Corn Disease Update: June 14, 2019

The disease situation in the MS corn production system has picked up over the past couple of weeks. Below are some of the more common diseases as they’re occurring in corn that has reached reproductive stages or is close to reaching VT.  Most of the diseases observed at present are not problematic and do not need to be addressed with a fungicide application.  Even though there is no disease threshold for aiding in the decision to apply a fungicide, observing a plant disease is in and of itself not a threshold for making a fungicide application.

Anthracnose leaf blight is a more common lower canopy disease.

Anthracnose leaf blight

In general, lower canopy diseases occur throughout the season. In some instances, the observed disease only appears on leaves below the ear leaf.  During 2019, I have received numerous telephone calls regarding anthracnose leaf blight occurring on lower leaves.  The symptoms associated anthrachnose leaf blight consist of lesions that are generally a 1 to 1.5 inches in length with a distinct yellow halo around the lesion.  Quite frequently, the lesions are only observed on the bottom three leaves, but can be observed during vegetative as well as reproductive growth stages depending on the environment that persists shortly after planting.  In the past, I have observed anthracnose leaf blight as early as V5-V7.  Leaves in the lower canopy are shaded and the humidity and temperatures in the lower canopy can allow anthracnose leaf blight to occur and persist throughout the season.  However, the fungus that causes this particular disease seems to prefer the environment in the lower canopy and will more than likely not be observed as high as the ear leaf.

Common rust can appear quite different throughout the corn canopy. Common rust in the lower canopy (top) appears different than in the upper canopy (bottom).

Common rust

The name should indicate that this is in fact the more common of the two rust diseases we observe on corn in Mississippi. Typically, the common rust fungus prefers cooler temperatures (< 85F) and tends to dry up when the temperature becomes hot.  During early reproductive growth stages, common rust can be observed throughout the canopy.  Be mindful that lesions and pustules in the lower canopy (several leaves below the ear leaf at VT/R1) can appear quite different than pustules above the ear leaf.  Common rust in the lower canopy can generally be observed to produce smaller lesions, the coloration of the spores from the pustule look lighter in color and can oftentimes be confused with southern rust (the much more potentially damaging rust in our production system).  Moreover, pustules formed in the upper canopy (ear leaf and above) tend to appear to have a little bit of a pattern and in general can be larger in size.  The spores that erupt through the leaf (pustule) tend to be maroon to dark brown in the upper canopy and do not always have a yellow halo.  In the lower most canopy (typically the bottom three leaves), the pustules associated with common rust can have a faint yellow halo and the spores that erupt through the pustule appear lighter than the more “normal” coloration described above.

Even though common rust may appear to look “bad” in some field situations, rare to levels of the disease on a single plant get greater than 1-2% and common rust is not a disease that needs to be addressed with a fungicide application in the MS corn production system.

Physoderma brown spot produces different symptoms on the leaf surface as compared to the midrib.

Physoderma brown spot

Prior to three years ago I don’t remember addressing Physoderma brown spot on the blog or in meetings. However, over the past several seasons I have received more calls and photos associated with Physoderma brown spot and have even observed the disease in some of the fields I have scouted during the season.  The symptoms associated with Physoderma brown spot can be observed on leaves, the leaf midrib, and on leaf sheaths.  Symptoms present on leaves appear quite different than those on the midrib and leaf sheaths.  The lesions do tend to appear in a band across the leaf; however, the pattern associated with this tends to have occurred as a result of when infection occurred.  Infection tends to occur when the leaves are in the whorl and this accounts for the banded pattern of lesions on the leaf surface.  On leaves, lesions begin yellow in color and then turn brown.  In addition, the area will grow in size as the lesions.  Typically, I look at Physoderma brown spot on leaves and think this is what it might look like if you were to rub some fine sandpaper on the leaves.  On midribs and leaf sheaths, oval brown blotches are the main symptom.  In some situations, these blotches can also occur on the nodes against the stalk.  In the years when I have observed Physoderma brown spot, I typically have had to look hard to find the disease within a single field.  Rarely have I observed the disease on more than 10 plants in a field situation.

General scouting thoughts

Oftentimes I have conversations with crop consultants and retailers about the importance of scouting specific areas within the canopy. Once the canopy closes in a corn field, disease will more often occur in the lower canopy with greater frequency simply as a result of increased shade and humidity and reduced temperature.  Once the corn plant puts an ear on (VT/R1), I shift my scouting to considering the leaves at and above my waist.  Leaves in the lower canopy are stressed, generally lack nutrition, will oftentimes fall off the plant before the end of the season, and the shade and added humidity of the lower canopy allows plant diseases to occur with greater frequency.  Once the corn plant sets an ear, focus on the leaves in the upper plant canopy.  Oftentimes, the diseases that occur on the lowest leaves will not move up to or above the ear leaf; however, there are exceptions to this situation.