Figure 1. Dark "freckles" associated with Goss's wilt lesions on corn (Figure 1a, top), may be more evident in leaves that are backlit by the sun or other bright light (Figure 1b). This symptom is a diagnostic characteristic of Goss's bacterial wilt and blight, which is already occurring in some Nebraska corn fields. (Photos by Kevin Korus)
Figure 2a (top) and 2b. Glossy bacterial "ooze" on the upper and/or lower surfaces of leaves with large lesions can be another sign that plants have Goss's bacterial wilt and blight.

Goss's bacterial wilt and blight was confirmed in numerous corn samples this week received from counties in south central and eastern Nebraska.

Plant wounding due to recent severe weather and history of widespread disease development of Goss's wilt across the state are likely to blame for the seemingly early development of the disease in corn fields again this year. The disease can develop at any time during the growing season, but it typically has not developed this early in eastern Nebraska. Its impacts on yield are worsened the earlier it develops during the plants' growth stages and the more leaf area that is affected by lesions. Development of the disease now on susceptible hybrids could have devastating impacts on yield if the disease continues to worsen in those fields.

The disease does not appear to be widespread at this time, but scouting is recommended, particularly in high risk fields, to determine if disease has developed and to what extent. To date, Burt, Hall, and York are the only counties from which samples were received with Goss's wilt. In addition, a few of the corn plants were systemically infected by the bacterium, which can ultimately lead to plant death, stand loss, and greater yield loss. Most of the plants submitted had larger, advanced lesions and were approximately at the V6 growth stage.

The initial infections may have occurred after one of the recent severe weather events that Nebraska experienced. In approximately 2006, the disease was confirmed in western Nebraska, northeast Colorado, and southeast Wyoming. Since then it has been confirmed in more than 60 Nebraska counties all across the state, and become important in Iowa, Illinois, and Indiana. Since its reemergence, the disease has also been confirmed in Ontario and Manitoba provinces in Canada, and as far south as the northern panhandle of Texas.

Scouting for Goss's Wilt

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, and
• fields with injured plants (due to hail, high winds, sandblasting, etc.).

When trying to make a diagnosis, look for two key features of the disease:

•  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 1a). 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 (Figure 1b). 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, it may appear sticky and brown, but once dry, it gives the leaf a glistening or sparkling appearance (Figure 2). 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 on this disease, see the UNL NebGuide, Goss's Bacterial Wilt and Leaf Blight of Corn (G1675).

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). However, several products are being evaluated in an effort to identify ways to reduce losses due to the disease.

Some bactericides are labeled for use in corn, but limited testing has been completed on corn and the Goss's wilt pathogen. However, when considering the application of these or any pesticides, consider that some products are contact products that will not be absorbed and moved systemically in plants. Contact products are unable to provide curative activity for the infections that are already inside plants. (This is in contrast to the foliar fungicides that have been used more commonly on corn during recent years. These foliar fungicides often have systemic activity and can  provide both curative and protective activity against fungal diseases like gray leaf spot.) However, the contact products can provide protection from new infections for one or more weeks, but will be washed off by rainfall or overhead irrigation and their protective properties lost. Some products also may be cost prohibitive, particularly considering that they may require multiple applications for long-term protection of leaves. Timely application of these products may provide some value to slow disease progression.

Information Resources

For the safety of yourself and others, and for the most effective results, read and follow the most recent pesticide label instructions.

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 County Extension Educator.

Tamra Jackson-Ziems
Extension Plant Pathologist
Kevin Korus
Plant and Pest Diagnostic Clinic Coordinator