Many in the agricultural community, as well as researchers annually rate corn for disease as a means to assess hybrid response, hybrid effectiveness, or potential disease level on field productivity. It can be difficult to rain the eye to accurately measure disease on foliage, and differences in the type and size of the structure or lesion associated with the pathogen varies significantly.
The four links below will direct you to disease area diagrams we developed in order to help you obtain accurate disease severity estimates in your fields. The method you use to assess disease severity may differ depending on the overall objective. The diagrams below are corn grey leaf spot, northern corn leaf blight, common rust, and southern rust. These can be printed, laminated, and taken to the field with you to assist your ratings.
Source: University of Illinois