Figure 2. Tar spot symptoms on leaves both on the lower and the upper canopy. (Photo Credit: Darcy Telenko)
While I have been out in Paulding county scouting in the last week, I have not noticed any tar spot in our cornfields as of yet. It could be there though as I am not walking in every field. I wanted producers to take note of what Tar Spot looks like and some monitoring from our neighbors in Indiana and information from a previous CORN New Article.
Figure 1. Map of counties confirmed for tar spot as of August 27, 2020. Orange counties have confirmed activity, gray counties where tar spot has been found in previous years.
From Purdue Extension:
Tar Spot – We have confirmed 13 counties with active tar spot as of August 27 in the 2020 season (figure 1). We are starting to find multiple fields, some moderately infested with tar spot, in northern Indiana. I have scouted fields that have 100% of the plants infected with stroma (black spots) ranging from a few on a leaf to 15% severity (figure 2).
Unfortunately, the sites where we are seeing the highest severity were those that were planted early and have had good moisture/irrigation all season. Luckily, at least the sites I have been in thus far, the corn has reached R5 and we hope it has minimal effect on yield. That being said, please keep a close eye on your late plantings. We have seen variable symptoms from a few stroma (black spots) on a leaf to many.
From Ohio State Extension – Pierce Paul, Felipe Dalla Lana da Silva – 2019 CORN Newsletter
Tar Spot, a new disease of corn caused by the fungus Phyllachora maydis, was reported for the first time in Ohio at the end of the 2018 growing season. At that time, it was found mostly in counties close to the Indiana border, as the disease continued to spread from the middle of the country where it was first confirmed in 2015. Over the last few weeks, there have been several new, confirmed reports of Tar Spot in Ohio, this time not only in the northwestern corner of the state but also from a few fields in central and south-central Ohio. As was the case last year, disease onset was late again this year, with the first reports coming in well after R4. However, some of the regions affected last year had more fields affected this year, with much higher levels of disease severity.
It could be that Tar Spot is becoming established in some areas of the state due to the fungus overwintering in crop residue from one growing season to another. This is very consistent with the pattern observed in parts of Indiana and Illinois where the disease was first reported. We will continue to keep our eyes out for Tar Spot, as we learn more about it and develop management strategies. You can help by looking for Tar Spot as you walk fields this fall, and please send us samples. 
What does it look like? Even though corn is drying down, if Tar Spot is present, you can still detect it on dry, senescent leaves almost as easily as you can on healthy leaves. So, please check your fields to see if this disease is present. “Symptoms of tar spot first appear as oval to irregular bleached to brown lesions on leaves in which raised, black spore-producing structures call stroma are formed… giving the symptomatic areas of the leaf a rough or bumpy feel to the touch… resembling pustules on leaves with rust. Lesions … may coalesce to cause large areas of blighted leaf tissue. Symptoms may also be present on leaf sheaths and husks.” As the name of the disease suggests, symptoms look like the splatter of “tar” on the leaves. In some cases, each black tar-like spot may be surrounded by a necrotic halo, forming what is referred to as “fish-eye” lesions.
What causes Tar Spot and how damaging is it? In the past, the greatest impact of this disease in terms of yield loss was observed when P. maydis-infected plants were co-infected with a second fungus called Monographella maydis. In other words, the damage tended to be much more severe when the two fungi worked together to affect the plant. So far, only the first fungus, P. maydis, has been reported in the US but based on work done in Illinois, this pathology alone is capable of causing substantial yield reduction on highly susceptible hybrids when conditions are favorable and infections occur early.
Where did it come from and will it survive and become established? At this point, it is still unclear as to how Tar Spot got to the US in the first place and how it continues to spread. The fungus is not known to be seed-borne or infect other plant species, so corn seeds and weeds are unlikely to be the sources of inoculum. However, the fungus can survive and be moved around on fresh and dry plant materials such as leaves and husks. In addition, since spores of the fungus can be carried be wind, it could be blowing in from neighboring states/counties/fields. Although not yet confirmed through survival studies, it appears that the fungus could be overwintering in infected crop stubble between growing seasons.
What should I do if I find Tar Spot? If you see anything that fits the description of or resembles (Picture) Tar Spot, please inform your state specialist, field specialist, or county extension educator, but most importantly, please send samples to my lab (1680 Madison Ave, Wooster, OH) for confirmation. We will also be using your samples to study the fungus in order to develop effective management strategies.
Read more about Tar Spot of Corn at:
https://cropprotectionnetwork.org/resources/articles/diseases/tar-spot-of-corn