Andrew Kness, Agriculture Agent | akness@umd.edu
University of Maryland Extension, Harford County
Before anyone panics, this disease has not been confirmed in Maryland; however, it is close by and could potentially spread to our state. As you’re scouting fields this summer, keep this one in the back of your mind.
Tar spot is a fungal disease of corn caused by Phyllachora maydis. It was first discovered in the United States in 2015. In Latin America where tar spot is more common, another fungal species, Monographella maydis, is known to occur in complex with P. maydis; however, only P. maydis has been found in the United States.
The initial occurrence of tar spot in 2015 was limited to a couple of counties in Indiana, but by 2018 it had spread across much of the US corn belt and caused serious economic damage. In 2020, tar spot was confirmed in corn fields in Lancaster County Pennsylvania (Fig 1). We believe this large geographic jump was due to seed that had infected corn residue that was planted into variety trial plots.
Symptoms of tar spot are primarily the presence of glossy black, raised lesions on the leaves (Fig 2). These lesions look like someone splattered paint or tar on the leaves, hence the name. These symptoms have been observed to occur from VT to maturity. Lesions are also visible on dried plant parts.
The fungus that causes tar spot overwinters in infected corn residue; however, it is unknown how well, or if, this pathogen will overwinter here in Maryland or if this disease will be of any economic importance in this region.
Currently there are no management recommendations for this disease for our area, because as stated, it is yet to be confirmed in Maryland and unknown if it will be a problem. For the 2021 corn crop, be on the lookout for this disease in your corn fields, especially if your have any corn hybrid trials planted in your field. If you suspect you found this disease, please contact Andrew Kness (410-638-3255, akness@umd.edu) and we will work with you to confirm identification.
