Corn Disease Update: July 12, 2020

The corn disease situation throughout MS continues to change as we get further into the season.  Increased moisture and more moderate than normal temperatures (up until 7/11/20) have resulted in an increase in the prevalence of foliar diseases throughout our corn crop.  However, for the most part the disease situation remains low for the most part.  Corn growth stages vary widely throughout the state with corn between southwest MS and the south Delta well into the dent stages and some corn around the middle of the Delta that was late planted

Common rust in the lower canopy can appear similar to southern rust. The more orange/brown coloration of the sporulation erupting through pustules is one key diagnostic feature between the two diseases.

Common rust

In MS, common rust is not a disease of consequence.  However, common rust can cause a tremendous amount of confusion when the pustules caused by the fungus sporulate profusely in the lower canopy.  For the past several years, and more so as a result of the “cooler than normal” temperatures for extended periods of time, common rust has continued to be observed in our corn crop a little longer than normal.  Common rust tends to occur throughout the canopy (from the bottom to almost the top of the plant) and in general the pustules are a little larger in size than southern rust pustules.  Moreover, the color of the sporulation that breaks through the leaf surface is darker orange, almost brown to maroon in color when compared with the sporulation that results from southern rust.  Common rust pustules tend to occur in groups as a result of infection that may have occurred prior the leaf expanding from the whorl.  But, with that in mind, not all common rust is observed with a “pattern” on the leaf.  Common rust pustules can also be observed to occur on the top and the bottom (underside) of the leaf throughout the canopy.  Common rust that occurs in the lower canopy, below the ear leaf, can be confused with southern rust because the pustules appear smaller than normal and the coloration of the sporulation is generally lighter than normal.  If you have questions or need help properly diagnosing them please call, I would be more than happy to help.

Curvularia leaf spot lesions can be observed throughout the corn canopy depending on the time of the season. Lesions are most commonly observed around tasseling states.

Curvularia leaf spot

Probably one of the most common diseases of corn throughout the MS production system over the past several years.  Expression of lesions can be confused with lots of other disease issues and over the past several years has been improperly diagnosed as eyespot.  Eyespot is not a disease that occurs in MS, likely since it gets so warm here.  Curvularia is generally observed in the upper most canopy when looking up into the canopy.  The small, sometimes pinpoint lesions have an almost “fish-eye” appearance and abundance of the lesions tends to differ between hybrids.  In most cases, lesions are small, much less than 1/8 th of an inch, but can coalesce on leaves.  Lesions tend to occur from the ear leaf up into the canopy and appear to be more aesthetic than anything else.  However, in years when the disease is especially widespread, the disease may appear to be a big problem.  I suspect that in field situations with reduced tillage practices as well as continuous corn there may be more widespread Curvularia leaf spot.  But, I think Curvularia leaf spot tends to follow rainfall patterns around the initiation of reproductive growth stages.  For information on the response of hybrids in our production system to Curvularia leaf spot from the 2019 OHT trials see:

Non-irrigated: https://www.mississippi-crops.com/2019/11/15/2019-msu-corn-hybrid-trial-disease-evaluations/

Irrigated: https://www.mississippi-crops.com/2019/12/04/2019-msu-corn-hybrid-trial-disease-evaluations-irrigated-locations/

Diplodia leaf streak lesions can form a concentric ring patter within the lesion and generally have a stark yellow halo. In addition, the presence of pycnidia (small, dark spots within the lesion) are a major difference between Diplodia leaf streak and NCLB.

Diplodia leaf streak

Becoming more common in our production system, Diplodia leaf streak can be most commonly confused with northern corn leaf blight (NCLB).  There are several major differences between NCLB and Diplodia leaf streak.  Lesions of Diplodia leaf streak can form in almost a band or pattern across the leaf.  The pattern tends to follow the expansion of the leaf from the whorl and should not be considered to be a true “pattern” associated with the disease.  The lesions tend to form along an area where infection occurred.  In addition to being confused with NCLB, Diplodia leaf streak can also be confused with the burn associated with foliar nutrient applications.  However, in most cases those applications tend to produce injury on the leaf margins.  The specific symptoms associated Diplodia leaf streak can be observed as lesions being surrounded by a stark yellow halo and in some cases the lesion will contain concentric rings.  In general, NCLB lesions do not have the stark yellow halo and for the most part lesions don’t have such a concentric pattern (but, I do realize there are exceptions to both of those situations as in some cases different genetics in our production system can result in some different symptoms).  In addition, the lesions associated with Diplodia leaf streak can be wider and are generally shorter than most NCLB lesions.  The one major difference that sets the two diseases apart tends to be the presence of a stark yellow halo in addition to the presence of pycnidia (dark structures within the lesions of Diplodia leaf streak).  In general, Diplodia leaf streak is another aesthetic disease and normally does not reach levels where it should concern corn farmers.  Rotation out of corn is likely the most beneficial management practice to reduce the likelihood of Diplodia leaf streak in subsequent seasons.

Physoderma brown spot is generally diagnosed by the presence of the brown lesions along the leaf midrib.

Physoderma brown spot

Over the past several weeks I have received a few phone calls related to Physoderma brown spot.  Physoderma brown spot has been a disease of limited incidence in our corn production system over the past several years, but certainly something that more have become acquainted with.  In most cases, Physoderma brown spot is a rare occurrence.  The name of the disease tends to describe the presence of the symptoms along the midrib (see photo at left).  The lesions that develop on either side of the midrib can be confused with rust, or even Curvular leaf spot.  In the recent past when I have encountered Physoderma brown spot the field has almost always been in corn for more than one season.  The fungus is reside-borne and the best way to prevent the disease from occurring in the future is to rotate.  Researchers in the mid-west, where Physoderma brown spot tends to be more problematic, suggest that fungicide applications are not beneficial on attempting to manage this particular disease.

Southern rust

Over the past several weeks, observations of southern rust have increased throughout southern MS (south of I-20, but in the western corner).  The bulk of the corn in the area where southern rust has been observed is at dent and beyond and out of the woods when it comes to a severe yield reduction.  Friday evening (7/10/2020) I received a text from a consultant with southern rust in Sharkey County.  That would mark the most northern progression of the disease this year.  However, corn in that particular situation was already at dent with the starch line beginning to move.  Moreover, I suspect there to be more southern rust observed over the next week to 10 days simply due to dry field conditions following the rainfall and the general progression of the disease through the corn crop.

Southern rust differs from common rust in the coloration presented by the sporulating lesions (see images).  Southern rust sporulation is lighter in color, the pustules appear smaller than common rust pustules and more often than not the pustules are present only on the upper leaf surface.  However, there are exceptions to that rule and diagnosing southern rust based on the presence of pustules on the top or bottom of the leaf is not the best method.  The best method to positively identify whether the rust on leaves is common or southern is with a microscope since the spores of common and southern rust are different in appearance.  In general, common rust prefers cooler temperatures, while southern rust can manage the higher temperatures, well into the low 100s based on research reports.

Southern rust of corn. Pustules appear smaller in size than common rust and the color of the sporulation erupting through the pustule is lighter (more light orange) than common rust.

The biggest question I get related to southern rust is whether or not to make a fungicide application.  Quite regularly, southern rust tends to appear towards the end of the season.  July 4 is our regular date of initial observation in MS.  I would say we were a little earlier than “normal” this year simply due to the general weather pattern and the hurricane that came through so much earlier this year.  With that in mind, keep an eye on the later-planted corn and judge fungicide application needs based on yield potential and whether or not southern rust is present.