Corn Disease Update: July 14, 2014

General leaf spot on a corn leaf as observed during the 2014 season.

Corn diseases continue to be observed throughout the corn production area in MS.  Generally speaking I think 2014 has been a lighter disease year than I initially expected.  Common rust, gray leaf spot, northern corn leaf blight, and southern corn leaf blight continue to be observed to some degree across MS.  However, several additional diseases have been observed over the past two weeks.

General leaf spot

I’ve received numerous telephone calls regarding leaf spots developing in the upper canopy.  Since I arrived in MS in 2007 I’ve observed similar symptoms in several corn fields.  Generally the spots develop in the upper most plant canopy and can be the result of two different scenarios.  First, Curvularia leaf spot can produce a similar symptom and is not an issue that requires a fungicide application.  Second, in some corn hybrids, the pollen that has landed on the leaf can be cannibalized by fungi.  The fungi end up removing moisture from a small area of the leaf around the cannibalized pollen that results in the small spot.  Neither of the two options outlined above require a fungicide application and in general an extremely small percentage of the total plant’s leaf surface is affected with the leaf spot.  Normally, when I’ve observed the leaf spot in a corn field, I generally observed the spots on the upper most leaves in the canopy as I walk through the field looking up into the leaf canopy.

Diplodia ear rot (or Stenocarpella ear rot)

A few fields have been observed to contain Diplodia ear rot at low levels.  Ear rots are not uncommon and generally occur in years where we receive rainfall during the flowering stage of development.  In the right environment Diplodia ear rot can be devastating, but in the years when I have observed the disease minimal yield loss resulted.  From a management standpoint, regardless of what fungicide labels indicate, no management practice will reduce the losses as a result of Diplodia ear rot, or any other corn ear rot for that matter.  Research conducted at Purdue University (based on a personal communication) suggests that not only are fungicides not efficacious on the fungus that causes Diplodia ear rot (Stenocarpella maydis), but fungicide applications are not able to reach the point of infection due to poor canopy penetration and the general aspect of the ear on the corn plant.

Diagnosing Diplodia ear rot and telling this disease apart from the general dried out leaf tips on some ears can be difficult.  In severe situations the ear leaf itself can develop a water-soaked appearance at the base of the stalk/ear.  In addition, the entire husk around the ear can turn brown and if the fungus inside the husk is mature enough dark pepper grains will develop on the outside of the husk and represent fungal reproductive structures.  In general, the entire ear below the husk will be covered with white fungal growth.  The key characteristics to tell Diplodia ear rot apart from the other ear rots are the brown, discolored husk, the white fungal growth on the entire ear, and the water-soaked ear leaf.

Diplodia ear rot, note the white fungal growth on the ear below the husk.

Diplodia ear rot, note the black reproductive structures on the outside of the husk.

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

Southern corn rust

Additional southern corn rust was observed in Coahoma, Holmes, Monroe, and Noxubee counties last week (see updated map on Disease Monitoring page).  Most of the corn in those particular situations has reached the R3, or milk stage of development.  Most reports of southern rust have indicated low levels of the disease.  As opposed to years past when southern rust has been observed in corn, the majority of the disease this year has been below the ear leaf.  Keep in mind that southern rust produces a lighter colored spore mass erupting from the pustule than common rust.  In addition, pustules will be observed on the top of the leaf whereas common rust will be observed on both leaf surfaces.  In 2010, fungicides applied as late as dent (R5) provided a reduction in yield loss compared to the nontreated (see: http://www.mississippi-crops.com/2011/05/29/the-corn-fungicide-dilemma-when-should-a-fungicide-be-applied-part-v-of-v-preventing-yield-loss-from-foliar-disease/).  However, in that particular situation, excessive southern rust was present on approximately 30% of the leaves at the ear leaf or above in a 40 foot plot.  Keep in mind that several factors should be weighed before making a fungicide application.  Consider yield potential as one of the most important factors.  Also, base irrigation type (pivot versus furrow irrigated) as an important issue since overhead irrigation can move the disease around the field and dramatically increase humidity in the plant canopy from the top down.  Scout fields for the presence of the disease rather than making a preventive fungicide application just because the disease is present in a particular county.  In years when I have observed southern rust in Washington County, MS I have oftentimes not been able to detect the disease on the DREC station.