Corn Disease Update: June 28, 2014

Corn diseases continue to be observed throughout the MS corn crop, albeit at fairly low levels.  Over the past week I have either received telephone calls regarding the diseases below or observed them first hand in fields where I’ve scouted for disease.  On Wednesday (June 25, 2014) I made a large loop to southeast MS to look at the corn growth stages as well as scout for southern corn rust.  Keep in mind that corn plants are large and in most cases, unless a lot of the leaf material is covered by a particular disease, a little bit of disease won’t require a fungicide application.  But, with that said, a disease threshold for any of the diseases listed below does not exist and fungicide application decisions should be based on several key factors (see outlined below).

Common rust

Over the past week to 10 days I haven’t observed near as much common rust in the fields I’ve scouted as I have two weeks ago.  Of the two rust diseases we observe on an annual basis, common rust is not a disease that will reduce yield and generally only covers a small portion of the leaf tissue affected.  Pustules tend to be larger than southern rust pustules, can occur on the top and bottom of the leaf tissue, and the sporulation that ruptures through the leaf tissue will generally be cinnamon to brown to russet in color.  In addition, common rust will generally be observed from the top to the bottom of the plant since infection by the fungus generally occurs early in the season.  Keep in mind that in some rare cases, old common rust lesions/pustules can appear to look similar to gray leaf spot.  Adding a picture of this particular scenario would be confusing.  A hand lens will help diagnose the situation since you’ll be able to clearly observe the shredded pustule as well as any remaining sporulation that ruptured through the pustule itself.

Mature gray leaf spot lesion. Note the parallel lesions and the absence of the lesion crossing the veins.

Gray leaf spot

Gray leaf spot (GLS) has continued to be observed in MS over the past few seasons, especially in situations where numerous years of continuous, reduced tillage corn have been grown.  GLS is generally observed in fields where continuous corn has been planted.  However, there are rare instances where the disease can be observed in first year corn fields, if those fields are in close proximity to fields of continuous corn.  Lesions have parallel sides, tend to be less than a half inch to an inch and a half in length and the lesion will not cross the leaf veins.  Sporulation of the fungus occurs on the underside of the leaf from the natural openings (stomates) that run in between the veins.  Observation of the lesion with a hand lens, on the order of a 20×, can allow you to see the dark pimple-like structures present in the stomates that produce an observable sign generally referred to as “ducks in a row”.

Not every field requires a fungicide application for GLS.  Just yesterday I observed two fields in western Tallahatchie County where GLS could be observed at extremely low levels, mostly in the lower plant canopy.  In some cases, the genetics of the corn hybrid itself will do a good job from a defensive standpoint against GLS and negate the need for a fungicide application by keeping the disease below the ear leaf.

Fungicide management alternatives for GLS should be based on several key factors:

-hybrid present (GLS defensive hybrid will tend to keep the disease below the ear leaf)

-number of years in corn

-presence of the disease on the ear leaf or above

-overall leaf surface area covered by the disease

-growth stage at time of disease observation

-presence and type of irrigation (overhead irrigation will tend to increase the spread of the disease)

Fungicides do a good job of preventing yield loss as a result of GLS.  However, once applied the lesions on the leaves won’t disappear and in most cases the disease will appear to get worse.  Fungicides do a good job of slowing the disease down, but won’t help recover the necrotic plant tissue.

NCLB lesion. Keep in mind the scale of the lesion compared to other maladies present on the leaf. Sporulation can generally be observed in the center of the lesion.

Northern corn leaf blight

As the corn crop matures we will likely continue to observe additional NCLB in corn fields.  In addition to observing NCLB over the past week I’ve received several calls regarding the injury associated with a pre-tassel nitrogen application producing a NCLB-type symptom.  Keep in mind that a pre-tassel nitrogen application can produce lesions that will appear similar to NCLB on leaf margins and in some cases along portions of the leaf that were exposed to the application.  The lesions that result will typically be invaded by secondary fungi that will produce symptoms quite similar to NCLB.

A fungicide application threshold for NCLB does not exist.  But, with that in mind, NCLB isn’t a fast moving disease and generally speaking I’ve rarely observed a field that I felt needed a fungicide application to prevent yield loss.  In the 7 years I’ve been here, I can remember one field where I suggested a fungicide be applied.  The field in question contained an extremely NCLB-susceptible hybrid and NCLB lesions were observed from the bottom of the plant canopy to the top of the plant canopy.  Every single leaf had more than one NCLB lesion in that case.  Since that time, I have not observed a field that had as much NCLB (except for my fungicide trial plots in Stoneville, MS last year).  In situations where a single lesion is present on a plant and the plants affected with lesions are widespread throughout the field I generally suggest that a fungicide not be applied.

