Alyssa Koehler, Extension Field Crops Pathologist; akoehler@udel.edu
We have been monitoring closely for the appearance of tar spot within corn fields. Tar Spot is a foliar disease of corn caused by the fungus Phyllachora maydis. It first showed up in northern Illinois and Indiana in 2015 and was found in Lancaster County at the end of the 2020 season and continued to spread to surrounding PA counties in 2021. This season it has been reported again in Lancaster and York counties in PA (Figure 1). Last week the first sighting appeared in Harford County, MD and samples have been sent to the USDA for official confirmation. The fungus produces small, raised, black bumpy lesions that look like specks of tar, giving it the common name of tar spot (Figure 2). These structures known as stroma can be on the upper or lower leaf surface and do not wipe off the corn leaf. In severe cases, lesions may also be observed on the leaf sheaths, husks, and tassels. While there are many diseases that have the common name of “tar spot”, particularly in ornamental plants, the pathogen that causes tar spot in corn is only known to appear on corn. If you suspect you have Tar Spot, please contact your county Extension Agent or submit a sample to the UD plant diagnostic lab for confirmation.
Figure 1. Tar Spot map as of 8/25/22 https://corn.ipmpipe.org/tarspot/
Figure 2. Slightly raised, black stroma of Phyllachora maydis https://cropprotectionnetwork.org/publications/an-overview-of-tar-spot