Based on various reports from around the state in 2008, probably the number one disease of corn that has been noted and mentioned has been common rust, caused by Puccinia sorghi. Common rust often forms throughout the entire plant because the infection took place when leaf tissue was still in the whorl. This is a distinguishing characteristic that separates common rust from southern rust (Puccinia polysora), which would have fewer pustules in the lower canopy. 

 

In most of our research and demonstration plots, we have found common rust throughout the plant, however, the severity of the symptoms has been considered low. In many cases, we have found only one or two pustules on the ear leaf, with a few more pustules on the leaves directly below or above, but still at levels we would consider to be 1% or less. Reports from around the state have varied, including areas where the severity has been higher. Given the differences in the reports, the goal of this article is provide a method to help in estimating the severity of common rust on the leaf surface. In Figure 1, four images are shown that were made using Severity.Pro software (F. W. Nutter, X. Zhang, and D. L. Heidfeld, Iowa State University) and illustrate 1%, 5%, 25%, and 50% severity of common rust on a corn leaf. The lesion type used for these examples was circular.

 

 

 

 

 

 

Furthermore, to estimate how many pustules were found for the different severity levels, a count was made on each leaf. For 1% and 5%, full counts were made on the leaves shown in Figure 1, while for 25% and 50%, a grid was placed over the leaf (each grid measured approximately 1 inch by 1 inch) and a sample of three counts were made and then the average number of pustules was used to calculate the total on the leaf surface. Results of these counts indicated that there were 162 pustules when the disease severity was 1%, 478 pustules for 5%, 1,905 pustules for 25, and 3,465 pustules for 50%. These results indicate that it takes a large infection before a substantial physiological loss of healthy green leaf tissue occurs. Another consideration of the severity of common rust is the type of pustule, meaning does it appear that the lesion appears is still active (e.g., the pustule would be surrounded by green tissue) compared with non-active (e.g., the pustule would be surrounded by necrotic tissue) pustule. Using the combination of these factors will help to determine the overall effect of your management tactics this year.