It is that time of year again. Soon corn will be in the ground, and the 2019 field season will be taking off. It is no surprise that I spent the majority of my time on the speaker circuit discussing tar spot in corn. We have learned a fair amount since then, but there are many more things that need to be researched and learned before we have excellent tar spot IPM management programs. However, there are a few points you should keep in mind this season that can help you determine your risk for tar spot and management practices that can help your bottom line.
The incidence of tar spot was fairly widespread last year. Incidence is simply asking the question, “Do I have any tar spot in my field?” Incidence does not incorporate the severity of infection. One could have a field with a high incidence of tar spot, yet the severity (number of lesions on leaves of plants) could be low. This link shows the tar spot incidence in 2018 :Tar Spot established in the United States-2018
If we were to estimate where the greatest severity of disease was last year, it likely tracked with the late season storms that pushed through the region in August and September. In Illinois, severity was greatest in the region North of I-90, and most severe in the north central part of the state. Increased severity likely means increased local inoculum for this season. If you are planting corn in a region that was hit hard by tar spot last season, your risk for disease is elevated compared to areas where disease was sparse or absent.
The fungus that causes tar spot overwinters in residue, and spores are released from the stromata (raised black spots on foliage, stalks, husks) at night during periods of moderate, humid weather. These spores spread locally and also can move at a minimum to nearby fields on rain and wind. If you are planting into a field of corn residue from plants that were severely affected by tar spot, you may be at increased risk for disease compared to if you are following a field that was in soybean last year or is tilled. That does not mean tar spot will not occur, as it can spread from nearby fields; however, planting after soybeans or tilled fields may reduce local inoculum levels, reducing disease onset and potentially severity. The later the disease starts, the less impact it is likely to have on your crop.
All commercially available hybrids are susceptible to tar spot, but some hybrids are more tolerant than others. No particular brand is better than another. Ask your seed dealer or check out Dr. Smith’s website for information pertaining to specific hybrids and tar spot response.
Scouting is critical for this disease. CCA’s and producers should ensure that fields are being scouted frequently and often, especially in the days/weeks approaching tasseling. If you notice tar spot showing up prior to VT, a fungicide may help. There are several products with a label or 2ee for tar spot suppression. Like rusts, this is an obligate fungus, and you want to ensure that the ear leaf and leaves above are protected during the critical periods of grain fill. You do not want to chase this disease-revenge sprays will not work.
Lastly, although tar spot is the hot topic, our most severe and widespread disease last year was, without a doubt, grey leaf spot. Do not lose sight of this disease and other diseases that are observed and encountered more frequently and consistently in Illinois. Tar spot is likely to be episodic, much like Fusarium head blight in wheat and white mold in soybeans. It may be a while before we see significant disease as we did in 2018 (I hope this is the case).