Early Season Corn Diseases and Non-disease Observations

Prior to Thursday (May 23, 2013) I had not been in vegetative stage corn scouting for the more “normal” foliar diseases.  In the six years I had worked for MSU I normally don’t get field calls regarding foliar corn diseases until after corn has tassled.

Before making a decision to apply a fungicide to diseased corn, or any row crop for that matter, a proper disease diagnosis is important.  Diagnosing corn foliar diseases can be confusing.  Several foliar diseases produce similar symptoms and in rare cases genetic abnormalities in a low number of plants can produce a symptom similar to important diseases.  I realize that using a pictorial guide to diagnose foliar diseases is sometimes one of the only options available.  However, in a lot of cases, the photos that are present in those guides don’t match the symptoms in the field (regardless of the disease).  In addition, some specific hybrids can respond differently to a specific pathogen.  For example, some diagnostic guides include photos of a gray leaf spot reaction indicative of the reaction that would be encountered on a resistant plant.  Most of the photos available are from inoculated trials.  Therefore, an excessive amount of inoculum has been sprayed onto a plant.

Since we have had atypical environmental conditions and in general a late-planted corn crop we could observe some foliar diseases that we don’t regularly observe.  Please keep this in mind as you are scouting corn fields.  If you need help diagnosing an unfamiliar disease please don’t hesitate to call.

Common rust is one of the most common diseases of corn. Early season rust is typically “common rust” whereas higher temperatures can be more closely associated southern corn rust.

Common rust

Common rust prefers cooler temperatures (61-77F) and high humidity (> 95%).  Even though common rust is present on all fungicides labeled for application to corn, rarely has the disease resulted in a yield reduction in the MS crop production system that would have necessitated a fungicide application.  One commonly confusing symptom of common rust will be the appearance of pustules on the lower most leaves of the corn plant when the plant reaches reproductive stages.  In those situations the pustules can appear smaller, more aggregated, and the color of the sporulation in the pustule itself will tend to be lighter in color.  Don’t confuse common rust with southern rust.  Southern rust prefers higher temperatures (77-80F; but can tolerate much higher temperatures up to and including > 100F) coupled with high humidity (> 95%), something we have not experienced to date this season.  Presently, no southern rust has been reported in the continental U.S.

Remember that common rust, if compared with southern rust, will produce larger pustules and in most cases the leaf tissue surrounding the pustule will appear to have a more shredded appearance.  In addition, the sporulation attributed to common rust appears more maroon or russet in color.

Northern corn leaf blight (NCLB)

Generally, NCLB is more of an issue in fields with a history of continuous corn production.  But, with that said, there can be situations where the disease can occur in first year corn fields if the environment has been conducive enough for infection to occur.  However, this year in particular, we’ve had extended periods of rainfall and cool temperatures that have increased the likelihood of finding NCLB earlier in the season.

Northern corn leaf blight lesion. Note darker area where sporulation occurs along the center of the lesion.

Lesions are generally shaped like a cigar and can range in size from an inch to more than two inches in length.  Sporulation generally occurs in the oldest parts of the lesion and can be observed with a 20x hand lens quite readily.  The spores are pretty distinctive, are generally gray to black, and when observed through a hand lens look like an elongated football on top of a tooth pick.

Similar to the comment above regarding listings on a fungicide label, NCLB is also a disease that appears on most of the fungicides we rely on for yield loss prevention in corn.  But, rarely has NCLB been a disease that has caused much yield loss in MS and in the years I have been here I have only observed a single field that likely would have benefited from a fungicide application.  Determine a specific hybrids disease package prior making the decision to apply a fungicide since there are a few hybrids that are more susceptible to NCLB than most of the other commercially available hybrids.  In addition, many lesions on a single leaf will be necessary for the disease to impact photosynthesis.  Keep in mind that when considering the overall size and length of a corn leaf that a single NCLB lesion likely represents less than 1% of the leaf surface area.  Generally speaking more than 15-20% of the leaf surface area covered with NCLB lesions might correlate with yield losses.

Scout those fields with a history of corn production more intensively for the presence of NCLB.

Bacterial leaf streak symptoms observed in a corn field in the south Delta.

Bacterial leaf streak

Prior to Thursday (May 23) I had not observed this disease in MS.  But, again, atypical year may mean we’ll see some atypical disease issues throughout the season.  The symptoms of bacterial leaf streak can appear similar to numerous fungal diseases that have not been regularly observed in MS that include: eyespot, northern corn leaf spot, Physoderma brown spot, yellow leaf blight, as well as some fungal diseases such as gray leaf spot that require several weeks to go through sporulation, infection, and lesion development.

Determining whether or not a bacterium or fungus is involved in the foliar disease symptoms requires a good microscope and some serious patience.  Looking for sporulation or the spore that may have caused the lesions to develop can require a good bit of time.  With the help of a colleague and some serious time invested we were able to observe bacterial ooze from some of the lesions which suggests a bacterial disease rather than a fungal disease.  Keep in mind, diagnosing a bacterial disease means that management practices will not be necessary.  A fungicide will not be a beneficial expense when a bacterial disease is diagnosed.

The particular field where this disease was observed had one prior year of corn production.  Since the corn is still in vegetative growth stages (approximately V7/V8) yield loss will not result from the bacterial disease.

Striping in a single corn plant is likely the result of a genetic abnormality in the plant most often referred to as “genetic stripe”.

Non-issues

Throughout the years I’ve heard numerous reports of Goss’s wilt, Stewart’s wilt, and numerous other disease issues in corn appearing in the MS crop production area.  To date, we have not observed these two important bacterial diseases in MS.  In most cases, drought stress or even sun scald are more likely symptoms that can be readily confused with Goss’s wilt.  Regardless of the appearance of the disease in the field it is oftentimes difficult to determine the specific disease, or whether or not the particular issue is in fact a disease, without the proper diagnosis.  In some cases, the proper diagnosis can require a laboratory as well as a good high-powered microscope.  Pictorial guides are available but should not be used to diagnose some particular issues.  Foliar diseases can appear to have different symptoms than the photos presented in corn disease guides depending on the genetics of the particular hybrid planted.

In the past I’ve mentioned a book called “Mutants of Maize”.  The subject of the almost 500 page book describes and presents all of the genetic abnormalities that can produce disease mimics as well as result from a single plant having a genetic abnormality that can produce a fairly typical symptom.  One of the genetic abnormalities presented in the book has to do with leaf striping.  The leaf striping symptom can be the result of no less than 7 genetic abnormalities in the corn plant.  If you see striping in a single corn plant it is likely the result of a genetic issue in the plant and should not be diagnosed as Stewart’s wilt, Aspergillus seedling blight, or a corn virus.  Plant pathology closely resembles the English language since there are tremendous exceptions.  Generally speaking, the simplest answer is correct……except when it comes to plant diseases.