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Agronomic Crops Network

Ohio State University Extension


Tar Spot Q&A

Q: Is tar spot a late-season disease in Ohio?

A: No, tar spot can develop at any time during the growing season. For the first few years after it was first reported in Ohio in 2018, tar spot was detected primarily towards the end of the season, giving the impression that it would likely be a late-season disease in the state. However, in 2021, symptoms were seen at the R1 growth state in some fields, suggesting that under the right set of conditions (moderate temperatures and extended wet periods), tar spot could develop much earlier than previously observed, if the hybrid is susceptible and spores are available. This is similar to what is being observed in neighboring states where the disease is established and considered to be endemic. So far in 2022, tar spot has been reported on V5-V6 plants in multiple Corn Belt states. This is not at all surprising, given that plants of any age are susceptible to infection by the tar spot fungus.

Q: Is tar spot mainly a problem in NW Ohio?

A: No, tar spot can develop anywhere in the state. From 2018 to 2020, tar spot was most frequently detected in fields in the NW corner of the state. However, in 2021 it was reported in more than 30 counties, including a few in the southern and eastern portions of the state. During the first few years, tar spot development in Ohio was likely driven mainly by spores blowing in from neighboring states. Consequently, the location and timing of symptom development depended on where and when spores landed and whether conditions were favorable for them to infect. Since fields in NW Ohio are closer to states from which spores are likely blowing in, tar spot tends to show up first and reach higher levels in that region. In addition, in 2021, some fields in the NW had more frequent rainfall during the months of July and August than fields in other areas, contributing to more tar spot in that region of the state; rain is one of the main drivers of tar spot. However, as the disease becomes established and widespread, fields in other regions of the state will be affected and symptoms with develop early, if the hybrid is susceptible and weather conditions are favorable.

Q: Why is rain or moisture so important for tar spot development and spread?

A: Moisture is important for spore production, release, and spread, and infection of the plant. Moisture is needed for the spores to ooze out of the stromata in which they are produced and then splashed into the air and carried by wind to new plants or fields. Moisture is also needed for spores to germinate and infect leaves, and for symptoms to develop.          

Q: Why do some fields under rotation with soybean and/or tillage still show symptoms of tar spot”.

A: Wind can carry spores over long distances between fields within counties and even between counties and states. So, the reason why some fields without a history of tar spot (did not have the disease previously) and some under rotation and tillage (with little or no corn stubble on the soil surface) still develop tar spot is because spores are picked up and transported from field to field within and across states. If these spores land on the leaves of a susceptible hybrid under wet, humid conditions, they will germinate and penetrate, and symptoms of tar spot will develop

Q: Can tar spot be controlled with fungicides?

A: Yes, based on data from neighboring states, fungicides do show promising results against tar spot. We continue to evaluate products and application timing for efficacy in Ohio. So far, fungicides with multiple active ingredients (AI) tend to be more consistently effective than single-AI fungicides, but no fungicide will provide 100% control of tar spot. In addition, fungicides are most effective when applied as soon as symptoms begin to develop and before the disease spreads. Applications made between R1 and R2 tend to give the best results.         

For more on tar spot:

Crop Observation and Recommendation Network

C.O.R.N. Newsletter is a summary of crop observations, related information, and appropriate recommendations for Ohio crop producers and industry. C.O.R.N. Newsletter is produced by the Ohio State University Extension Agronomy Team, state specialists at The Ohio State University and the Ohio Agricultural Research and Development Center (OARDC). C.O.R.N. Newsletter questions are directed to Extension and OARDC state specialists and associates at Ohio State.