Corn Foliar Disease Update: June 15, 2011

At present, low levels of foliar disease continue to be reported from some corn fields throughout the Delta.  Over the past week more common rust has been identified throughout the corn canopy.  I’ve received several telephone calls questioning whether or not southern rust has been observed in MS.  Currently I am not aware of ANY southern rust in MS.  In fact, the only observed southern rust in the southern U.S. is in the panhandle of Florida, specifically in Jefferson County, FL.  However, at this time of the year it can be difficult to tell the difference between common rust and what could be considered to be southern rust at the field level.  Typically I rely on microscopic identification because if rust pustules are only found on the bottom 3 to 4 leaves of the corn plant they are almost always common rust at this time of year.  Keep in mind it will be extra difficult to determine if the rust is common or southern based solely on the number of sporulating pustules that have ruptured or even the overall appearance (i.e. color of spores).  See the attached photo of common rust on lower leaves.  More often than not the environmental conditions in the lower corn canopy are more conducive for the development of common rust pustules.  For southern rust to occur the spores will have to blow into our production area.  Having had corn in a particular field last year does not necessarily mean that southern rust will be identified.  The fungi that cause common and southern rust do not overwinter in our country and have to be blown into MS for the disease to occur on an annual basis.

Most people that have called over the past two weeks have commented on just how little disease is occurring in MS corn fields.  Northern corn leaf blight (NCLB) has been confirmed in few fields.  However, with the hot and dry conditions we continue to encounter throughout the state NCLB will likely not be able to cause tremendous issues if identified.  Also, as I’ve mentioned previously this season there are many things that can be misidentified as NCLB.  Stress related to the high temperatures, lack of moisture, herbicide burn, and urea burn can all manifest themselves to produce a lesion that can appear similar to NCLB.  If you need help identifying the particular lesions on your corn leaves please call.

With regards to fungicide application:  I am not an advocate of the at tassel fungicide application timing (or sometimes referred to as a “plant health” application).  A fungicide should be applied to manage the potential yield loss risk associated with a particular foliar disease.  Presently, that level of risk is extremely low due to the extreme temperatures and drought that we’ve encountered throughout much of the state.  The greater data set, assembled by members of the MSU extension and research community, suggests that a fungicide is most reliable when used in a situation where a foliar fungal disease threatens yield.  For more information regarding this data set see the five part series included on this blog starting with “The corn fungicide dilemma: when should a fungicide be applied? Part I of V, yield in the absence of measureable disease”.   The additional topics of that series present data on irrigated vs. non-irrigated situations, green leaf material following a fungicide application, stalk integrity/standability, and yield in the presence of disease (specifically southern rust) following fungicide treatment.