Corn Foliar Disease Update: June 24, 2011

Note the parallel type lesions and size of the lesions highlighted by arrows of sun scald on a corn leaf.

At present there is still little to no foliar disease in the majority of the corn crop.  Many of the calls I’ve received over the past 7 to 10 days have dealt with sun scald, herbicide injury or urea burn rather than foliar disease.  However, yesterday I received a few calls from the south Delta regarding Northern corn leaf blight (NCLB) and questions regarding gray leaf spot (GLS).  Common rust and NCLB continue to be the predominant diseases present in corn throughout the state but at extremely low levels.  But, with that said, a single field with GLS has been identified in the Greenwood, MS area although this appears to be an isolated and somewhat interesting case.  Southern corn rust has not been detected in Mississippi or any other state in the region for that matter and is still only present in the panhandle of Florida. 

Note the parallel margins of the GLS lesions.

With regards to GLS, first and foremost keep in mind that the photos contained in many of the diagnostic books contain images of the “early” or immature lesion symptoms of GLS.  In addition, some of the photos contained in the numerous corn field guides present images of GLS on hybrids that are tolerant to the disease.  The reaction of a particular hybrid will differ with regards to whether or not the hybrid is susceptible or tolerant.  Moreover, it is difficult to determine the disease from early symptoms.  Initially symptoms will develop in the lower canopy but again much of this will depend on the particular hybrid.  The single most important thing to remember is even hybrids with good disease packages can produce GLS symptoms.  Also, the most mature lesions will have parallel edges on the top, bottom, and the sides of the lesion.  Moreover, in some cases sun scald or drought stress can produce a lesion that is similar to GLS.  But, keep in mind symptoms from sun scald or drought stress will likely produce lesions that will ultimately be longer, wider, cross the veins, and on the top leaves first if it is related to sun scald (see attached photos).  In addition, lesions can consume large areas of entire leaves.  Most GLS lesions will be on the order of ¾ of an inch to an inch in length and NOT cross the veins.  Also, if you hold the leaf up to the sunlight to view the lesion, a GLS lesion will have a yellow halo around the outside.  I’ve included some photos to attempt to address this topic.  Additionally, if you have a good hand lens in the truck, something along the lines of a 20-30x lens if you observe the underside of the lesion on the bottom of the corn leaf the structures that produce spores (conidiophores) will be present in lines along the underside of the leaf.  The fungus that causes GLS infects through the stomates so the presence of conidiophores along a line with the stomates can sometimes be present (see photo with arrows).  There are some in the pathology world that refer to this as “ducks in a row”.  Also, remember, many other foliar diseases can produce symptoms that will be similar to GLS.  Southern corn leaf blight and northern corn leaf spot can produce lesions that appear similar to GLS.  However, SCLB will produce margins that are NOT parallel and a bit wavy in appearance and to my knowledge northern corn leaf spot has not been identified in MS.

Note the rows of conidiophores produced on the underside of a corn leaf by the GLS fungus highlighted by arrows.