Corn Diseases to Watch for While Scouting Corn

Tom Allen, Extension Plant Pathologist
By Tom Allen, Extension Plant Pathologist May 25, 2012 11:58

Corn Diseases to Watch for While Scouting Corn

Scouting corn for disease can be difficult.  Unlike most of the other crops we are accustomed to farming in MS you can’t simply look out over a field of corn, notice an area that has an unfamiliar appearance, and check the area for the presence of disease.  You have to walk numerous rows of corn and get an idea as to the presence of disease.  Typically, when I walk corn fields prior to tassel and/or ear emergence I observe all of the exposed leaves in the corn canopy for the presence of foliar disease.  Once the ear is emerged, and generally around the time brown silk (R2) occurs, I’m focused on the ear leaf and above to the top of the canopy.  Much like wheat, protecting the ear leaf from foliar disease is the most important step.  Keep in mind, once disease occurs a fungicide can still be applied and make an economical difference (see: http://www.mississippi-crops.com/2011/05/29/the-corn-fungicide-dilemma-when-should-a-fungicide-be-applied-part-v-of-v-preventing-yield-loss-from-foliar-disease/).  Even though the data attributed to the particular trial contained is from a single year, fungicides can still make a tremendous difference if and when disease manifests itself in a corn field.  The most important factor is application timing.  The most economically beneficial fungicide timing is one whereby yield loss is prevented and the cost of the fungicide plus application is exceeded.  However, at present the best application timing in the presence of disease to make an economic benefit is not completely known especially since the application timing may differ depending on the particular disease present and amount of disease encountered in a particular field.  The best timing once a disease has occurred and whereby the fungicide will provide an economical return was previously thought to be 2 weeks prior to black layer.  However, the particular statement regarding 2 weeks prior to black layer was based on older fungicide chemistries and the work that was conducted did not include the newer classes of fungicides.  Ideally, more work is necessary to determine if that has changed.

 

To give you an idea regarding the specific diseases that I focus on when scouting corn there are several descriptions listed below along with photographs.  At present I have only encountered common rust in our corn crop (as of 8 am on 5/25/12), but I expect this may change at any time.  Keep in mind, that at the field level it can be difficult to determine which specific disease is present especially regarding the rusts.  If you find a leaf sample and are in question as to the particular rust present please feel free to call.  Oftentimes I receive a call after the leaf has been discarded and I end up spending hours walking a field following the telephone call only to determine another leaf with the disease that was observed.  When in doubt, don’t discard the leaf.

Common rust on corn leaf. Early in the season, in some rare cases, excessive sporulation and a greater number of common rust pustules can make one think they've encountered southern rust. Note the darker spore color erupting through the pustule.

Common rust – At present, common rust is the only rust that has been detected in the MS corn crop.  Over the past couple of weeks I’ve encountered common rust that has appeared quite similar to southern rust.  When common rust covers a large area (or larger than what is more generally observed) of the affected leaf oftentimes the first reaction is to call it southern rust simply due to the sheer quantity of rust on the leaf.  However, considering the coloration of the spore mass contained within the pustule as well as the location on the leaf will generally allow you to determine which particular rust is present.  More often than not, the most prominent feature of common rust is the location of the lesions on the leaf.  Regularly the lesions will be present closer to the base of the leaf (up against the stalk) due to the location of the leaf in the whorl at the time of infection.  Common rust spores are more cinnamon in color, thus darker than those associated with southern rust.  But, and I can’t stress this part enough, if excessive rust pustules are present on juvenile corn leaves (e.g. in the very bottom of the canopy, generally on the first true leaves) common rust can look just like southern rust and be incredibly difficult to distinguish at the field level.  Also, common rust can occur on BOTH leaf surfaces so pustules will appear to rupture through the top and the bottom of the leaf.  Keep a sample if you find a leaf and are in question as to the particular rust present.  From a management perspective, in the 5 years I’ve been in MS I have not seen a common rust situation whereby a fungicide was necessary or would have benefited the farmer.

 

Southern corn rust. Note the presence of lighter colored spores erupting through the lesion on the leaf surface.

Southern rust– Southern rust is the more damaging of the two rusts encountered in our production system.  Typically, but not always, southern rust is more plentiful on the leaf surface and will almost always be present only on the upper leaf surface.  Historically speaking, southern rust is typically more of an issue later in the season.  However, in 2010 we experienced a severe epidemic that was first identified in June so it is not out of the question to find southern rust early in the season.  At present we have only just begun to encounter environmental conditions that would be considered conducive for the development of southern rust (temperatures in the 90s).  Remember, the rusts present in our corn system have to blow into MSfrom somewhere else they do not have the ability to overwinter in MS.  Diagnostically, southern rust lesions appear to be smaller than those caused by common rust and the spores erupting from the pustules are a lighter orange in color (think Tennessee Volunteer orange versus Auburn orange).  However, as I’ve stated in the past it can be difficult to determine the particular rust present if you only have ONE of the diseases to observe.  I maintain a leaf sample in the lab specifically to make comparisons between the spore shape and size early in the season to verify the presence of southern rust over common.  Fungicides are the best management alternative in the event they are considered necessary.  The infection can be slowed down by a carefully timed fungicide application at the right growth stage and the application can be economically beneficial.  I continue to scout corn to detect southern rust but at present have not observed any of the disease in our corn crop.  At present, only one commercially available corn hybrid is resistant to southern corn rust and we do not have the particular hybrid planted in MS.

Northern corn leaf blight. Note the darker center where sporulation has started to occur.

