Tar spot is beginning to show up in a few corn fields across the state, but based on some of the samples received and questions asked, there is still some uncertainty about the correct diagnosis or identification of this disease, particularly during the early stages of its development. When fully developed, the stromata are easy to see and feel on the surface of leaves (or ear husks later in the season) as slightly raised black spots resembling sprinklings of tar that give the disease its name. However, for the untrained eyes, early diagnosis my require observations of the spots under a dissecting and/or compound microscope.
Early and correct diagnosis are important for tracking the progress of tar spot and deciding whether a fungicide application is warranted. Several fungicides are available for managing this disease, but the level of efficacy varies with the product, the number of active ingredients (AIs) in the fungicide, and application timing. In general, fungicides with two or three AIs of different modes of action tend to be more effective against tar spot then products with a single AI, and treatments tend to be most effective when applied at R1 (silking). Incorrect diagnosis could lead to unnecessary fungicide applications or applications being made when it is too late. See the article by Karhoff et al in this issue for more on fungicide use on corn.
If you find plants with symptoms resembling those of tar spot, please send samples to the plant disease clinic for confirmation or contact your local county extension educator or field specialist. Even if you have the experience and are sure you have the disease in your field, still send us samples or inform to the clinic. We are using this information to monitor the spread of tar spot across the state, from county to county. Visit the clinic website at https://ppdc.osu.edu for more information on handling and submitting samples. You can also contact Dr. Paul, your state specialist, at paul.661@osu (330.347.5878) or our diagnostician, Dr. Francesca Rotondo, by email at rotondo.11@osu.edu or ppdc@osu.edu, or by phone at 330.263.6721.