Corn Disease Update: June 7, 2013, and Specific Information Regarding Northern Corn Leaf Blight

Increased observation of corn foliar diseases have occurred throughout the southern Delta over the past couple of weeks.  Much of the corn south of Highway 82 has either started to tassel or will be tasseling in the next 7 to 10 days.  First and foremost, I can’t stress how important it is to determine the specific hybrid in each field.  Knowing the particular disease package will help address whether or not a fungicide application might be beneficial.  Most hybrids have good tolerance to foliar diseases; however, there are some in our corn production system that tend to be great yielders but have weak disease packages and may be predisposed to foliar diseases.  Some defensive corn hybrids, with excellent disease packages, do a good job of keeping yield-limiting foliar diseases lower in the canopy (below the ear leaf) and only allow limited, single lesions to progress above the ear leaf.  The more offensive hybrids, or “race horse” hybrids, in most cases, have a weaker disease package and lesions can generally be observed all up and down the plant.Additional information regarding the general time of disease occurrence in corn can be found at:

https://www.mississippi-crops.com/2013/04/20/corn-disease-calendar/

Common rust of corn. Note the dark colored sporulation that erupts from the pustule.

Common rust

More common rust has popped up in some field situations.  Common rust has not previously been a treatable issue in the past and I don’t expect that to change this year.  Common rust is the less aggressive of the two rust diseases we regularly observe in MS corn fields.  More often than not common rust will be observed on corn leaves following a flush of nitrogen or rapid new growth in the corn plant.  In addition, common rust will produce lesions on BOTH surfaces of the corn leaf: top and bottom.  Southern rust will only produce pustules on the top of the leaf; however, I have seen an exception or two in the past that were uncommon occurrences.  Temperatures have not been conducive for the occurrence of southern corn rust so at this stage in the season we are only observing common rust.  Remember, common rust on the lowest leaves in the plant canopy will have a different appearance with smaller pustules, and the spores that erupt from the pustule itself will appear lighter in color than the more normal russet or maroon spore masses that occur higher in the canopy.

Additional information can be found at: https://www.mississippi-crops.com/2013/05/24/early-season-corn-diseases-and-non-disease-observations/

Northern corn leaf blight (NCLB)

I’ve observed several fields and have had several calls regarding increased incidence of NCLB.  Most of the fields with observable symptoms of NCLB are south of Highway 82 in some of the oldest corn.  Even though the fungus that causes NCLB can be more of a concern in fields of continuous corn we are observing NCLB in first year corn fields.  In some cases more than one lesion has been observed on a single corn leaf, especially in situations where continuous corn has been planted.

NCLB prefers cooler temperatures and a wetter environment with high humidity, similar to the environment we’ve experienced for the past several weeks.  However, with that in mind, if the temperature would shift and become warmer (mid-90s) the majority of the NCLB we have in our corn crop would either slow down or shut down completely.NCLB specifics:

-infection by the fungus occurs following 6-18 hours of free moisture (dew or other) on a leaf surface

-germinating conidia (spores) prefer temperatures between 66 and 80F

-lesions typically develop within 7 to 12 days

-secondary spread within a field can occur from lesions present within a field

-inoculum can overwinter on stubble in a field

Deciding to spray a fungicide in response to NCLB is a difficult choice.  In the six years I’ve been with MSU I have only suggested that one field be sprayed due to a susceptible hybrid, continuous corn, and numerous lesions on single leaves.  Remember, determine what hybrids are planted in each field on your farm and make fungicide decisions based on the hybrid’s disease package, number of years in corn, irrigation type (overhead versus furrow), overall yield potential (especially since we have a tremendous amount of corn that looks shorter than in previous years and has had some early nutritional issues), and percent leaf area covered.  Yield loss has not been reported to occur until 15-20% of the leaf surface area has NCLB lesions.

Specific corn disease reactions to fungicides and labeled products can be located at: https://www.mississippi-crops.com/2013/05/31/labeled-fungicides-for-corn-disease-management/

Single leaf, lower in the canopy containing numerous NCLB lesions (Photo courtesy of consultant).

One of the biggest issues to consider regarding NCLB has to do with the percent leaf surface area covered by the disease.  I am often asked about “treatment thresholds”.  Regardless of the disease, treatment thresholds don’t exist for several reasons.  The environment is the most essential key for any plant disease to occur and judging how a disease will respond to a change in the environment is difficult.  Yesterday, (June 7) I took some basic measurements of some corn leaves that had NCLB lesions to determine the percent leaf area affected by the disease.  The leaves I had were quite long but in general tended to be 39 inches in length.  Realizing that a corn leaf tapers at the tip, if we based the length on 37 inches, the leaf was 2.75 inches wide.  The resulting leaf surface area would be approximately 101.7 square inches (37 inches × 2.75 inches = 101.7 square inches).  A single NCLB lesion occurring on a leaf that measured 2.5 inches long by 0.25 inches wide, would result in 1.6 square inches of leaf surface area covered.  Overall, a single NCLB lesion on a leaf of that size would cover approximately 1.6% of the leaf (1.6/101.7 × 100%).  If the plant in question had 10 leaves (most plants have more, simply using the number for easy sake of comparison), only 0.16% of the total leaf surface area of the single plant would be affected by NCLB.  Even if three lesions were present (and based on the sizes above covering approximately 4.8% of the leaf’s surface) then only 0.48% of the plant’s total leaf surface area would be affected.  One lesion, per leaf, per plant is hardly a justification for making a fungicide application based on leaf surface area affected…..especially if a tolerant hybrid is planted.  Unless the plant is extremely susceptible to NCLB let the genetics within the hybrid manage the disease.  Save the fungicide for something more aggressive (should it happen) such as southern corn rust, especially since no resistant hybrids exist in our cropping system to southern corn rust.

Scouting for NCLB

In general, NCLB will occur on some of the lower leaves in the canopy, especially in continuous corn situations since the inoculum overwinters on residue at the soil level.  But, the leaves at the ear leaf level and above are the ones to be most concerned about.  I regularly tell folks that once an ear sets on the corn plant, focus on the disease occurring above your belt.  In some cases the diseases can move up the plant but protecting the ear leaf and those leaves above the ear leaf is the most important management practice.