Corn Disease Update: June 26, 2018

Tom Allen, Extension Plant Pathologist
By Tom Allen, Extension Plant Pathologist June 26, 2018 18:28

Corn Disease Update: June 26, 2018

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A little longer post than I would like, but things appear to be heating up in the presence of foliar corn diseases in our production system over the past 10 days. Some of the diseases outlined below are becoming more common in the MS corn production system.  Multiple calls have been received regarding Curvularia leaf spot in our production system.  In my brief career, I have only observed Curvularia leaf spot in three or four of the last 11 seasons.  One year I was able to make observations following fungicide applications (2016).  Fungicides were not effective at reducing the symptoms associated with this particular leaf spot.

As a general statement and more with regards to scouting corn for diseases, try to focus on lesions on the ear leaf and above. Lesions on the bottom two leaves of the plant are common once corn sets an ear.  In general, the lesions on the bottom two leaves are not going to be an issue since corn moves so quickly once the plant reaches tassel stages.

Common rust of corn presents pustules that produce a darker colored sporulation. Be mindful this may appear different in the lowest parts of the corn canopy.

Common rust

To this point in the season, common rust would be the main rust encountered in corn fields. So far, I have observed some photos of common rust, and only been able to detect the disease in one corn field near Grace, MS.  To date, southern rust has not been observed in ANY of the southern states.  Be mindful, that common rust in the lower canopy can appear quite similar to southern rust.  Check before you make a fungicide application to manage common rust.  Common rust is not a disease with yield-limiting capabilities.  The warmer temperatures will likely shut common rust down so let the environment manage the disease.

In general, common rust will produce larger pustules than southern rust. The sporulation that comes out of the pustules is normally darker in color and tends to be dark orange.  However, and this is where the confusion can occur as a result of this disease, sporulation from pustules in the lowest part of the canopy (on leaves below the ear leaf) can appear lighter in color.  The best way to determine the specific rust involved is to either submit a sample to the diagnostic laboratory in Starkville or call me to come and look.  Verifying spore shape and size under high power magnification is one of the best ways to confirm the specific rust fungus involved.

Curvularia leaf spot

Curvularia leaf spot can be commonly observed when looking up into the corn canopy this season on leaves in the upper canopy.

Lesions mostly in the upper canopy with gray centers and maroon margins. Normally the area on a leaf infected will be an inch in diameter with several small lesions present.  However, in some rare cases the area infected can tend to the majority of a leaf or two.  The lesions as a result of Curvularia leaf spot are caused with excessive dew or sporadic rainfall.  Over the past five seasons I have observed an increase in this particular disease, but the disease does not occur every year.  During 2015 I evaluated corn plots that were sprayed with fungicides since that season there were so many questions about Curvularia leaf spot.  Essentially, fungicides were not effective at managing this particular disease, but with that statement the disease did not dramatically increase.  In general, what you see now is all you will get with this particular disease.  I suspect that some hybrids are more sensitive to this fungus than others.  However, it is also likely that the disease is more of an issue in certain areas of fields where the environment is more conducive such as situations where the dew remains on plants or areas of fields are shaded for longer periods of time.

Diplodia leaf spot

Diplodia diseases in corn have become more common over the past several years. Stark yellow halo around these Diplodia leaf spot lesions.

Over the past several years I have observed an increase in the amount of Diplodia-associated diseases. Several diseases can be related to multiple species of Diplodia.  A leaf spot (see image) and a leaf streak are probably the two most common diseases observed in the MS corn production system.  I have only observed Diplodia leaf streak on a scattered number of acres in MS.  But, over the past several seasons, Diplodia leaf spot has become more common.

Lesions associated with Diplodia leaf spot generally occur in the middle of the canopy. The initial symptoms associated with the disease can appear prior to tasseling, but in general the symptoms I have observed over the years have occurred shortly after tasseling.  Lesions tond to be a half in to an inch in length, can have concentric rings within the lesion, and have a stark yellow halo around the edge of the lesion.  In some cases, the lesions can appear to have a pattern across a line on the leaf.  The fungus infects the plant when leaves are still in the whorl, so the pattern is as a result of time of infection.  Normally, the lesions stay an inch to an inch and a half.  Lesions that appear similar to these that can extend for 18 inches or more tend to be the leaf streak phase.