Northern corn leaf blight (NCLB) – NCLB produces long, cigar-shaped lesions that in general can be up to 3 inches in length.  The lesions can be confused with urea burn or other associated injury or even drought stress.  One of the more common characteristics of NCLB lesions is the dark, sporulating fungi present within the lesion itself.  However, dark fungal growth can occur as a secondary issue in response to other injury on the developing corn leaf.  Having a hand lens (× 20 will generally be sufficient) can greatly aid in distinguishing whether or not you have NCLB or another, secondary fungus present on an injured corn leaf.  The spore produced by the fungus that causes NCLB itself will look like a cigar sticking out of the top of a pole.  The spore will have several septations that while not easily observed with a hand lens may help in identifying what it is you have in the field.  In addition, one other disease can produce a lesion similar to NCLB and that is Diplodia leaf streak.  However, I’ve only encountered the disease one time in the years that I’ve been in MS, north of Water Valley, MS, and I would suggest that Diplodia leaf streak should only be an issue in fields with a continuous history of corn production.  From a management perspective I have only suggested one fungicide application to a field as a result of NCLB detection.  In most cases, the presence of NCLB will be more apparent in a field situation with:

-continuous corn production since the fungus overwinters on corn stubble

-on a hybrid with a weak disease package

However, as with most foliar diseases, if we experience some wind driven rain the disease can be observed in fields that have not had corn production simply because the fungus can be airborne and travel great distances.  Keep in mind, that many of the photos contained in the available diagnostic guides show the best example of NCLB lesions but lesion size, shape, coloration, and appearance will differ depending on the particular genetics of the corn hybrid present.  Presently, we are not experiencing what could be considered conducive weather for the development of NCLB.  However, over the last month and a half I have observed some NCLB lesions on Johnsongrass which can also be infected by the fungus.  But, I would not consider that to be a good diagnostic guide to check Johnsongrass on the field edge since much like corn there are numerous issues that can produce a similar lesion type, the least of which can be a nutritional deficiency.

Gray leaf spot lesions. Not elongated lesions and not the necrotic areas along the vein. Keep in mind in situations where either several diseases are occurring at one time or corn is non-irrigated there are situations where sun scald or leaf burn can look similar to GLS.

Gray leaf spot (GLS) – I hesitate to spend too much time talking about gray leaf spot (GLS), but it is becoming more prevalent in our production system and can be devastating in a conducive environment.  Historically, GLS has been more of a problem in the Ohio River Valley; however, increased corn acreage in MS and the Mid-south has increased the levels of disease we encounter on an annual basis.  In my opinion, GLS is a greater concern in situations where either continuous corn has been common and/or continuous corn has been coupled with minimum or no-till.  Most of the GLS that I’ve encountered in MS has been in situations where a tremendous amount of stubble has been left in the field following several years of continuous corn.  The lesions associated with GLS will be elongated, narrow, tan to brown in color, not cross the veins, and can measure anywhere from 1/3 of an inch to 2 ½ inches in length. One of the most diagnostic characteristics of the disease can be observed on the underside of the leaf.  Sporulation, and the structures that produce spores (conidiophores) are generally produced on the underside of the leaf.  With a hand lens, look for the presence of dark structures in a row on the underside of the leaf.  The fungus that causes GLS infects the plant through the natural openings on the leaf at the stomates.  While a × 20 lens is generally enough to see the other fungal structures listed above, I would suggest a × 30 lens to look for the structures characteristic of GLS.  Much like NCLB, GLS can be identified in fields that have not had a history of corn simply due to spores moving on air currents.  However, I would spend a good bit more time scouting for GLS in fields that have previously had corn and may have some sort of reduced tillage.  Some hybrids planted in MS have decent tolerance to GLS; however, in situations where continuous corn and no-till are the predominant management scheme a hybrid with a decent disease package may not respond quite as well in situations where stress or 5 or 6 years of corn have been planted.  In addition, depending upon the particular hybrid planted there are certain situations where a limited amount of GLS will move up above the ear leaf but may have more disease in the lower canopy simply due to the loss of nutrients in the lowest leaves as the season progresses.  In trials conducted last year, fungicides (strobilurin and strobilurin + triazole) were applied at approximately dent and did not produce an economical response on several different corn hybrids.  However, quite a bit of disease was already present and it is likely the fungicide was applied too late.  Additional trials are planned for this season at the same location where GLS has been the predominant disease for at least 3 years.

Lesion mimic (mutant) producing a GLS-type lesion.

Lesion mimics (or mutants) – One of the things I’ve not mentioned previously is the presence of lesion mimics.  In some cases, and notably depending upon the particular genetics within a given corn hybrid, corn can sustain a physiological response to the environment that will produce a lesion similar to some of the diseases encountered in our production system.  The lesion mimics themselves are not a disease but are likely oftentimes confused with a true disease causing organism.  Over 50 different mimics have been identified in maize (as of 2007).  In fact, there is an entire book devoted to the topic.  The book, “Mutants of Maize”, contains 468 pages on the topic of mimics and contains the known mutants attributed to this particular phenomenon.  If you want to read something shorter on the particular topic, I refer you to a short article that can be downloaded from: http://www.apsnet.org/publications/apsnetfeatures/Pages/MutantsofMaize.aspx

Oftentimes, when dealing with plant diseases we cannot put our finger on the particular causal agent of a leaf spot or lesion on a particular corn hybrid.  The genetic composition of the corn plant can make this difficult especially since, for example, in some cases a lesion mimic can occur whereby a GLS-type lesion is produced (see photo).  As stated in the linked article, the production of a lesion mimic can result from something as simple as the response of the corn plant to light or temperature.  I have simply included this section on mimics to provide you with a basis for how confusing corn disease diagnoses can be in certain situations.

 

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Tom Allen, Extension Plant Pathologist
By Tom Allen, Extension Plant Pathologist May 25, 2012 11:58
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