NCLB lesions are typically an inch in length, and a quarter of an inch in width.  However, size and appearance of the lesion will greatly depend on the environment present in the field as well as the hybrid’s response to the fungus.  The center most portion of the lesion will more than likely produce the spores necessary for the disease to infect additional leaf tissue.  A hand lens can be used to observe the differences between an injury as the result of a nitrogen application and NCLB lesions.  The NCLB lesion will be observed to have small, fine, dark green to black hairs with the spore on the top of the hair (conidiophore).  Keep in mind that lesions that run along the vein for the entire length of the leaf or appear on the order of 5 to 6 inches in length are likely not produced as a result of NCLB.

Southern corn leaf blight. Not irregular appearance of lesions as compared to those of GLS.

Southern corn leaf blight

On Wednesday, I observed a fair amount of southern corn leaf blight (SCLB) on lower leaves throughout corn fields in southeast MS.  Southern corn leaf blight is not a normal occurrence, but I suspect the cooler than normal and wetter than normal conditions we’ve experienced over the past month has increased our observation of SCLB.  SCLB can look similar to gray leaf spot.  However, in most of the cases I’ve observed since arriving in MS, the SCLB will tend to stay low in the canopy and typically not be observed above the ear leaf.  Telling GLS apart from SCLB can be difficult and requires observing numerous lesions with a hand lens.  When SCLB lesions first appear and begin to more mature they will tend to have parallel margins, similar to GLS.  However, generally speaking SCLB lesions will tend to be a little bit more irregular in shape and appearance on the leaf than GLS.  Lesions can be longer than GLS, on the order of 2 inches depending on the hybrid planted and tend to cross the veins which is a major difference between GLS and SCLB.  The majority of the hybrids planted in MS are likely fairly tolerant to SCLB, but check to make sure how the hybrid is claimed to perform in the company literature.

To definitively tell the difference between GLS look at the lesions for the presence of “ducks in a row” on the underside of the leaf.  SCLB will more often than not sporulate on the top of the leaf as opposed to GLS where the spores will be observed on the underside of the leaf.  Since the early 1970s, SCLB has not been a major problem in corn production systems due to enhanced genetics and a reduction in the monoculture of the same hybrid on large numbers of acres.

Southern corn rust. Note yellow halo around pustules in the upper portion of the photo.

Southern rust

On Wednesday I scouted corn in several southeastern MS counties (Forrest, George, Green, Jackson, Perry, Stone).  A consultant in the area had mentioned he observed southern rust a week to 10 days ago.  I scouted field corn as well as sweet corn where present and was able to find low to moderate levels of southern corn rust in all of the fields that I scouted.  Similar to the monitoring situation for soybean rust, observations of southern corn rust are reported to a website and the counties where the disease is observed are colored either red (indicating the disease has been observed/confirmed) or green (indicating the disease has not been observed/confirmed) for that particular county.  The website can be located at: http://scr.ipmpipe.org/cgi-bin/sbr/public.cgi.  However, in addition to that website, stay tuned to the Mississippi Crop Situation and check the “Disease Monitoring” tab at the top for observations of important yield-limiting diseases of corn and soybean.

Southern corn rust differs from common rust in several key symptoms.  Southern rust will almost always only be present on the upper leaf surface (top).  Pustules are smaller in size and the sporulation that ruptures through the leaf tissue is generally more orange in color.  Looking closely at the lesions on the leaf you may be able to distinguish a yellow halo around the pustule (see photo).  In addition, southern corn rust prefers higher temperatures and will generally be first observed on the ear leaf and above since it occurs later in the season and blows in from somewhere to our south.

Over the past several years I’ve heard people suggest that “southern corn rust will kill a plant in 7 days and should always be sprayed with a fungicide”.  Since I’ve been working in corn I’ve not observed a situation where southern corn rust has killed the plant even as much as 35+ days post-observation.  In addition, a tremendous amount of disinformation involving plant lodging as a result of southern corn rust seems to have been discussed throughout the state.  Lodging, as a result of southern rust, will generally only occur if infection occurs prior to tasseling in vegetative corn stages.  Since arriving in MS, I’ve not observed this occurring in our corn crop.

At present, southern rust has ONLY been observed in southeast MS.