By Champion Seed Agronomist Brandon Hulme
Tar spot is a fungal disease in corn caused by the fungal pathogen known as Phyllachora maydis. The disease causes black, tar-like specks to form on the leaves. The disease was discovered in Mexico and Central America and moved to the U.S. in 2015, infecting fields in Wisconsin, Illinois, Iowa, Minnesota, Indiana and Michigan.
2021 was an ideal year for tar spot and an unfortunate one for corn growers. Tar spot infection favors high humidity with long periods of dew on the leaves. Storms and wind can move the tar spot spores from one field to the next. These conditions were prevalent throughout most of the growing season last year and therefore we saw early infection of tar spot and continued to see widespread reports of the disease further west than we ever had before.
Unfortunately, tar spot is here to stay. We have seen some level of infection every year since 2015, indicating that it does overwinter in the infected debris. How prevalent and how widespread the disease is year to year will depend largely on if we have the high humidity and long durations where the leaves are wet due to rain or dew.
Currently, no corn hybrid developed for the U.S. has complete tar spot resistance, but some germplasm for other parts of the world does. Further research into this is being done and bringing that resistance to U.S. germplasm may be achievable down the road. In the meantime, we must rely on observations where tar spot was present and rating the tolerance to the disease as some hybrids appear to be more susceptible than others.
Yield impact from tar spot infection has been inconsistent. How early the infection enters the field, how severe the infection is, if the leaf tissue dies, and if other diseases are present in the field, all play a major role in the yield hit that may or may not come from tar spot.
Most fields I looked at in 2021 where tar spot was the only identifiable disease had healthy green leaves with some black speckling and saw little-to-no yield hit or standability issues. In fields where another disease was identified in conjunction with tar spot such as gray leaf spot or common rust, the plant health was considerably less and yield was impacted, especially grain fill and test weight. My recommendation here is to build a fungicide into your program if you are in an area where tar spot has been severe or a field that had tar spot in the last two years. Even if the fungicide does not completely control tar spot, by increasing overall plant health and reducing the infection from other diseases, the corn plants should be able to tolerate the tar spot infection better and reduce the negative yield impact.
Champion Seed has tar-spot ratings for many of its hybrids on the tech sheets found on our website. Selecting a hybrid with increased tolerance to the disease and adding a fungicide at the VT stage to your corn-management program will give your fields the best chance to withstand the spread of the spores that cause tar spot.
If you have more questions or think your fields may be infected by tar spot, reach out to a Champion Seed sales representative or agronomist.