Tar Spot of Corn Confirmed in Maryland
Tar spot, a foliar disease of corn caused by the fungal pathogen, Phyllachora maydis, was reported and confirmed for the first time in Maryland. Tar spot has been present just over the Mason-Dixon Line for a couple of years now where it was first discovered in Lancaster County PA in 2020 and has since spread to several neighboring counties. It was first detected in the United States in Illinois and Indiana in 2015 and has rapidly spread throughout the Corn Belt. The sample collected from a Harford County farm on August 19 is the first confirmed case in Maryland.
Lesions (which are actually reproductive structures called stromata) from tar spot look like someone splattered black paint or tar on the leaves (Figure 1); they are typically raised and they cannot be washed or scraped off of the leaf. It is not too late to scout fields for tar spot; if you suspect that you could have this disease, please contact your local agriculture agent or you can use the iPIPE website to report your sighting: https://corn.ipmpipe.org/reporting-form/. We will then follow up with you to confirm what you are seeing is tar spot. There are several look-alikes that can be confused with tar spot, such as insect frass, bird droppings, and some other foliar diseases, so proper confirmation is essential as we want to monitor the spread of this disease in our state.
Tar spot is believed to spread locally via wind-blown spores and dragging infested corn residue on combines or equipment to new fields.
Right now most corn is probably far enough along to not worry about it too much this year. Once corn reaches mid to late dent, yield losses have been reported to be less severe. However, corn that is infected at early dent and earlier, there is the potential for significant yield loss (50% reported under ideal conditions). Right now it is good practice to scout some fields for tar spot and be prepared to make management adjustments for when it moves into your area.
Tar spot has the potential to cause significant yield loss on susceptible corn hybrids if infection occurs early enough. In 2021, it was estimated that tar spot reduced yields more than any other foliar disease of corn in the US. It thrives under cooler temps (60-low 80s), wet/humid, cloudy conditions, which we just experienced in our area around the time tar spot was detected in Harford County.
Time will tell just how severe this may be in our area or to what extent it will play long-term, but growers should have it on their radar, especially for the 2023 crop and beyond; particularly for those counties in Northern Maryland bordering PA. Considering it seems to be quite happy in the corn belt and in PA, it's probably not leaving us anytime soon and this was likely an unavoidable situation. Hybrid resistance to tar spot plays a large role in managing this disease. The fungus that causes tar spot does not infect any other plant species other than corn and it persists year-to-year in infected corn residue. Foliar fungicides applied around VT/R1 also play a role in managing the disease.
There is still a lot we need to learn in terms of tar spot disease epidemiology and management for our region. If you have any questions or concerns, please contact me (firstname.lastname@example.org, 410-638-3255).
This article appears in September 2022, Volume 13, Issue 6 of the Agronomy news.