How to Tell Northern Corn Leaf Blight Apart from Urea Burn or Some Other Malady

Tom Allen, Extension Plant Pathologist
By Tom Allen, Extension Plant Pathologist June 12, 2013 21:21

How to Tell Northern Corn Leaf Blight Apart from Urea Burn or Some Other Malady

Northern corn leaf blight lesion.

Northern corn leaf blight lesion.

Note there are numerous photos associated with this particular post to aid in determining the difference between northern corn leaf blight lesions and injury associated with a urea application.  I’ve received numerous calls regarding the difference between NCLB lesions and those created by urea burn or herbicide injury.  Mature NCLB lesions will tend to sporulate profusely.  However, observing the reproductive structures produced within the lesion, the spores themselves, requires a good hand lens and some photos for comparison.  Keep in mind that most of the foliar diseases that affect corn in our production system are caused by a fungus that produces a dark-walled spore (e.g., diplodia leaf streak, grey leaf spot, northern corn leaf blight, southern corn leaf blight) except for the rust diseases.  However, there are some differences in lesion type as well as where sporulation can be observed with each of the associated diseases.  For example, grey leaf spot produces a much different lesion type and the associated sporulation is present on the underside of the leaf.  In addition, most of the saprophytic, secondary fungal organisms that can infest injured corn leaves also produce a similar spore so telling the two apart can prove difficult.  When attempting to tell apart NCLB from a urea injury the biggest issue occurs when urea burn, or the other types of injury that may have occurred on the corn leaf, become infested by a secondary organism.  Generally speaking, NCLB lesions, when mature, will have a fuzzy appearance.  The sporulation that occurs in the center of the lesion, upon closer inspection, will have the appearance of small, fine, dark hairs in the middle of the lesion.  Using a 20x hand lens is generally the best way to observe the lesions for sporulation.   Look at several lesions on several leaves to make a determination as to the specific situation in each corn field.

Typical lesion pattern associated with urea burn or other malady.

Typical lesion pattern associated with urea burn or other malady.

In some cases, depending on the specific growth stage at the time of application, urea injury will form a distinct pattern on the parts of the leaf affected (see photos).  NCLB will typically not occur in a pattern and will more likely be observed at random areas on a leaf.

Use the included photos and associated symptom comparison table (NCLB table to compare to urea burn) to determine what type of lesion you might be observing and realize that in some cases the environment, time of day when application was made, number of years in corn, or number of days following a urea application can all alter the appearance of lesions in the corn canopy.

 

 

 

 

Urea injury on a corn leaf with secondary saprophytic fungal growth in the two lesions.

 

 

 

 

 

 

 

 

 

Fungal reproductive structures (conidiophores) and spores of the NCLB fungus in a mature lesion.  Best portion of the photo is in the upper third of the image.

 

 

 

 

 

 

 

 

 

 

 

Urea injury or other associated malady on corn leaf.

Urea injury or other associated malady on corn leaf.

 

 

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Tom Allen, Extension Plant Pathologist
By Tom Allen, Extension Plant Pathologist June 12, 2013 21:21
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