Southern Corn Rust Detected in MS Corn Crop

Southern rust was confirmed on a single corn leaf from Humphreys County on Tuesday evening (7/2/2013).  A consultant made the initial observation and the disease was confirmed by microscopy in the laboratory early Wednesday morning.  Southern rust, depending on the growth stage at infection can become a devastating disease.  However, corn that has reached dough (R4; 24-28 days after silking) or is close to dent (R5; 35-42 days after silking) can generally be considered to be out of the woods when it comes to southern rust; especially when infection levels are extremely low.

Southern rust of corn. Note light colored sporulation that occurs on the upper leaf surface.

Differentiating common rust from southern rust

At this stage of the season, a lot of common rust can still be observed.  However, in some cases common rust pustules can be misidentified as other diseases without the aid of a hand lens when the pustules become hollowed out and sporulation is difficult to observe.  Common rust can occur from the bottom of the plant through the top of the canopy, but, in general the disease will tend to be in a small, localized areas on the leaf.  Southern rust will tend to be observed all over the leaf, pustules will be plentiful and in some cases (as the leaf that was initially observed) the pustules will be surrounded by a faint yellow halo.  In addition, common rust will typically occur on both the top and the bottom of the corn leaf.  Conversely, southern rust will generally occur on the upper leaf surface.  Common rust pustules appear larger and the sporulation that erupts from the lesion is a maroon or russet color.  Southern rust pustules appear smaller in size and the general color of sporulation is light orange.

Southern rust myths

Over the past few seasons I’ve heard numerous people ask how fast southern rust can kill a corn plant.  Last year in particular I heard people state that “southern rust could kill a corn plant in 7 days”.  I’ve dealt with southern rust every year since I arrived in MS and have not observed corn plants that died as a result of a southern rust infection.  In fact, I have only seen a single instance of southern rust drastically reducing yield (on the order of a 50% reduction in yield) in a field of corn planted following wheat.  Late-planted corn is generally most at risk since the disease favors high temperatures and high humidity and generally enters our corn production system at some point in July/August.  Even during 2010, when southern rust was detected early in June, and fungicide trials were conducted on corn that had widespread infection, the corn plants were not killed as a result of southern rust.  However, fungicides were able to produce a yield benefit when applied at dent in a field of corn that was already significantly infected (see: https://www.mississippi-crops.com/2011/05/29/the-corn-fungicide-dilemma-when-should-a-fungicide-be-applied-part-v-of-v-preventing-yield-loss-from-foliar-disease/ for additional information regarding southern rust fungicide trials conducted in MS).

Stalk integrity or increased lodging as a result of southern rust is an often overstated situation.  For increased lodging to occur as a result of southern rust, infection of the corn plant would likely need to occur prior to tassel (VT).  Once leaf tissue is covered by the disease, cannibalization of the stalk can occur and reduce the stalk’s integrity, resulting in increased lodging.  See the above link regarding lodging information in the trial conducted in Stoneville during 2010.

Common rust (left) compared to southern rust (right). Note the main differences in color of the sporulation.

Disease management

Fungicides should be used judiciously to manage southern rust.  All corn hybrids are considered to be susceptible; however, scouting for the disease is the most important step.  Once identified, determine if a fungicide is necessary based on expected return on investment, overall expected yield, and growth stage of the corn.  Keep a watchful eye on young corn fields and recognize that even though southern rust can move fast it can requires several weeks for inoculum to build up in a single field.  Also, make sure you properly diagnose southern rust since in some instances common rust in the lowest part of the plant canopy can appear extremely similar to southern rust.

Southern corn rust monitoring

Monitoring for southern rust continues to be important for corn farmers.  To keep abreast of the situation continue to monitor the “Disease Monitoring” maps on the Crop Situation Blog (https://www.mississippi-crops.com/disease-monitoring/) for current observations of southern corn rust that will be updated as new information becomes available.