General Corn Foliar Disease Update: June 8, 2011

Tom Allen, Extension Plant Pathologist
By Tom Allen, Extension Plant Pathologist June 8, 2011 23:06

At present, and generally due to the hot and dry conditions, little if any foliar disease is present in the Mississippi corn crop.  On Friday (June 3) I scouted fields in the Tchula/Thornton area.  The particular fields were dryland corn fields and aside from suffering due to lack of moisture there was little disease present.  Monday (June 6), I scouted a dryland and irrigated field south of Greenwood.  In the four fields, low levels of common rust (3 of the fields) and northern corn leaf blight (NCLB; 1 field) were identified.  Foliar disease can be expected, especially if corn plants have reached tasseling/pollination stages.  In addition, I’ve started to receive calls regarding NCLB in some fields.  However, there are many situations where other foliar “spots” continue to be classified as disease. 

When scouting corn at this time of the year it can be difficult to determine the difference between foliar fungal diseases, herbicide drift/injury, and sun scald/drought injury.  To compound factors there are a lot of diagnostic guides that show “early” symptoms of many of the diseases in our cropping system.  I find it difficult to determine if a particular spot will result in a specific disease without having confirmed that the spores of the fungus are present in the plant tissue.  Remember, the inoculum came from somewhere and it is possible from within the field you are standing in.  Keep looking; if you don’t find the most mature lesion type for the particular disease then it is possible the spot is either a different disease or not a disease at all. 

With regards to the lesions caused by fungal disease, herbicide drift/injury, and sun scald/drought injury keep in mind that ANY of the lesions could have sporulation detected within the lesion.  Secondary fungal infections can and frequently do occur on plants that have been injured.   However, whether or not a lesion will produce spores will likely depend upon the period of time between when the lesion occurred and when it was first observed.  More often than not the spores that are produced are dark so simply saying that something like NCLB produces “dark spores within mature lesions” can be confusing and not necessarily aid in distinguishing between NCLB and a lesion caused by herbicide injury for example.  This season in particular we are encountering a tremendous amount of herbicide drift/injury in the corn crop (see attached photos).  In some cases this makes determining if a leaf spot is inconsequential or one that may require an action difficult at best.  Given the current environmental conditions, hot and dry, most foliar fungal diseases (save for the rusts) will likely begin in the lower canopy.  Temperatures won’t be nearly as extreme on lower leaves and in cases where more than one year of corn have occurred in a particular field a potential source of inoculum for a disease such as NCLB will be in closer proximity to leaf material.  But, with that said it is still possible for a disease to begin in other parts of the canopy given that spores can be windborne and brought into a field via rain.  More often than not herbicide drift or injury will be on the top leaves of the corn plant but can be found throughout the entire plant canopy.    

Specifically, the two most prevalent diseases in our corn production area:

Common rust – I have not seen common rust result in a yield limiting situation in MS.  Most notably, the fungus that causes common rust prefers cooler temperatures, so let the high temps manage the disease at this time of year.  Last year in particular it was difficult to determine if the rust that was observed on the lowest corn leaves was common or southern rust.  The difference between the spores produced by the two fungi is the best way to tell the two diseases apart since the spores are different shapes and sizes.  The best way to do this is with a light microscope.  At the field level there are a few characteristics to focus on:

-common rust pustules appear to be larger in size than southern rust pustules

-common rust pustules that have ruptured have spores that will appear more russet or cinnamon in color than the more orange color of southern rust spores

-common rust typically appears to produce a lower amount of spores from ruptured pustules when compared to southern rust

Northern corn leaf blight – Generally speaking, NCLB should be more of a problem in fields that have had a history of more than one year of corn.  However, this is not to say that it isn’t possible to find NCLB in fields without a history of corn production nor to suggest that fields with a history of corn will have a problem with NCLB or another foliar disease produced by a fungus that could have overwintered in stubble remaining in the field.  One thing to remember, there are many other situations that can appear to be similar to NCLB.  Over the past few seasons I have observed urea burned leaves, drought stress, sun scald/leaf scorch, or, particularly this season herbicide injury that have all appeared to be similar to NCLB lesions.  In addition, there are cases when injured corn leaves can become infected with a secondary fungus that will lead to dark spores within the center of the lesion that can then appear similar to NCLB. 

-focus on whether or not a pattern exists to the foliar lesions present, lesions caused by foliar nutrition applications can typically be found on areas of the leaf that were not fully expanded until after the aerial application

-drought stress or sun scald can produce a lesion that in some cases will appear to be much larger than NCLB lesions

-the best diagnostic feature is to have soe compare spores from a lesion to determine if it is in fact NCLB versus a secondary fungus invading another non-disease foliar lesion

-NCLB lesions are cigar-shaped in appearance, can develop torn leaf tissue when fully mature, and when recognized will have almost an olive-drab appearance in the middle of the lesion where sporulation is occuring

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Tom Allen, Extension Plant Pathologist
By Tom Allen, Extension Plant Pathologist June 8, 2011 23:06
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