Both southern and common rust have been confirmed in multiple corn fields across the state, but as is usually the case in Ohio, the latter is much more wide-spread than the former, and most of the affected fields are in the southern half of the state. Southern rust is characterized by the presence of small, circular, light orangish pustules predominantly on the upper surface of the leaves, whereas common rust produces larger, more elongated, and darker (cinnamon-brown) pustules on both leaf surfaces.
Common Rust Southern Rust
Most of the fields that were planted early are now between R1 (silk emergence) and late-R2 (brown silk), and as such, are less likely to be heavily impacted by rust. Common rust in particular tends to become less severe as the season progresses as it prefers cooler conditions. By contrast, late-planted fields, most of which are still between V10 and V18, are at greater risk for yield reduction due to rust. Southern rust can be very damaging if the hybrid is susceptible, symptoms develop early (before tassel), and the weather stays warm and wet during pollination and grain fill.
Under favorable conditions rust can spread very quickly on a leaf, from one leaf to another, and from one plant to the next. The rate at which it spreads is heavily dependent on the amount of spores available (blowing in from the south or from a nearby affected field) and how long it takes for each set of new pustules to produce new spores. If it stays wet and warm over the next week or so as late-planted fields approach R1, several new crops of spores will be produced, new infections will occur, and more leaves will become damaged before grain fill is complete.
Several of the commonly used foliar fungicides will provide good to excellent control of both rust diseases. However, timing is extremely important. The best results are often seen when applications are made as soon as the first few pustules are observed. This helps to reduce the rate of disease development and spread by preventing new pustules from developing and reducing the number of spores available to infect healthy leaves. Give priority to protecting the latest-planted fields as they are at the greatest risk for damage and yield loss.