A couple of years ago, tar spot made news as a new corn disease in Indiana. Plant pathologists thought it was a novelty and likely not a major threat. After 2018, they’re taking another look. Tar spot caused considerable damage and yield loss in parts of Indiana, Illinois and several other Midwestern states.

Darcy Telenko, the new Purdue University Extension plant pathologist, is taking the disease seriously enough that she wanted it included in the 2019 Purdue Corn & Soybean Field Guide. The Purdue guide has been a crop scouting staple for many people for two decades. It’s also available in app versions — one for corn and one for soybeans, at minimal cost. Both are based on the print guide.

If you haven’t updated your copy that’s in the glove box of your pickup, or in your tractor or sprayer cab, do it now. Corey Gerber, Purdue agronomist and director of the Crop Diagnostic Training and Research Center, says copies are available through the Purdue Extension store at edustore.purdue.edu.

The newest edition of the Purdue guide includes two pictures of tar spot. Telenko says symptoms include small, black, raised spots that can appear on both sides of leaves, on leaf sheaths and on ear husks. The disease can appear anytime between July and October.

Yield impact is unknown, Telenko reports in the Purdue guide. Based on early work with this disease, which is caused by a fungus, it appears that some hybrids are more susceptible than others. You can find another reference on tar spot, BP-90-W, in the Purdue Extension store.