August 21, 2009

As of today (August 21), Goss’s bacterial wilt and blight of corn has been confirmed in 24 counties (see map) stretching from eastern to western Nebraska. The gram-positive bacterial causal agent of the disease, Clavibacter michiganensis subsp. nebraskensis, was confirmed in symptomatic samples submitted to the UNL Plant and Pest Diagnostic Clinic.

For the past four to five years, the disease has been a serious problem in western Nebraska, eastern Colorado and Wyoming, reappearing after almost two decades of its near disappearance. In this tri-state region it has become the most severe disease problem in corn. Last year was the first time the disease was confirmed in Nebraska counties statewide in approximately two decades. In addition, the disease developed in other Midwest states, including South Dakota, Iowa, Illinois, and for the first time, Indiana.

Goss’s wilt is likely much more widespread than the 2009 map of Nebraska counties indicates. For example, many of those counties where the disease has been confirmed this year are not the same Nebraska counties where the disease was confirmed in 2008. One possible explanation is that disease development is consistent with parts of the state that received the most severe weather earlier in the season, especially hail, high wind, and sandblasting that created the wounds necessary for infection. Since bacteria don’t blow long distances by wind, the bacteria were already present at the time of wounding. Disease may develop in other areas, but at a lesser incidence and severity and go unnoticed. In addition, as more people become familiar with the appearance of the disease symptoms, fewer are submitting samples for diagnosis to the UNL Plant and Pest Diagnostic Clinic, so the map may not be representative of all counties with the disease. Goss's wilt is likely present in many counties not indicated on the map. We recommend that producers, consultants, etc., continue to monitor for this and other diseases.

Symptoms

The disease may become systemic and kill entire plants (Figure 1) in severe cases, as is occurring in parts of western Nebraska. This results in the most severe yield losses. The stalks of systemically infected plants may be cut into cross-sections or longitudinally (Figure 2) to observe internal discoloration evident of systemic movement of the bacteria. Otherwise, it is most common to observe the foliar blight phase of the disease, which has been seen in eastern Nebraska more recently.

Disease symptoms are becoming more evident now as they increase in severity and larger portions of the leaves begin to blight and die in more severe cases. Although the leaf blight lesions may appear similar to some other diseases, there are two diagnostic features that may be used to differentiate them from other diseases. Specifically, this is the only known disease to cause “freckling” or dark green to black discontinuous water-soaked spots near the expanding edges of the lesions. These “freckles” may be accompanied by secretion of bacterial exudate or “ooze” on the leaf surface giving it a glossy appearance (Figure 3). Lesions may continue to expand and blight entire leaves (Figure 4).

Management

Unfortunately, since this disease is caused by bacteria, it cannot be controlled with foliar fungicides, which have been much more widely used in Nebraska during the past two years for control of foliar diseases caused by fungi. And, the pathogen overwinters very well in infected crop residue that is not broken down. As a result, if you have had the disease in a field in the past, it is likely to develop again under the right conditions.

Crop rotation, tillage of debris to promote breakdown (when practical), and use of resistant hybrids have been the most effective means of controlling this disease. Differences in yield between susceptible and resistant hybrids with the leaf blight phase of the disease have reportedly been as high as 65 bushels per acre in southwest Nebraska in recent years. With fall approaching, the time is right to start thinking about selection of resistant corn hybrids to plant in fields with a history of Goss’s bacterial wilt and blight, particularly in high risk fields, such as continuous corn and/or conservation tillage systems where crop residue is maintained.

Tamra Jackson
Extension Plant Pathologist
Amy Ziems
UNL Plant & Pest Diagnostic Clinic