No. 10 Article 2/June 8, 2012

Corn and Soybean Diseases: Update and Outlook

Other than some issues with seedling diseases a few weeks ago (see issue 8, May 25), relatively few diseases have been observed on corn and soybean in Illinois so far this season. Weather is the major factor influencing plant diseases, and the relatively dry weather of late has contributed to the lack of diseases being seen. In this issue I’m providing an update and outlook on several diseases of corn and soybean.

Diseases Being Observed

Anthracnose leaf blight of corn. Where anthracnose leaf blight is being seen, it is most often in fields that are "corn on corn," especially in areas where rainfall has been a little more abundant. I addressed this disease in issue 6 (May 10). When corn plants begin to develop rapidly as the temperatures warm up, they tend to "outgrow" this disease, with new leaves generally not being affected.

Holcus leaf spot of corn. Symptoms of holcus leaf spot are showing up on corn in a few areas in the state. The symptoms look very similar to paraquat herbicide injury, with light-colored round spots on the leaves. Some spots may have edges that look water-soaked. Fungicides have no impact on holcus leaf spot because it is caused by a bacterial pathogen. The disease is not likely to spread to noninfected leaves.

Left: Symptoms of holcus leaf spot on corn leaves. (Photo courtesy Dr. Jason Bond, Southern Illinois University). Right: Septoria brown spot lesions on a unifoliolate soybean leaf.

Septoria brown spot of soybean. In areas that have been receiving rain more frequently, Septoria brown spot, observed as small brown circular lesions on leaves, can be seen on soybean (especially the unifoliolate leaves). The disease will only spread upward on the plant when rain splashes spores of the fungus onto new leaves. The relatively dry weather much of Illinois has been experiencing does not favor continued development of Septoria brown spot.

Diseases That Could Develop

Charcoal rot of soybean and corn. In areas where conditions are very dry, charcoal rot could be an issue later in the season. This disease has been more prevalent in hot and dry years. Both soybean and corn can be affected by charcoal rot, whose symptoms include premature dying and wilting.

Sudden death syndrome of soybean. Illinois soybean fields that were planted early (late April) followed by cool and wet weather are at highest risk for sudden death syndrome (SDS; see issue 4, April 27). However, because rainfall throughout the growing season is important for the development of SDS, the dry weather being experienced in much of Illinois reduces the risk.

Rust diseases. Southern corn rust and soybean rust have already been observed in the southern United States in 2012. Many factors will influence the presence of these diseases in Illinois this year, including the buildup of spores in southern states, weather systems that move spores north to the Midwest, and local weather in Illinois when (and if) spores arrive. The spread of southern corn rust and soybean rust can be monitored online at the IPM-PIPE website (; Figure 1). Cooperators in several states use observation data to color-code maps to indicate the presence or absence of disease.

Figure 1. Observations of southern corn rust (red areas) in the United States as of June 6.

--Carl A. Bradley

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