July 9, 2010

Several diseases are showing up in Nebraska corn fields. Producers and crop consultants should scout regularly to monitor for their development and spread.

Diseases Caused by Fungi

Several foliar diseases are developing across the state. The following foliar fungal diseases can be managed with foliar fungicides on susceptible hybrids. These diseases can be mimicked by some bacterial diseases (see further below), so making an accurate diagnosis is critical to avoid making unnecessary fungicide applications.

Gray Leaf Spot
Gray leaf spot is developing again in many fields and continuing to advance further up the plant in susceptible hybrids. While fungicides have been beneficial for managing this disease in susceptible hybrids, they often can be avoided in resistant hybrids. Fungicide trial results from the University of Nebraska-Lincoln Corn Pathology program are available at the Plant Disease Central website in the Management Trials section. The results of some of these trials indicate that during some years, delaying fungicide applications for two to four weeks after tasseling may provide the greatest return. For more information about gray leaf spot of corn, see the UNL NebGuide, Gray Leaf Spot of Corn.

Figures 1 and 2. Anthracnose leaf blight lesions tend to be reddish-brown in color and may have tiny whisker-like fungal projections, called setae.

Anthracnose Leaf Blight
Anthracnose may occur in any of three phases: leaf blight, top dieback, and stalk rot. All of these phases occur in Nebraska, but the foliar blight phase usually has not been a serious problem. Recent weather conditions have probably contributed to the development of anthracnose leaf blight in some fields (Figures 1 and 2). Note the presence of reddish-brown lesions in the photos. Lesions may become large and coalesce. The fungus that causes anthracnose produces whisker-like projections, called setae, that may be visible on the surface with a high magnification hand lens (≥20X).

Figure 3. Common rust is developing in Nebraska corn fields and can be characterized by the development of reddish-brown sporulating pustules on both the upper and lower leaf surfaces.	Figure 4. Bacterial stalk rot may develop in fields where corn has stood in water. Slimy lesions may begin at either the bottoms or tops of the plants and produce a foul odor.

Common Rust
Common rust also has developed in many parts of Nebraska. Not to be confused with the more aggressive southern rust disease that caused significant yield loss in 2006, common rust spores and pustules tend to be reddish-brown (Figure 3) in color during this time of the season and develop frequently on both the upper and lower leaf surfaces. For more information on the rust diseases of corn and help identifying them, please see the UNL NebGuide, Rust Diseases of Corn in Nebraska.

Diseases Caused by Bacteria

Goss’s Wilt and Blight
Goss’s wilt has been confirmed in southwest, south-central, and northeast Nebraska corn fields. Some of these fields did not have hail, but plant leaves were wounded by recent high winds, providing infection points for the bacteria. Goss’s wilt may appear similar to several other diseases. It’s important to accurately diagnosis it as foliar fungicides will not control it. For more information, please see the UNL NebGuide, Goss’s Bacterial Wilt and Leaf Blight of Corn.

Bacterial Stalk Rot
Bacterial stalk rot has also developed lately and can probably be blamed on the warm and flooded conditions. Affected plants wilt rapidly and are overtaken by a slimy lesion (Figure 4) that may begin at either the top or bottom of the plant. Plants with bacterial stalk rot also may have a foul odor. For more information, please see the UNL NebGuide, Common Stalk Rot Diseases of Corn.

If you are in doubt about the identity of a disease or cause of another plant problem, you may submit a sample to the UNL Plant and Pest Diagnostic Clinic (P&PDC) for diagnosis. For more information about these and other plant diseases or for submission forms and instructions, visit the Plant Disease Central website.

Tamra A. Jackson
Extension Plant Pathologist, Lincoln
Amy (Ziems) Timmerman
Coordinator, UNL Plant and Pest Diagnostic Clinic, Lincoln

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