Corn Foliar Disease Update: July 8, 2011 – Southern Corn Rust Detected at LOW LEVELS

Tom Allen, Extension Plant Pathologist
By Tom Allen, Extension Plant Pathologist July 8, 2011 07:04

Corn Foliar Disease Update: July 8, 2011 – Southern Corn Rust Detected at LOW LEVELS

Flecking on a corn leaf is not indicative of a disease that has yet to produce symptoms.

All continues to be mostly quiet on the corn foliar disease front.  The majority of field calls continue to be sun scald with an occasional foliar disease, mostly northern corn leaf blight.  Little if any common rust is present and in most cases can be observed on the lowest leaves which at this point in time are no longer essential to corn development.  Keep in mind that on lower leaves and early in the stages of symptom expression common rust can be easily confused with southern rust especially in furrow irrigated fields with good canopy closure.  The high levels of humidity will allow common rust to thrive lower in the canopy.  In some cases, the common rust pustules with spores erupting on the lowest leaves will not have developed the characteristic russet or dark cinnamon color that is indicative of common rust.  In addition, pustules may be more numerous and smaller in size which will make them difficult to tell apart from southern rust.  Southern rust pustules will more often than not first be detected higher in the canopy since they are wind and rain borne.  When scouting corn at stages once the ear has emerged focus on lesions present on the ear leaf and above.  Most of the corn crop that I’ve observed has been at dent or a little further along.  The low levels of foliar disease present at this time of the season should not be considered to be a threat to yield since it would require several weeks for infected leaf tissue to impact yield.

Southern rust update

Southerm corn rust. Notice the bright orange sporulation present in erupted pustules (courtesy of B. Spinks).

Low levels of southern corn rust were detected in a commercial corn field east of Alligator, MS by a consultant yesterday morning (July 7, 2011).  Southern rust was detected on two leaves (see photo).  The field had reached the dent stage and the starch line had progressed approximately 5-10%.  At present this would be the only southern rust that has been detected in MS and does NOT mean that a fungicide application is necessary.  Most corn has reached dent and previous research on fungicide applications has been conducted in situations where southern rust was detected at substantial levels. Essentially, a fungicide application made in the absence of disease at dent will NOT pay for itself and thus would not result in a yield increase.  Moreover, I’ve received a few calls suggesting that the “flecking” present on corn leaves in the upper part of the canopy is indicative of a rust infection that has yet to produce pustules.  There are many situations that can produce flecking and this should not be considered to be early symptoms of southern rust.  The commercially available hybrids planted in MS do not contain the Rpp genes that would be required to result in this type of flecking in response to southern rust infection.   

Flecking on a corn leaf is not indicative of a disease that has yet to produce symptoms.

Conversely to the first southern rust detected in 2010 we are essentially 4 to 5 weeks later in the season this year.  Corn is much further along this year than it was in 2010 and in most situations we will likely outrun any yield loss that could be attributed to southern rust.  If you need help diagnosing the particular rust on corn plants please feel free to call.  Remember, common rust has not been reported to be a yield reducer in MS.

To stay up to date on the southern corn rust situation and follow not only current suggestions but locations where the disease has been identified go to:

http://sba.ipmpipe.org/cgi-bin/sbr/public.cgi?host=Corn&pest=southern_corn_rust

The web page is formatted similarly to the soybean rust monitoring situation.  However, pay careful attention to the colors on the map as there are more colors present since races of the fungus have been determined to be resistant to the Rpp 9 gene which codes for resistance and is only present in one hybrid that is commercially available (Pioneer 33M52).  This hybrid is not typically planted in MS but a close susceptible relative, 33M57, has been planted in MS in the past.

Foliar disease versus sun scald and moisture stress

I still continue to receive calls regarding sun scald.  Sun scald can and will occur in irrigated fields just as it will in non-irrigated fields.  In situations where the lesions have been present for a period of time they can become infected by secondary fungi.  The secondary fungi will typically be black and will easily rub off on your clothing as you walk through the field.  Oftentimes the lesions that occur from sun scald can be confused with many other foliar diseases.  Keep in mind that lesion size may vary depending upon corn variety or the length of time between when the symptoms first occurred and were observed.  In some cases the lesions can be long, extending more than 4 inches and oftentimes can begin at the leaf margin or leaf tip and extend back towards the plant (but not always).  Few diseases in MS can cause lesions of that length, especially in situations where corn has not generally been a continuous crop.  In most cases, the photos present in diagnostic guides don’t include a lesion length or scale for comparison. 

Moisture stress and sun scald on corn.

At this time of year I regularly hear that a disease is “firing the corn plant up”.  With the hot and dry conditions we’ve experienced this summer there has been little foliar disease present.  More often than not corn that takes on this appearance is suffering due to lack of moisture regardless of the irrigation situation.  While disease will typically begin on the lowest leaves and move up the plant, especially in situations where corn has followed corn in a field, moisture stress or sun scald will generally begin at the top of the plant and move down.

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Tom Allen, Extension Plant Pathologist
By Tom Allen, Extension Plant Pathologist July 8, 2011 07:04
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