By John Schoenhals, Pioneer Field Agronomist, Northern Ohio
Tar spot is a relatively new corn disease for Ohio, as well as the rest of the United States. It is caused by a fungus and appears on corn leaves (as well as husks under severe cases) as small, raised black bumps that cannot be rubbed off.
Tar spot has historically been present in corn-growing regions of Mexico and Central/South America. These areas are often at higher elevations, with a similar climate to much of Ohio and the Midwest Corn Belt. Tar spot was found in the U.S. for the first time in 2015 in Illinois and Indiana. The disease was first found in Ohio in 2018, and has been found in much of the state in 2021.
Periods of moderate temperatures (60-75 degrees F), high humidity (above 85%), and leaf wetness exceeding 7 hours (heavy dews, foggy mornings, frequent rainfall) present the most ideal environment for disease development. If tar spot is present and these environmental conditions exist, disease progression can be rapid.
While it is unclear what initially brought tar spot to the Midwest, this disease can and does overwinter on infected corn residue in Ohio. When favorable conditions exist in the spring, spore production begins. Rain splash and wind causes spores to move and infect growing corn leaves. It takes about 14 to 20 days from the infection of a corn leaf until black spots (fruiting structures) appear on the leaf. These black spots then produce more spores, allowing the disease cycle to continue. Spores can be moved in the wind, and multiple cycles of spore production can result in rapid spread of the disease within and between fields.
Tar spot is an aggressive disease that can rapidly lead to plant health deterioration when pressure becomes severe. The exact mechanisms of tar spot are not clearly understood, but as tar spot pressure increases, corn leaves become covered with black spots which often leads to large areas of necrosis (brown tissue). This directly impacts the ability of the plant to capture sunlight for photosynthesis to complete grainfill. As leaves shut down and photosynthesis becomes inefficient, plants begin to move sugars stored in the stalk to meet the demands of the ear. This cannibalization leads to stalk rots, ultimately ending in standability concerns. In addition, plants may die early, leading to yield loss and low test weight. Portions of northern Ohio are experiencing severe tar spot complications this fall.
Due to the amount of inoculum (ie, infected corn tissue) currently present in Ohio, tar spot management decisions for 2022 will begin this fall. The following are the most important management factors to consider.
- Hybrid selection: There are great differences in plant health and tar spot tolerance between hybrids. Complete resistance to tar spot does not currently exist in commercial hybrids, but selecting genetics with strong tolerance to tar spot and good staygreen is a valuable management tool. Yield differences between hybrids of over 50 bushels per acre have been attributed to differences in tar spot tolerance under severe pressure.
- Fungicides: Harvest data this fall will help refine this for 2022, but it is clear that fungicides are a valuable tool in managing tar spot. Using fungicides as a preventative measure (ie, applying prior to visible symptoms) has shown much more promising results than applying once symptoms are present. A 2-pass fungicide program may be warranted in some cases. New planter-applied fungicide products are also showing promise for helping to manage tar spot and other diseases.
- Residue management/crop rotation: Since tar spot overwinters in infected corn residue, anything that reduces residue can be of value; however, it will not be possible to eradicate tar spot with tillage. High residue and corn-back-to-corn scenarios will pose a higher risk of tar spot, so hybrid selection and fungicide programs will be of heightened importance.