Corn Disease Update for June 22, 2012: Southern Rust Detected in Localized Areas

Tom Allen, Extension Plant Pathologist
By Tom Allen, Extension Plant Pathologist June 23, 2012 08:45

“Plant Health”

From what I’ve heard it seems that a lot of farmers decided to make VT/R1 fungicide applications in the absence of disease.  The topic of “plant health”, or fungicide application in the absence of disease, appears to have been a cyclical one.  MSU conducted 3 years of research trials and determined that making a fungicide application in the absence of disease (regardless of product tested) was not economically or mathematically beneficial.  I realize that some believe the application of the fungicide keeps the plant greener for an extended period of time and/or lends itself to stronger stalks.  I still have not observed these particular characteristics and have had additional fungicide plots conducted since 2008 with numerous products.  Fungicides are excellent products when used in the correct environment, generally when disease threatens yield.  At this point in my career I have not seen data that leads me to believe the response of corn to a fungicide would benefit in reducing drought stress, keeping the leaves on the plant greener for an extended period of time, increasing nutrient uptake, reducing the likelihood of lodging, preventing green snap, or increasing yield on a consistent basis in the absence of observable disease.  At the end of this season I am hopeful to have quite a bit more information on some of the above topics.

Specific fungicide timing

Put quite simply, there is no particular fungicide application timing in corn.  In my opinion, the best timing would be when a disease has been identified in the field that might result in a yield reduction.  However, over the past two growing seasons environmental conditions have tended to be hot and dry and little if any yield loss resulted from disease except for a few isolated cases.  Hot and dry conditions are typically not conducive for the development of many foliar diseases.  The one exception is southern corn rust (see below).  The southern corn rust fungus prefers situations where extended periods of hot and dry weather occur.  However, the corn crop is much more advanced this season and we will likely escape with little to no yield loss as a result of the disease we have had in most corn fields.

Adjuvant burn

Urea burn or other foliarly applied nutrients can burn the leaf and develop the appearance of some foliar disease lesions. Leaf tearing in a pattern is one diagnostic characteristic of the nutrient burn as opposed to disease.

Over the past few weeks I’ve had several field calls regarding “adjuvant” burn in corn that was treated with a fungicide and a product that was sold as an “adjuvant”.  In two of the cases the product was comprised of nitrogen that when applied produced a burn-like symptom similar to some foliar diseases.  Typically, 2 weeks following some of the foliar applications is required for symptom expression.  In some cases adjuvant burn can appear similar to some plant diseases, particularly gray leaf spot and northern corn leaf blight.  Lesions can, in some cases, become invaded by secondary organisms that will produce dark fungal growth in the center of the lesion.  But, most of the secondary organisms will produce round, pepper-type specks in the lesion that are not indicative of GLS or NCLB.  Moreover, rarely will a disease infect an entire field in 7-14 days especially since over the past 2 weeks we’ve had a limited number of days with cloud cover and while we’ve received tremendous rainfall in some parts of the state the typical environment has continued to be hot and dry.  Keep in mind that applying some products over the top of corn during the hottest part of the day can tend to increase the likelihood of a burn on the leaf.  Also, adjuvant burn will tend to start in the top of the plant whereas most of the diseases we deal with, particularly those in situations where corn has been cropped continuously in a particular field, will start lower in the canopy and move up the plant.

Sun scald

Note the circled areas where sun scald will develop the appearance of some northern corn leaf blight lesions.

Over the past week I’ve started to observe sun scald in numerous corn fields.  Sun scald can be the result of drought, reduced irrigation, irrigation in a skip-row pattern that hasn’t wicked across the middles, or can be much more prominent in non-irrigated situations.  Keep in mind, the lesions that are a result of sun scald can be misidentified as disease since they will be elongated and can favor northern corn leaf blight.  In addition, if saprophytic (secondary) fungi grow in the lesion a disease can be misdiagnosed as the culprit.  Sun scald can in severe cases takes on a “pattern” on the leaf and/or tends to start at the leaf tip and move back towards the stalk.  Over the years I have heard numerous individuals refer to this particular situation as “firing up the plant”.  In some rare cases a large area will have sun scald and the plants within the area will all appear to have died.  Typically a large area in a field is the result of a soil characteristic at that location and in most cases is a sandier area.  One important thing to observe: more often than not disease will start in the lower canopy and move up the plant (but not always) and sun scald will typically begin in what appears to be the top of the plant and move down.

Northern corn leaf blight (NCLB)

Typical NCLB lesion shown with a pen to provide scale. Normally lesions will measure from 1 to 3 inches in length with the length greater than the overall width of the lesion.

