Southern Rust of Corn in Wisconsin

Damon Smith, Extension Field Crops Pathologist, Department of Plant Pathology, University of Wisconsin-Madison

If you are like me, you have been paying attention to reports from the southern and other Midwest states indicating that southern rust of corn is making its way further north this year, a first in a couple of years. We have been watching and have indeed confirmed southern rust on field corn in Columbia County Wisconsin this week. You can follow current southern rust updates on the iPiPE site. Now that southern rust has been confirmed in Wisconsin, it is time to NOT PANIC. We just need to be diligent in scouting over the next several weeks so that we can make, first, silage harvest decisions, then grain harvest decisions. Grain yield reductions are greatest if southern rust moves in prior to the “milk” (R3) growth stage in corn. Much of the crop in Wisconsin is nearly past this, so the good news is that impacts on grain yield should not be significant. However, there could be impacts on silage corn harvest as southern rust, like other foliar diseases, can dry the crop prematurely. See my previous article on how silage corn might be impacted by foliar disease. I also would like folks to know which fields southern rust has been a problem this year for grain corn, and prioritize those fields for harvest first, as standability might be an issue in some of these fields.

Southern rust is caused by the fungus Puccinia polysora. Symptoms of southern rust are different from common rust in that they are typically smaller in size and are often a brighter reddish/orange color (Fig. 1). Pustules of southern rust also typically only develop on the upper surface and will be be more densely clustered. Favorable conditions for southern rust development include high humidity and temperatures around 80F. However, very little free moisture is needed for infection to occur. Southern rust is typically a rare occurrence in Wisconsin. When it does occur, it is usually in the southern and south-western portions of the state, with epidemics initiating late in the season. With that said, southern rust has made it to south-central Wisconsin this year. Spores of this fungus have to be blown up from tropical regions or from symptomatic fields in the southern U.S. The fungus can not overwinter in Wisconsin. While southern rust epidemics can be rare events in Wisconsin, the disease can be serious when it occurs prior to the R3 growth stage. Therefore close monitoring of forecasts and scouting are needed to make timely in-season management decisions.

Management of Southern Rust

Traditionally resistance was used to manage southern rust. However, in 2008 a resistance-breaking race of the southern rust fungus was confirmed in Georgia. Thus, most modern hybrids are considered susceptible to southern rust. Rotation and residue management have no effect on the occurrence of southern rust. The southern rust fungus has to have living corn tissue in order to survive and can not overwinter in Wisconsin. Fungicides are typically used to control southern rust in parts of the U.S. where this is a consistent problem. Efficacy ratings are available for fungicides against southern rust on the Corn Fungicide Efficacy Table. As I said previously, should southern rust make its way to Wisconsin prior to the “milk” (R3) growth stage in corn, it could cause yield reductions. A helpful matrix can be found here to help make the fungicide application decision, in order to limit impacts by the southern rust fungus. If you need assistance in identifying rust on corn, leaf samples of corn plants can be sent in a sealed plastic bag with NO added moisture to the University of Wisconsin Plant Disease Diagnostic Clinic (PDDC). Information about the clinic and how to send samples can be found by CLICKING HERE.

 

Common rust is caused by the fungus Puccinia sorghi and is extremely common in Wisconsin, but often results in little yield loss. Most field corn hybrids planted in Wisconsin are very resistant to the disease. Symptoms can include chlorotic flecks that eventually rise and break through the epidermis to produce pustules of brick-red spores (Fig. 2). Typically these pustules are sparsely clustered on the leaf. They can also appear on other parts of the plant including the husks and stalks. Management for common rust primarily focuses on using resistant hybrids. Remember resistance is not immunity, so some pustule development can be observed even on the most resistant hybrids. Some inbred corn lines and specialty corn can be highly susceptible to common rust. Under these circumstances a fungicide may be necessary to control common rust. Most of the hybrids I have scouted this season have some pustules, however incidence and severity is relatively low. Therefore, a fungicide application to control common rust wasn’t needed to manage common rust for most of these hybrids in Wisconsin.

Other Useful Resources about Rusts on Corn

WisCONTEXT Article on Southern Rust

Ohio State University Article on How to Differentiate Common Rust from Southern Rust

Video by Dr. Tamra Jackson-Ziems of the University of Nebraska – Identifying Rust Diseases of Corn

References