What is all the Buzz about the Brown Spots on Corn?

With the wet weather, there seems to be more scouting occurring in fields, which is good.  Many have asked about the brown spots that they have been finding on corn stalks, leaf sheaths, and leaves. 

Purple leaf sheath (Picture taken by Matt Montgomery)

Purple leaf sheath (Picture taken by Matt Montgomery)

The pictures above show an abnormality called purple leaf sheath, which is a harmless discoloration caused saprophytic fungi and bacteria developing on pollen and particulate matter lodged between the stalk and leaf sheath.  The Compendium of Corn Diseases states the following about purple leaf sheath:
"Irregular, purplish brown blotches and spots of varying size form on the leaf sheaths, usually after silking.  This harmless discoloration occurs when saprophytic fungi, such as yeasts and Fusarium spp., and bacteria develop on pollen and other particulate matter lodged between the stalk and leaf sheath or after an infestation of aphids.  The stalk under the sheath is not discolored or infected.  This is not a new disease and there is NO NEED TO BE ALARMED!

Purple leaf sheath may be confused with Physoderma brown spot, which is a fungal disease that can infect corn stalks; however, Physoderma brown spot usually also causes symptoms on the leaves as seen in the pictures below.

Physoderma brown spot (Picture taken by Matt Montgomery)

Bands of lesions across the leaf caused by Physoderma brown spot (Picture taken by Matt Montgomery)

Physoderma brown spot caused by Physoderma maydis is NOT usually considered to be a threat to corn.  This disease is often seen after abundant rains and high temperatures, and infection takes place when spores are splashed into leaf whorls.  This disease might be more apt to show up in fields that consist of reduced tillage or corn on corn.  Some hybrids may consist of more resistance than others and of course, tillage can help bury the disease inoculum.  Again, there is not need for alarm if you are seeing this disease in your field.