliable feature for diagnosis.
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 (G1680). In the laboratory, spore appearance is examined microscopically to make a definitive diagnosis.
Rust pathogens of corn 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. Warm, humid, wet weather 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 can be more problematic than common rust and some other foliar diseases because of its aggressiveness and ability to reproduce quickly, producing large numbers of spores under favorable weather conditions.
The common systemic foliar fungicides used for gray leaf spot management also can be used to control the rust diseases. Crop stage, disease development, and history of fungicide use should be considered when making decisions about fungicide applications. Many fields that have matured quickly may not need a fungicide application if one was made recently or plants are approaching dent. Most of the common foliar fungicides in use now provide at least three weeks of protection from foliar fungal diseases. However, corn fields that are delayed due to late planting or replanting will be at higher risk if the disease spreads. Scouting is strongly encouraged to monitor disease progression.
Research on Effectiveness of Late Season Treatments
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 that applications made at dent (R5, more on stages ) for moderate to severe gray leaf spot control were only economical in about 50% of the treatments at current prices. No applications made late season in 2009 under minor disease pressure were economical. The results from these and other later season fungicide application timing trials are available in the Management Trials section of the Plant Disease Central website or accessed through Disease Management in the Corn section of Crop Watch.
Trials conducted by Iowa State University evaluating late-season fungicide applications between 2007 and 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 bu/ac. However, when disease coverage averaged more than 5% of the ear leaf, the yield increase was almost 9.5 bu/ac, making the application economical. For more results from these trials, see the Iowa State University Integrated Crop Management News article published July 7, 2010.
Some fields are at higher risk for damage and yield loss than others. Hybrids vary in their genetic resistance/susceptibility to the disease, although most are susceptible to southern rust. Also, fields that were planted late, such as fields replanted following hail and flood damage earlier this season, will be more vulnerable than others and should be monitored closely to determine if a fungicide is needed to protect them. Leaf diseases that develop during grain fill can cause substantial loss of leaf area and lead to stalk rots and standability problems later in the season, all of which can cause yield loss. Fields should be scouted regularly to monitor for disease development and spread.
Goss’s Bacterial Wilt and Blight
Figure 3. Goss’s bacterial wilt and blight continues to develop across the state. The freckles on the end and adjacent to expanding lesions (on the right side of the lesion) are a diagnostic clue used for identification. But, the freckles shouldn’t be confused with black spots in the older dead leaf tissue (to the left in the photo) which are other secondary fungi colonizing dead leaf tissue.
Corn samples testing positive for Goss’s bacterial wilt and blight have been confirmed in many counties across the state.
Remember to look for two key features of the disease when trying to make a diagnosis (Figure 3):
- 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 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. For more information, see the UNL NebGuide, Goss’s Bacterial Wilt and Leaf Blight of Corn (G1675).
For information about these and other diseases, their identification, and management, see the UNL Extension Plant Pathology team’s website, Plant Disease Central, the CropWatch website, or contact your local UNL Extension Educator.
Extension Plant Pathologist