As we have moved into our late season disease assessments for 2008 in corn around the state, we have noted the following diseases throughout the year (Figures 1-6):
Common rust (Puccinia sorghi)
Eyespot (Kabatiella zeae)
Northern corn leaf spot (Bipolaris zeicola)
Northern corn leaf blight (Exserohilum turcicum)
Anthracnose (Colletotrichum graminicola)
Gray leaf spot (Cercospora zeae-maydis)
Late Season Foliar Diseases of Corn
Paul Esker, Extension Plant Pathologist
As we have moved into our late season disease assessments for 2008 in corn around the state, we have noted the following diseases throughout the year (Figures 1-6):
- Common rust (Puccinia sorghi)
- Eyespot (Kabatiella zeae)
- Northern corn leaf spot (Bipolaris zeicola)
- Northern corn leaf blight (Exserohilum turcicum)
- Anthracnose (Colletotrichum graminicola)
- Gray leaf spot (Cercospora zeae-maydis)
Figure 1. Common rust of corn (Image source: P. Esker, UW-Madison).
Figure 2. Eyespot of corn (Image source: C. Grau, UW-Madison).
Figure 3. Northern corn leaf spot (Image source: P. Esker, UW-Madison).
Figure 4. Northern corn leaf blight (Image source: P. Esker, UW-Madison).
Figure 5. Anthracnose leaf blight symptoms on corn (Image source: P. Esker, UW-Madison).
Figure 6. Gray leaf spot of corn (Image source: G. Munkvold, Iowa State University).
Overall, disease pressure has been light and variable depending on location. In areas where we know there is a history of anthracnose, pressure has been higher. Also, we have had more reports of common rust from around the state. The reports have ranged from severity at very low levels (a few pustules on a single leaf) to those with higher levels on the ear leaf, especially in the eastern part of the state. Also, these observed differences in severity have often been hybrid specific.
In terms of disease management, we are for the most part past the optimal stages (from tasseling through blister) where one would consider the application of a foliar fungicide. However, there are still reports of fields being sprayed and this again an excellent time to emphasize that if a field is to be sprayed, a check strip is important in order to assess the overall impact of the fungicide application through active scouting of treated and nontreated (i.e., check strips) sections of the field. Also, based on our own observations from the field this year, differentiating among eyespot, northern corn leaf spot, and gray leaf spot has been a little more difficult. These three can easily be confused with one another, but do have some distinguishing characteristics:
Eyespot symptoms typically include small, circular lesions or spots that are surrounded by a red and yellow halo and are often found in zones or patches on the leaf surface.
Northern corn leaf spot symptoms differ depending on the race (five occur), but of those races, Race 3 is most important. Symptoms often include narrow, linear lesions that can be found on the leaf blades, sheaths, and husks. Lesions are grayish-tan with a pigmented border and multiple lesions often form along veins.
Gray leaf spot lesions are often yellow to tan in color at the earliest stages. These can be similar to other diseases expect there is a faint watery halo (Fig. 7). As the lesions expand, they will become tan to brown and rectangular in shape.
Figure 7. Early gray leaf spot of corn symptoms (Image source: P. Esker, UW-Madison).