Corn Disease Update: June 7, 2012

Common rust

Common rust, note cinnamon-colored sporulation as well as elongated pustules.

I continue to receive calls regarding the rust present in the lower canopy of numerous corn fields.  At present, common rust is the only rust present in MS corn.  However, I will not be surprised if this changes over the next week to 10 days.  But, based on how far along some of the corn is throughout the Delta (some fields approximately R2+) I am not concerned about southern rust at this point.  Again, remember, common rust on the lowest leaves of the corn plant can appear similar to southern rust.  In addition, after observing a sample this morning (6/7/2012) for what may have been considered gray leaf spot or even southern corn leaf blight, keep in mind there are situations where the oldest common rust lesions will develop a much larger lesion around the pustule that can appear quite similar to other important foliar diseases.  At this point in the season, and once an ear is setting on the corn plant, focus on scouting fields from your belt up.  Leaves in the lower part of the canopy may become infected but they are generally nutrient-stressed and not contributing to the plant’s photosynthesis.  Keeping the ear leaf clean is the most important step.  But, with that said, corn can still sustain some disease and produce outstanding yields.  Common rust should not be considered a threat to yield.

Gray leaf spot (GLS)

Early symptoms of gray leaf spot, note the defined yellow halo around the rectangular lesions.

Ever since my first year in MS (2007) we continue to encounter more and more GLS on an annual basis.  I attribute a lot of this to continuous corn production in some fields in addition to a conducive environment in some specific locations.  The first observed GLS this year was from around the Minter City area.  Over the past two seasons this has continued to be a “hot spot” for the disease.  When scouting for GLS keep in mind that the symptoms of the disease may differ depending upon the hybrid’s specific tolerance.  The most mature lesions will have the characteristic blocky appearance at the ends, be brown to gray in color, not cross the veins, and in general the length of the lesion can be much longer than the width.  Typically, mature lesions can measure 1” to 1.5” inches in length.  The width of the most mature lesions will likely be less than 1/16^thof an inch.  Young lesions will have a characteristic yellow halo around the developing lesion and when the leaf is held up to the light the lesion will look rectangular.  Fungicide applications can be beneficial in response to infection on susceptible corn hybrids.  Last year, at one particular

Mature gray leaf spot lesions.

location, two fungicides were applied by air on 3 different hybrids in response to a heavy infection of GLS at approximately dent.  In short, fungicide applications made at that late stage in response to GLS were not effective at preventing yield loss when compared to the untreated strips as the disease had already infected most of the aboveground plant material.  Fungicides containing a strobilurin component (either a straight strobilurin or a strobilurin + triazole) will likely be the most beneficial in response to GLS.  While fungicides containing just a triazole (e.g. propiconazole, prothioconazole, tebuconazole, tetraconazole) are labeled to manage GLS, rankings from university trials for ALL labeled triazoles are not available at this time.  However, fungicide timing will be important, but, more data is needed to make more informed management suggestions with regards to proper fungicide placement.  Several fungicide trials will attempt to get some of this information this season.

Northern corn leaf blight (NCLB)

Mature northern corn leaf blight lesion.

Over the past week I’ve observed two fields with mature NCLB lesions.  Neither of the fields required a fungicide application.  Diagnostically, NCLB lesions will be elongated, and cigar-shaped.  Typically the center of the mature lesions will produce sporulating fungi.  A hand lens can generally aid in the diagnosis of the particular disease.  The fungal growth in the lesion will appear like dark hairs.  However, remember that lesions produced by leaf injuries can have a secondary fungal growth that will also produce dark fungal structures.  I realize that many of the diagnostic guides available describe and/or show photos of young NCLB lesions.  I have never observed anything but the most mature lesions that we are most familiar with in MS.