Little if any NCLB has been observed this season.  In some of the locations with continuous corn, where I would expect to see a good bit of NCLB I haven’t seen a single lesion.  The majority of the corn crop, save for a few isolated situations, has likely reached dent and can be considered “out of the woods” from yield loss that could be attributed to NCLB.  In addition, hot and dry environmental conditions are not a conducive environment for NCLB to thrive.  In certain situations, as corn ages and gets closer to physiological maturity I would not be surprised to see more NCLB regardless of the environment.  Keep in mind that several of the other issues mentioned above can also develop the appearance of NCLB, namely, elongated cigar-shaped lesions.  However, in most cases, NCLB lesions will not have a pattern associated with them and can occur anywhere in the plant canopy.  More often than not, the disease will first be detected below the ear leaf, but since the fungus can be airborne it is possible to first detect the disease on the ear leaf or above.  While rating plots in Stoneville this week I was only able to detect a total of 4 NCLB lesions in my plots.

Gray leaf spot (GLS)

I am still picking up a lot of GLS in isolated areas.  I think the greatest concentration is in an area between Satartia and Minter City.  For whatever reason, this area tends to be a little bit more humid throughout the summer.  In some cases GLS has been observed in first year corn fields.  Prior to 2012, GLS had almost always been observed in fields with a history of corn production.  While I realize that some fields have already been sprayed with a fungicide, fields with the disease present in the lower canopy that are approaching (or have reached dent) would not benefit economically from a fungicide application.  In addition, a lot of corn hybrids have the ability to keep the disease lower in the canopy.  Remember, the lowest leaves on the corn plant are shaded from the sun, nutritionally deprived at this point in the season, and while they can serve as a source of inoculum unless you are observing the disease (and I don’t mean 1 or 2 lesions) on the ear leaf or above then I would not suggest a fungicide be applied.  Stalk integrity will only become an issue if the disease infected the corn plants prior to tassel.  The most typical GLS lesions are 0.5 to 1.5 inches in length, rectangular, don’t cross the veins, and can have a yellow halo around them when a young lesion occurs.

Southern corn rust

Southern corn rust, note the smaller lesion size when compared to common rust and the overall color of the sporulation that erupts through the leaf.

On Wednesday (6/20/12), Trent LaMastus detected a small amount of southern rust in a corn field in Sharkey County, north of Holly Bluff.  Typically, we don’t encounter much southern rust until July.  However, since 2010 we’ve detected the disease in June.  But, unlike 2010, our corn crop is much more advanced than it was that year.  The majority of corn fields have reached the milk stage (R4), some have reached dent (R5), and only a few at this point are somewhere between VT and R1.  At this point I am quite doubtful that a fungicide will need to be applied to manage southern corn rust but if you have a particular situation where you are concerned about your corn and/or what particular rust you are observing pick up the phone and call (662-402-9995).  Depending on how much rust you are observing I can normally (but not always) tell the rust based on a picture.

Even though southern rust can be aggressive keep in mind that there are field situations where it can be easily confused with common rust.  I scouted around the Holly Bluff area yesterday (6/22/12) afternoon and found fields with differing levels of common rust that in some cases looked quite similar to southern rust.  Some fields had almost no common rust and a few others had common rust that had moved up to almost the ear leaf.  The best diagnostic characteristic to distinguish the two diseases from one another is the presence of pustules on the bottom of the leaf.  Common rust will produce pustules on BOTH sides of the leaf.  Remember, abnormal common pustules in the lower canopy can produce sporulation that will appear lighter in color.  See the attached photo for some help distinguishing the two diseases based on the size of pustules and general color of the sporulation that erupts through the leaf surface.

Common rust (left) compared to southern rust (right). Note difference in color of the sporulation as well as the overall size and shape of the pustules associated with each disease.

During 2010 one of the most common questions regarding southern rust had to do with maintaining stalk integrity.  Generally speaking, stalk integrity is only an issue when a disease infected the plant prior to tassel and had a long period of time to rob the plant of nutrients, degrade the leaves, and stalk cannibalism ended up in an issue where the stalk lost integrity and lodging resulted.  At this stage, I do not foresee an issue where stalk integrity will be degraded as a result of a southern rust infection.

To keep up to date on the southern corn rust issue and observe where the disease has been identified based on county level resolution (much like with soybean rust) log onto:

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Tom Allen, Extension Plant Pathologist
By Tom Allen, Extension Plant Pathologist June 23, 2012 08:45
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