A few weeks ago, tar spot, a new disease of corn caused by the fungus Phyllachora maydis was reported for the first time in the US, first in Indiana and then in Illinois. It was later found as far east as Allen County, Indiana, bordering Paulding County in northwest Ohio. So, although tar spot has not yet been confirmed in Ohio, it is quite possible that it may be present in the northwestern corner of the state.
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. According to Dr. Wise, my counterpart at Purdue University, “Symptoms of tar spot begin as oval to irregular bleached to brown lesions on leaves in which black spore-producing structures 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."
What causes Tar Spot and how damaging is it? Tar spot is caused by the fungusPhyllachora maydis, but the greatest impact of this disease in terms of yield loss occurs when P. maydis-infected plants are infected by a second fungus calledMonographella maydis. So far, thankfully, only the first fungus has been reported in the US (IN and IL). In regions such as Mexico where Tar Spot has been known to cause substantial yield losses, the two fungi act as a team, with Phyllachora maydisfirst infecting the plants, followed by infection with Monographella maydis. Damage tends to be most severe under cool, humid conditions at high elevations.
Where did it come from and will it survive and become established? At this point it is unclear how Tar Spot got here. It 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 by water and wind, there is some speculation as to whether it came in on a tropical storm. Since Tar Spot is generally considered a tropical disease (common in Mexico, parts of South America and the Caribbean), it is unlikely that the fungus will survive the harsh Midwest winter and become established here. However, we’ll have to wait and see and do the research to learn more about this disease.
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, and most importantly, please send samples to my lab (1680 Madison Ave, Wooster, OH) for testing and verification.