Corn Foliar Disease Update: June 26, 2013

The majority of the corn crop in the greater part of the Delta has reached tasseling stage (VT) and some fields are beyond tasseling as of this morning (6/24/13; likely R1 to R2).  Sunday afternoon (6/23/2013) I observed corn fields north of Highway 82 and south of the Highway 49 turn at Lula, MS that would likely require several weeks before reaching VT.

Generally speaking, disease in the corn crop has been light.  But, with that in mind, remember when scouting, once the corn plant has set an ear focusing on the ear leaf and above is the best strategy to determine the potential yield loss that could result from a foliar disease.  Also, be aware that numerous fields received a pre-tassel nitrogen application.  Some of the injury/lesions that result from a pre-tassel nitrogen application can appear similar to some foliar diseases.

Typical common rust pustules exhibiting the “shredded” leaf tissue and russet color of sporulation.

Common rust

Common rust continues to be the only rust observed throughout the MS corn production area as of 6/25/2013.  I’ve been on the phone consistently with counterparts in LA and haven’t heard that southern rust has been identified/observed.  Currently, this is great news for MS corn farmers.  However, based on observing the general stage of the corn crop throughout the Delta (as of 6/23/2013) some corn fields that haven’t reached tassel yet could be at risk.  But, only time will tell.

Common rust, while a disease of corn, is generally a minor issue and does not reach treatable levels in corn.  Keep in mind that the disease will appear different than the “normal” pustules when observed on the lowest leaves in the corn canopy.  Southern corn rust can be the more devastating of the two corn rust diseases in the MS corn production system.

Northern corn leaf blight (NCLB)

Regardless of where you are in MS, NCLB appears to be a more common disease occurrence this season.  Some of this can be attributed to more continuous corn.  One other factor has likely been a cooler, wetter spring.  NCLB doesn’t do well during periods of extended high temperatures.  So, as temperatures climb throughout the remainder of June, NCLB will be less and less of a factor.  Lesions on ear leaves and above are the ones that could likely reduce yield.  However, rarely have I seen NCLB result in a situation that needed to be addressed with a fungicide.  But, susceptible hybrids should be scouted regularly especially if planted in a continuous corn situation.

Southern corn leaf blight lesions in the lower corn canopy.

Southern corn leaf blight (SCLB)

In the six years that I’ve been in MS I have only observed SCLB two other times.  Rarely is SCLB an issue in our production system.  However, in field situations where continuous corn have been planted and susceptible hybrids are planted the disease could be observed.  Two races of the fungus that cause SCLB occur; however, the less virulent race, Race O is not as virulent as Race T and therefore not as great a concern.  SCLB was last a major problem in the 1970s.  Since that time, corn breeders have done a marvelous job of breeding for tolerance to SCLB.

Lesions of SCLB can be confused with several other, more common diseases in our production system such as gray leaf spot (GLS) and NCLB.  Lesions, when most mature, can be elongated and be easily confused with gray leaf spot.  However, lesions are not typically as parallel as those caused by the GLS fungus.  But, SCLB lesions can cross the veins, whereas GLS lesions cannot.  Moreover, sporulation on GLS lesions occurs on the underside of the leaf and SCLB sporulation is similar to NCLB and is more clearly defined on the upper surface of the leaf within the lesion.  However, the three times I have observed the disease the lesions were clearly defined and generally concentrated in the lowest parts of the canopy where humidity was ample enough for the disease to occur.

Goss’s wilt of corn. Note water-soaking of leaf tissue towards the bottom of the lesions. Photo courtesy of LSU AgCenter.

Goss’s wilt

I want to address this particular situation in LA to attempt to head off a mass of telephone calls (see: http://agfax.com/2013/06/24/louisiana-corn-gosss-wilt-suspected-first-time-ever-in-state/ to read further information regarding the situation in LA).  Annually folks will walk fields in MS and think they have observed Goss’s wilt.  Historically, this particular disease has only been an issue in the upper Midwest (see map at http://www.iowafarmertoday.com/news/crop/goss-s-wilt-expands-reach/article_8c562a6c-d4ac-11e0-8559-001cc4c03286.html?mode=image&photo=0 for the historical range of Goss’s wilt).  However, in 2011, the disease expanded to some new parts of the country, moving a little east to Indiana and a little south and west to Texas.  But, to my knowledge, a positive sample has never been confirmed in MS.  Diagnostic photo books contain pictures of Goss’s wilt and if you are flipping through the book to make a diagnosis on the turn row and you have an irrigation issue (or a nutrient issue) in the corn field the photos of Goss’s wilt will appear to be quite similar.

Currently, we have not confirmed Goss’s wilt in MS, nor has a field been observed with suspicious symptoms that could be considered to be Goss’s wilt.  The situation in LA has been confined to Madison Parish, LA, and based on the information I have received first hand from the LSU AgCenter, as well as some individuals that have viewed the field(s) and worked with the disease, the particular spots in fields are confined to three specific hybrids (see link above) and small, localized areas within fields.  Keep in mind, Goss’s wilt can be a seedborne issue.  Inoculum did not travel from Nebraska to LA and drop in the small in the fields in question.  Seed produced in a field with either a history of the disease or a single year event that was harvested and then planted in a field is likely the cause of this particular disease observation.

One important thing to consider.  Goss’s wilt produces a diagnostic symptom within the corn leaf tissue referred to as freckling.  In our production system, if a secondary fungus invades a lesion this can be misdiagnosed as freckling.  The freckles as well as elongated, water-soaked lesions as highlighted in the attached image are the best way to determine the difference between environmental injury or Goss’s wilt.  If you have one of the hybrids listed in the information on the internet (linked above from LA) and you see some strange please don’t hesitate to call.  But, keep in mind, regardless of what the situation may be, if it is Goss’s wilt, absolutely nothing can be done once the infection process has begun.

Non-disease issues 

Nutrient deficiency in lower corn canopy. Note the yellowing tissue mixed with necrotic tissue.

Nutrient stress

Over the past week limited nutrient deficiencies have been observed in some corn fields.  Generally speaking, nutritional deficiencies that are the result of poor application practices, newly cut fields, or soil textural changes can be mistaken for plant diseases. In these cases the crop is generally running out of nitrogen and/or potassium, which in turns leads to necrotic tissue in the lower canopy. However, the majority of the time the appearance of the foliar injury can appear similar to a disease.  But, nutrient deficiencies generally have patterns, and more often than not move from the tip of the leaf back towards the plant stalk.  In addition, affected leaves will turn necrotic from the bottom of the plant to the top of the plant, with the upper canopy showing distinct visual nutrient deficiency symptomology.  If there is no patterned symptomology of a nutrient deficiency, drought stress could be responsible for causing the necrotic tissue.

Irrigation stress

Irrigation stress can manifest itself as a foliar disease or be confused as nutrient stress.  In situations where either long periods of time have gone between irrigation events or when there are observable differences in soil types within rows, irrigation stress can be the issue at hand.  Keep in mind that irrigation stress can be confused with diseases such as Goss’s wilt (see above).  In general, plants under heavy irrigation stress can die from the top down or even the middle of the canopy to the bottom of the plant.  However, this will appear different than nutrient stress.  Plants that are nutrient stressed will generally turn yellow, or lighter green in color and leaves will die at the bottom of the plant and move up.