August 1, 2008

Corn Disease Update

Southern Rust

 

Figure 1. Southern rust in corn.

Figure 2. Common rust in corn.

Figure 3. Gray leaf spot in corn.

Southern rust (Figure 1) was confirmed on corn Thursday (July 31) in samples submitted from Thayer County to the UNL Plant and Pest Diagnostic Clinic. There also are unconfirmed reports of the disease in York and Fillmore counties. Warm temperatures, high humidity and showers forecast for early next week are expected to promote development and spread of the disease. Rust diseases produce large amounts of spores that are easily moved by wind for long distances.

This is the third consecutive year that southern rust has been reported in Nebraska, having caused substantial yield loss in south central Nebraska in 2006 and losses to a lesser extent in 2007. Southern rust severity can increase rapidly during periods of high humidity or wetness. Corn fields should be monitored regularly for its spread. Southern rust was reported at low severity in a field in Manhattan, Kansas (D. Jardine, Kansas State University) one week ago and continues to be severe in some southern states.

The characteristics used for differentiating between common rust and southern rust are described and illustrated in the NebGuide, Rust Diseases of Corn in Nebraska (G1680). The simplest and most reliable way to differentiate the diseases without a microscope are to examine both leaf surfaces for spore development. Southern rust spore production is usually limited to only the upper leaf surface and tends to be more tan/orange in color than that of common rust until the spore type changes and turns black later in the season.

Common Rust

Common rust (Figure 2) continues to be unusually severe in parts of the state (southwest Nebraska to northeast Colorado and northeast Nebraska), warranting fungicide applications in some areas. High temperatures are expected to slow progress of common rust, but warmer night temperatures will exacerbate southern rust development. Historically southern rust been the more aggressive rust disease in Nebraska. Common rust can produce spores equally well on both leaf surfaces, in contrast to southern rust, and tends to be more brick red/brown in color until the spore type changes and turns black later in the season.

Gray Leaf Spot

Gray leaf spot (Figure 3) continues to increase in severity and has reached the ear leaf in some areas. Kansas officials have reported epidemic levels of disease in some areas of that state, but the spores are not expected to move by air as well as those of the rust diseases. Gray leaf spot requires several days for obvious lesion development after fungal infection, so the disease can advance without being immediately obvious and is probably one to two leaves higher on the plant than is apparent.

Fungicides

Fungicide applications made several weeks ago likely are no longer providing protection from fungal foliar diseases. The preventative activity for foliar fungicides ranges from two to three weeks. The pre-harvest interval for some fungicides (in the triazole class) restricts their use after the development of brown silks so pay close attention to the label restrictions on the most recent version of the product label. The pre-harvest intervals and other foliar fungicide label information are summarized in the NebGuide, Rust Diseases of Corn in Nebraska.

Hybrid response to foliar fungicide applications for control of gray leaf spot has been variable, but has been most successful in susceptible hybrids under disease pressure and more variable in the absence of disease. Results from 2007 foliar fungicide trials conducted in Nebraska are available at the UNL Extension Plant Pathology Web site, Plant Disease Central, list in the left navigation bar under Management Trials:Efficacy.

Tamra A. Jackson
Extension Plant Pathologist
Amy Ziems
UNL Plant and Pest Diagnostic Clinic