Gray leaf spot forms parallel lesions that generally range from a half inch to a little longer than an inch.

Gray leaf spot

Over the past 7-10 days I have observed several fields with gray leaf spot symptoms and received numerous telephone calls regarding the presence of gray leaf spot. Normally, and more often than not, gray leaf spot occurs in fields of continuous corn.  However, in some rare situations where the environment remains conducive for extended periods of time gray leaf spot may occur in first year corn fields.

In my experience, hybrids with a good gray leaf spot package will not allow the disease to get much above the ear leaf. However, this season in particular there may be some hybrids with a weak gray leaf spot package.  Check the particular disease rating on the hybrid in question.  Make a fungicide application based on:

-specific growth stage

-presence of the disease within the upper canopy (ear leaf and above)

-number of years in corn

-specific gray leaf spot resistance ratings

Gray leaf spot lesions are generally an inch or less in length, have parallel margins and do not cross the leaf veins. When held up to let light through them a faint yellow halo can oftentimes be observed.  One of the most diagnostic features are the presence of the fungal reproductive structures on the underside of the leaf.  These can be observed with a 20x hand lens and will look like little black pimples growing in a row.  The black pimples are the infection from the fungus into the stomates of the leaf.

Northern corn leaf blight.

Northern corn leaf blight

Following the 2017 season I suggest this has been one of the lightest NCLB years in my career. To date, I have observed four (yes, 4) NCLB lesions in two fields.  I think the environment has not been conducive to this point in the season for the development of NCLB (hot and dry).  In addition, and more commonly, I don’t see NCLB occurring in fields prior to tassel/R1.  But, with that statement in mind, I don’t see an issue from NCLB this season.  The majority of the hybrids planted in our production system have a pretty good disease package when it comes to NCLB.

Lesions as a result of NCLB tend to be ¾ of an inch to 1.5 inches. However, in some rare cases lesions can be longer than this on some particular hybrids (this is where NCLB can be a little confusing).  Be mindful that one lesion per leaf can be common and should not be considered a disease threshold for triggering a fungicide application.  Corn leaves are large and when you factor in the entire surface area, one lesion is much less than 1% of the plant.  In addition, this particular fungus does not seem to like high temperatures and will shut down during the day.  Over the past several seasons I have known individuals to draw margins around lesions with markers.  Lesion expansion may occur at night, but the period of a conducive environment is cut dramatically when temperatures hit the mid-90s during the day.

Presence of the two main symptoms associated with Physoderma brown spot: 1) the small yellow lesions in a band across the corn leaf and 2) the more regular purple to black oval lesions along the midrib.

Physoderma brown spot

I have hesitated to even discuss this disease because it has been so infrequently observed in MS. Prior to 2017, I had only observed the disease one time.  However, this year I have received multiple calls and questions concerning the presence of Physoderma brown spot in reproductive corn.  Even though the fungus associated with the disease can cause a stalk rot, we have not observed the stalk rot phase in MS.

Physoderma brown spot can manifest itself in two different forms. Firstly, small, yellow lesions can occur in a band across the corn leaf, generally in the middle of the canopy.  Be mindful that the small, yellow lesions on leaves can appear quite similar to spider mite injury (which is not a common occurrence in corn).  Secondly, and likely the most diagnostic feature associated with the disease tends to be the brown to black oval lesions that can occur along the midrib and also on the leaves wrapped around the corn stalk itself.

Hybrids likely differ in their sensitivity to Physoderma brown spot. However, the environment encountered in many of the fields where the disease has been observed, with frequent rainfall, has likely contributed to the occurrence of the disease.  Even though fungicides are labeled for the disease, no unbiased sources of data exist as to the specific growth stage timing to manage the disease nor whether or not the application will result in a yield benefit.  Honestly, Physoderma brown spot appears more as an aesthetic malady than a truly pathogen yield-limiting situation.

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Tom Allen, Extension Plant Pathologist
By Tom Allen, Extension Plant Pathologist June 26, 2018 18:28
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