Southern corn rust detected in Mississippi corn
Southern rust of corn. Note light colored sporulation that occurs on the upper leaf surface. Southern rust was confirmed on a single corn leaf from Humphreys County on July 2, 2013. A consultant made the initial observation and the disease was confirmed by microscopy in the laboratory July 3. 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.
Common rust (left) compared to southern rust (right). Note the main differences in color of the sporulation. 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 Mississippi 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: http://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).