Fungicide considerations for corn diseases: Scouting is key

Share Tweet Email

We are entering the time window in Kansas where corn producers should be scouting fields and assessing the need for a foliar fungicide application. Several fields in eastern and central Kansas are in V10-V14 development stage, with a few weeks to go until pollination. On the western side of the state, the early to mid-May planting dates are progressing toward V10 development stage towards next week but still have several weeks to go before flowering time.

Now is the time to be out scouting for the following fungal diseases of corn:

  1. Gray leaf spot (Figure
  2. Southern rust (Figure 2, left)
  3. Common Rust (Figure 2, right)

Figure 1. Early development of gray leaf spot lesions showing a distinct yellow halo. Photo courtesy of Doug Jardine, K-State Research and Extension.

Figure 2. Typical symptoms of southern rust (left) and common rust (right). Photo courtesy of Rodrigo Borba Onofre, K-State Research and Extension.

Gray leaf spot scouting. Begin scouting for gray leaf spot in corn about two weeks before expected tassel emergence. Gray leaf spot is characterized by rectangular lesions that are 1-2 inches in length and cover the entire area between the leaf veins. Early lesions are small, necrotic spots with yellow halos that gradually expand to full-sized lesions. Lesions are usually tan in color but may turn gray during foggy or rainy conditions. The key diagnostic feature is that the lesions are usually very rectangular in shape.

Southern rust scouting. Southern rust is typically first reported in Kansas in mid-July. Pustules will appear on the upper leaf surface (unlike common rust which can be found on either side of the leaf). Pustules will be scattered on the leaf surface and spores will appear orange and will rub off on fingers (and clothes!). Severe infections can be seen on the leaf sheaths.

Factors that influence corn yield response to fungicide applications

Years of fungicide application research clearly demonstrates that the single best time to apply a fungicide to corn for gray leaf spot control is from VT to R1. A single application at V7 – V8 will not hold up against late-season pressure. Those who choose to put a fungicide down with their last herbicide treatment will most likely have to apply second cover at VT – R1 if there is any gray leaf spot pressure at all. A VT – R1 application may also provide some suppression of southern rust, should it arrive early enough to cause yield loss. There is evidence that foliar fungicides can control southern rust when applied prior to R5, when dealing with a susceptible hybrid and high disease pressure. Some fungicides that are good to excellent for gray leaf spot are also very good for southern rust control.

Summaries of multi-year university research about fungicide efficacy can be found here:


Disease risk factors to consider when weighing the benefits of a fungicide application:

Susceptibility level of corn hybrid. Seed companies typically provide information on the susceptibility of their hybrids to gray leaf spot and southern rust. In general, hybrids that are more susceptible to fungal foliar diseases will have a greater response to a foliar fungicide (if disease pressure is high enough).

Previous crop. Because gray leaf spot survives in corn residue, the risk of disease increases when corn is planted back into a field that was in corn the previous year. Fields with a history of gray leaf spot should be closely scouted. Southern rust, on the other hand, blows in from the south each year. It is important to watch regional updates about southern rust pressure in the state.

Weather. Rainy and/or humid weather generally is most favorable to gray leaf spot. In growing seasons when these conditions prevail, the risk for disease development increases. Southern rust is favored by warm days and nights (> 80 degrees) as well as high humidity.

Field history. Some field locations may have a history of high foliar disease severity. Fields in river bottoms or low areas or surrounded by trees may be more prone to having gray leaf spot.

Current disease management guidelines suggest the following criteria for considering an application of foliar fungicide:

  • For susceptible hybrids (those with the lowest rating within a company’s lineup): Fungicide applications should be considered if disease symptoms are present on the third leaf below the ear or higher on 50 percent of the plants examined.
  • For intermediate hybrids (those with an average rating within a company’s lineup): Fungicide applications should be considered if disease symptoms are present on the third leaf below the ear or higher on 50 percent of the plants examined, if the field is in an area with a history of foliar disease problems, if the previous crop was corn, if there is 35 percent or more surface residue, and if the weather is warm and humid.
  • For resistant hybrids (those with the best rating within a company’s lineup): Fungicide applications generally are not recommended.

According to the data from Illinois corn fungicide trials, if at least 5 percent of the ear leaf area is affected by disease at the end of the season, a foliar fungicide applied at VT and R1 would likely have been beneficial. Using the disease risk factors and scouting observations collected just before tassel emergence will help you predict how severe disease may be several weeks after the VT to R1 stages, and help you decide whether to apply a foliar fungicide.

If no disease is present or pressure is low, I recommend holding off on the R1 application since efficacy will begin to wane in three to four weeks, just as late season pressure may begin to develop. Data exists that would suggest that if pressure begins to develop later, an R2 application can be economical and will provide protection later into the grain fill period. This later application could also protect against any late-season southern rust pressure.

Distinguishing between gray leaf spot and bacterial streak

Bacterial streak, identified as a new corn disease in the U.S. in 2016, is now active in most of western Kansas. While yield loss potential for this disease remains unknown, we do know that it can be misidentified as gray leaf spot, resulting in unwarranted fungicide applications. Fungicides will not have any effect on bacterial streak. Keep in mind that gray leaf spot typically has very sharp edges defined by the leaf veins, whereas bacterial streak will have a wavy edge that can cross the leaf vein (Figure 3). Also, when backlit with light, gray leaf spot lesions will have an opaque appearance while bacterial streak lesions are more translucent (Figure 4). If you are unsure about symptoms, please contact the K-State plant disease diagnostic clinic via email at

Figure 3. Comparison of sharp-edged gray leaf spot lesions (right) with wavy-edged bacterial streak lesions (left). Photo courtesy of the University of Nebraska.

Figure 4. Gray leaf spot lesions (left) have an opaque appearance whereas as bacterial streak allows light to more easily pass through giving it a translucent appearance (right). Photo by Doug Jardine, K-State Research and Extension.



Rodrigo Borba Onofre, Postdoctoral Research & Extension Fellow

Doug Jardine, Emeritus Plant Pathology

Ignacio Ciampitti, Farming Systems

Tags:  corn fungicide foliar fungicides foliar diseases