June 27, 2008

Figure 1. Common rust of corn can be identified by red sporulation and pustule development on both the upper and lower leaf surfaces and is being reported in parts of Nebraska.

During the past few days common rust (Figure 1) and anthracnose have both been confirmed in corn fields in south central and eastern Nebraska. Development of these diseases is a few days to weeks earlier than "normal" in Nebraska and can be attributed to frequent and prolonged periods of wetness this year. In addition, gray leaf spot (GLS) has been reported (unconfirmed) in some eastern counties already.

Don't confuse the common rust developing now with the more devastating southern rust that hit Nebraska fields especially hard in 2006 and to a lesser extent in 2007. Southern rust is favored by warmer temperatures and we usually see it develop in late July-August. For more information and photos on how to differentiate between the two rust diseases refer to the NebGuide, Rust Diseases of Corn in Nebraska.

Many people are considering making a fungicide application in response to these diseases. There are several points to consider before making your final decision:

1. Weather — If these wet weather conditions persist, then disease will probably continue to increase and could reduce leaf area. 
2. Yield impact — Disease and loss of leaf area will have a much greater impact on yield after tasseling than now. 
3. Window of protection — Most fungicides provide approximately 14-21 days of protection. During recent years, gray leaf spot and southern rust have been our most severe fungal foliar diseases and impacted yield in late July through August. If this trend is repeated in 2008 and if weather conditions persist, then fields may require an additional application(s) of fungicides later in the season when it is most critical for protecting leaf area and yield. So if you only intend to apply once, then the need may be greater later. 
4. Input costs — Considering the increase in corn prices and higher input costs already invested in this year's corn crop, how many fungicide applications are economical on commercial corn? Considering that fungicides cost an average of $15-20/ac an increase of 2-3 bu/ac in yield will be necessary to cover the cost of each fungicide application. According to the results from fungicide trials conducted across 12 states in 2007, at today's corn prices, a single fungicide application at tasseling would have led to a profitable yield increase at approximately 60% of the 168 locations. 
5. Late season diseases — Early season rains and the delayed corn planting dates could have a substantial effect on late season development of GLS and other diseases, such as southern rust. Younger leaf tissue tends to be more susceptible to foliar diseases and is especially observed in later planted corn which matures later in the season when these diseases are typically more active.

For those considering pre-tassel applications of foliar fungicides this year, be advised that some company recommendations have changed for them, particularly for the use of tank mixes and adjuvants with fungicides to prevent potential plant injury and are summarized in Foliar Fungicides on Corn: Label Changes and Potential Phytotoxic Effects in the March 7, 2008 issue of Crop Watch.

	Figure 2. Goss's wilt and blight on corn can be identified by the presence of dark green leaf "freckles" (pictured) and shiny secretion on the surface.

Bacterial Diseases

Goss's bacterial wilt (Figure 2) has also been identified in central Nebraska (A. Ziems, UNL Plant & Pest Diagnostic Clinic), as well as in northeast Colorado again this year. With the widespread storm damage on corn this year, we are at higher risk for this disease because it requires wounds to infect the plants. For more information, refer to the UNL NebGuide, Goss's Bacterial Wilt and Leaf Blight of Corn, G1675.

In addition, another bacterial disease has been reported again this year in southwest Nebraska and is called chocolate spot. This disease has a similar appearance to Goss's wilt on the leaves, but has been a minor disease and has historically occurred in Wisconsin fields deficient in potassium. In the past, this disease has subsided with the onset of hot temperatures.

Tamra Jackson
Extension Plant Pathologist