Last September tar spot, a corn disease that had not been previously found in the continental US, was confirmed in corn samples collected in Indiana and in several northern Illinois counties.This disease occurred late enough in the growing season that yield loss was not of concern, but many questions remained. Before its 2015 Midwestern appearance, tar spot was only found on corn grown under very humid conditions in Latin America and Puerto Rico. This disease, caused by the fungus Phyllachora maydis, is characterized by leaf lesions that contain raised, black structures called ascoma. Ascoma resemble small drops of tar (Figure) and house reproductive structures called perithecia which produce ascospores. These ascospores can cause additional infections during the growing season.

In addition to P. maydis, a fungus called Monographella maydis can be found in tar spot lesions in Mexico. In Mexico, it is the two fungi working as part of the tar spot 'complex' that result in significant leaf damage and yield losses. University of Illinois Plant Clinic staff members were unable to isolate M. maydis from any of the Illinois tar spot lesions.

P. maydis was not expected to survive the northern Illinois winter as similar to the rust pathogens, it is an obligate parasite and requires living plant tissue to survive. University of Illinois personnel set out to test these expectations through survival observations. In October, extension commercial agriculture educator Russ Higgins collected symptomatic leaves from a late-planted corn trial at the former Northern Illinois Agronomy Research Center. In order to easily recover the leaves in the springtime, they were placed in mesh bags. The bags were staked to either the soil surface or buried 3 to 5 inches deep to expose the leaves to conditions that would mimic either no-till or fall tillage, respectively. In April, after the leaves experienced the northern Illinois winter, they were recovered and delivered to the University of Illinois to the plant pathology lab of Dr. Santiago Mideros. Researchers were unable to observe live perithecia or ascospores under a microscope. Provided that (similar to in its home in Mexico) this pathogen is not able to survive the winter on any of the perennial grasses in or near corn fields in Illinois, this preliminary survival study suggests that farmers may have one less disease to worry about in 2016.

If you suspect that you have tar spot in your corn in 2016, please send a sample to the University of Illinois Plant Clinic.