Understandably, tar spot has been the focus of our attention this year, as it has been detected in more than 20 counties. It is a disease that is relatively easy to identify based on visual signs and symptoms, but as we approach the end of the season, it may become increasing difficult for untrained eyes to tell tar spot apart from late stages of some other disease. Yes, tar spot, as the name suggests, is characterized by the presence of raised, black, tar-like spots called stromata predominantly on leaf blades (A). However, not all raised, black, tar-like spots on a leaf are tar spot.
Two other diseases that produce raised, blackish spots on leaves towards the end of the season are southern rust (B) and common rust (C). Both are very prevalent this year in fields with tar spot. Yes, it is true that rusts, as the name suggests, give leaves a typical yellowish-orangish rusty color, but this is the color of urediniospores, only one of several types of spores produced by corn rust fungi. As the crop begins to dry down and temperatures drop, the rust fungi will produce a different type of spore called teliospores, and these develop in raised, black, structures called telia.
In other words, rust pustules usually change from their typical rusty color to a black, tar-like color as they age. So, do not automatically conclude that you have tar spot or tar spot is the only disease affecting your crop simply because the lesions are black. Take a closer look and send samples to a lab for examination if you are unsure. This is particularly important if you are trying to compare hybrids for susceptibility to tar spot or rust, and if you want to determine whether the fungicide you applied is effect against one or both diseases.
Misdiagnosis my lead to errant conclusions. Here are a few tips to help you tell the difference between tar spot and rust telia. Tar spot stromata do not rupture the leaf or have a split on the top. In addition, they cannot be easily broken or rubbed away with your fingers. Rust telia, on the other hand, usually break or rupture the upper surface of the leaf tissue (D). In other words, they usually have a split on top and if you rub them with your finger, the spores are released, leaving your finger with a dark-rusty to blackish tinge.