Figures 1a (left) and 1b. Corn plants systemically infected with Goss's wilt may be evident in clumps or singly in the field and may have varying degrees of foliar symptoms. Figure 2. (right) Systemic infection by the Goss's wilt bacteria is indicated by the presence of discoloration in the vascular bundles when stalks are cut cross-section.
June 22, 2012
Goss’s bacterial wilt and blight continues to be reported across the state. The disease has been confirmed in samples from more than 12 counties scattered across the corn-growing areas of the state. Not all fields are affected and the disease severity and incidence in the affected fields ranges from barely detectable to up to 30% of plants showing symptoms. Disease symptoms are not always evident from the road. At this stage it may be necessary to actively search for symptoms in the field.
Most of the fields appear to have one or more factors in common, putting them at elevated risk for Goss’s wilt development.
Fields should be scouted with special attention paid to:
Hybrids that are sensitive to Goss’s wilt
Fields with a recent history of the disease
Fields planted with corn after corn
Fields with plants that have been injured (due to hail, high winds, sandblasting, etc.).
Plants that were wounded by hail during the past month, particularly at or prior to V6 growth stage, are displaying the most advanced disease severity. Early development of disease symptoms tends to have the greatest impact on yield.
This year, systemic Goss’s wilt also has been confirmed in plants from numerous fields in eastern Nebraska. Systemic Goss’s wilt is a more severe phase of the disease where the bacteria move in the vascular tissue throughout the plant. Plants affected early by systemic disease are much more likely to die prior to ear development. Systemically infected plants may have little to severe foliar disease (Figures 1a and 1b). A cross-section cut through the stalk may reveal discoloration of the vascular bundles, indicating the pathogen is present (Figure 2). Systemically infected plants may develop individually or in clumps in the field. The symptoms of systemic Goss’s wilt are NOT unique to this disease. They may mimic those caused by other diseases, especially those caused by bacteria, such as bacterial stalk rot. Co-infections with one or more pathogens also are possible.
Symptoms
Remember to look for two key features of Goss’s wilt 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 spreading lesions (Figure 3a and 3b). 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. Freckles may be more evident when backlit by bright light, such as the sun. 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.
Figures 3a and 3b. The presence of “freckles” or small water-soaked spots can be visible with early lesion development (3a) and on the edges of mature lesions (3b).
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 dry, 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 Goss’s wilt testing, 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.
Management
The most effective disease management is with the selection and use of Goss’s wilt-resistant hybrids and crop rotation to reduce the amount of overwintering inoculum (bacteria). Unfortunately, some seed companies are reporting that resistant hybrids may succumb to the systemic wilt phase of the disease. While there is evidence that crop rotation and fall tillage may offer some reduction in overwintering bacterial inoculum and subsequent disease severity, they will not eliminate the disease, particularly in fields hard hit by a wounding event, such as hail.
Currently, research trials are evaluating several products to identify how to reduce losses from the disease. The UNL Corn Pathology laboratory does not have any results from experiments testing the effects of foliar products on active disease infections where lesions are already obvious. Limited data are available on the use of a bactericide early after inoculation and infection and results were summarized in the June 9 Crop Watch article, Goss's Wilt of Corn Confirmed in Multiple Locations across Nebraska.
Information Resources
For information about Goss’s wilt 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.
Tamra Jackson-Ziems Extension Plant Pathologist Kevin Korus Coordinator, UNL Plant & Pest Diagnostic Clinic
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