Corn Disease Update: June 24, 2017

The corn disease situation had been mostly quiet until the last 7 to 10 days.

Common rust

A moderate and humid environment for the majority of 2017 have greatly increased the observed levels of common rust. In the past, common rust has been slowed down by increased temperatures.  However, this year, with a limited number of days with temperatures in the 90s, common rust has appeared to be more severe than in the recent past.  Fungicides have not been suggested to manage common rust in the past.

For aid in identifying the differences between common and southern rust see:

Keys to Field Identification of Common Versus Southern Rust in Field Corn

Diplodia leaf streak

Over the past several years the observations of Diplodia leaf streak throughout the season have generally increased. Identifying Diplodia leaf streak can be difficult as the disease can be easily confused with northern corn leaf blight.  Lesions as a result of infection generally begin about the time the ear begins to form and can have a slight pattern across the leaf surface as a result of the timing of infection.  As lesions mature they tend to develop a bright yellow halo.  In severe situations the most mature lesions can measure longer than 12 inches.  However, I have only observed one field with lesions this size and the majority of the lesions observed this year have been much shorter, on the order of 1 to 4 inches in length.  The most mature lesions develop sporulation that looks quite different than northern corn leaf blight and can greatly aid in identification.  Sporulation looks like tufts of black hair within the lesion.  Management of the leaf phase of the disease is not warranted as fungicides are not considered to be effective on the fungus.

Diplodia ear rot

The fungus that causes Diplodia leaf streak can also cause an ear and stalk rot. However, observing the leaf phase does not mean that plants will develop one of the other disease phases.  More often than not I have observed a corn field to contain the leaf phase and no ear rot or the ear rot and no leaf phase.  But, in my experience in MS I have only observed Diplodia stalk rot in two in the last 10 years.  Fields in continuous corn are generally most at risk and as is normally the case with most diseases if the field has had a history of the disease then it is likely that particular disease will continue to be observed.  So far this year I have received one call from a corn farmer that has had a history of Diplodia ear rot.  Due to the increased moisture this season he has already identified Diplodia ear rot in some of his fields.  In addition, I have received photos and observed only one other field this season exhibiting Diplodia ear rot.  Ears develop a white fungal growth underneath the husk.  In situations where ear leaves look wet, brown, completely dried out, or simply dead, check the ears for ear rot.  Management of the ear rot phase is not warranted with fungicides.

Northern corn leaf blight (NCLB) this season has produced lesions that appear longer than normal in some situations.

Northern corn leaf blight (NCLB)

In the past, warmer temperatures have helped slow down NCLB. However, this year in particular, NCLB has enjoyed the cooler than normal temperatures and the ample moisture received throughout the MS corn production area.  NCLB lesions tend to be an inch in length to at most 3-4 inches.  However, this year in particular, a likely a result of the conducive environment, some NCLB lesions appear to be longer than normal.  The specific germplasm planted in a field can also dictate lesion development and expression.  Lesions do not typically have a yellow halo as can be expressed by Diplodia leaf streak lesions.  Sporulation generally occurs in the oldest part of the lesions and viewed with the aid of magnification will like small trees with the spores present sticking out the top.

In most cases, determining the amount of NCLB in the field can be difficult. Over the past week I have looked at some fields with patchy NCLB that tended to occur along tree lines or other areas of the field where dew was present for extended periods of time during the day.  Once outside of those areas observing NCLB was difficult.  One or two lesions on a single leaf on one plant between a limited number of plants in a field should not be considered to be a “lot” of NCLB.  The large lesions sometimes make it difficult to discriminate and I think oftentimes the disease may appear to be more severe in a short distance within a field.  However, as the corn plant matures through reproductive growth stages the level of any disease generally increases.  Therefore, as corn plants reach more advanced reproductive growth stages approaching dent, the level of NCLB will likely increase in a given field.  Making a fungicide application at advanced reproductive growth stages is not economically beneficial and in most cases due to what is normally a slow level of disease progression the corn plants will more than likely outrun any potential yield losses as a result of NCLB.

Southern rust can produce tightly grouped clumps of pustules on infected leaves that result in light orange sporulation erupting through the leaf epidermis.

Southern rust

Over the past ten days, southern rust has been observed in three counties in MS. Two of the observations were on field corn (Claiborne and Clay counties) and one was on sweet corn (Webster County).  In all three cases, small amounts of southern rust were observed on a limited number of plants in each field.  It is more than likely that additional southern rust will be observed once fields are dry enough to scout following the tropical storm.  Fields with a short period of time until reaching dent will likely outrun losses as a result of southern rust.

Scout and make decisions to apply a fungicide to manage southern rust rather than making blanket fungicide applications. Fungicide applications at dent have not generally been considered to be economically beneficial.  In addition, on an annual basis, rumors regarding the potential of corn to die as a result of southern rust in a short period of time or plants to lodge are quite common.  Corn plants do not die in 7-10 days after the initial observation of southern rust especially in situations similar to those that have been observed over the past week where a light level of infection was observed.  In addition, I have only observed corn to lodge as a result of southern rust when the corn was planted in July and likely infected prior to the VT growth stage.  Corn planted within the normally suggested planting window has not been observed to lodge as a result of southern corn rust in my program in the last ten years.