A recent study shows U.S. corn growers are facing an elusive yield robber in the rapid spread of tar spot infection. The pathogen infecting their fields is a multi-headed threat from related, but genetically different, species.

Tar spot is a condition blamed on a fungal infection of Phyllachora maydis and has been known to cause yield losses of up to 60 bushels per acre in untreated fields. It appears as small, raised black spots scattered across corn leaf surfaces. These fungal fruiting structures interfere with photosynthesis.

Costly Disease

First noted in the U.S. in 2015, the disease spread rapidly across much of the Corn Belt, costing an estimated 241 million bushels of production between 2018 and 2020. Today, the disease has been detected throughout the Corn Belt eastward to Pennsylvania and in the southeast in Georgia and Florida.

Until recently, scientists thought the disease was new to the U.S. and that it was not capable of infecting species other than corn. However, a study published in Ecology and Evolution in April 2022 complicates that view after studying field samples of corn residue and several other grass species.

The study indicates five genetically distinct Phyllachora species — not just Phyllachora maydis — are at work, three of which are currently found in the U.S.:

• Species 1 — Found only in corn from field samples in Indiana and Ohio.
• Species 2 — Found in dried corn samples from Columbia and Puerto Rico, as well as corn field samples from Puerto Rico, Mexico…