Identifying Rusts In Corn

Nathan Kleczewski Ph.D
Extension Specialist- Plant Pathology
Assistant ProfessorDepartment of Plant and Soil Sciences

Over the past week several individuals reported the presence of rusts in Delaware corn fields.  Indeed, rust is present on our corn trials located in Georgetown, DE.  However, this was not common rust (CR), but Southern rust (SR).  How can you tell the difference between the two?  1) SR tends to sporulate mostly on the upper leaf surface; CR can be found on both upper and lower leaf surfaces.  2) Pustules of SR are orange to light brown in color and generally round ; CR are brick red or dark brown and often elongated or oval.  3) SR often is found on leaves in patches; CR tends to be evenly distributed on a leaf. 4) Common rust does best between 61-77 degrees F; Southern rust does best between 77 and 82 degrees F.  It is not uncommon to see both rusts on the same leaf.

Southern rust on the ear leaf of a corn plant. Pustules often are only on the upper surface, are round, orange, and grouped in patches.

SR needs to blow in from the south, often Latin America, every year.  This season it arrived sooner than usual and was a concern early in the southern states.  It was first detected in Virginia roughly a week ago, and I suspect it moved in to Delaware and the surrounding region soon thereafter.  Typically these diseases start in one or few “epicenters” in a field where the environment is slightly more favorable for disease development.  Over time and given the right environment, the disease may spread through and between fields.  Therefore, it is easy to miss the start of a rust outbreak if fields are not thoroughly scouted.

Our hybrids likely do not have much resistance against SR, so some growers may be concerned with the appearance of this pathogen in our fields.  First, if your corn is at or near or past dent you are likely at the point where within season management is not really an option and likely not economical, particularly at $3.60 a bushel.  Currently I do not recommend fungicides for southern rust, especially if you are at or near R5 and have already applied a fungicide around VT/R1.  Late planted fields that have not received a fungicide application might see some benefit, particularly if the field is irrigated.  Before making spray decisions carefully scout your fields to see if southern rust may be present and if any other issues may warrant your attention.

Source : udel.edu