A new fungal disease could rival the devastating impact that tar spot has delivered to US corn acres since the pathogen established itself in the Midwest, according to a new report from Farm Progress.

The warning lands at a moment when tar spot itself remains one of the most closely watched yield threats in American corn production. Discovered in Illinois and Indiana in 2015, the disease has since spread across the Corn Belt and into Canada, cutting yields by as much as 60 bushels per acre in severe outbreaks. Any newcomer described as comparable in potential damage demands immediate attention from growers and crop advisers.

Farm Progress, which published the assessment, did not name the pathogen in its headline, but the framing alone signals how seriously plant pathologists now view the shifting disease pressure in corn. Fungal pathogens have repeatedly demonstrated their ability to move from curiosity to major economic threat within a handful of seasons — tar spot being the defining recent case.

Why does the comparison with tar spot matter?

Tar spot's history explains the alarm. For decades, the disease was confined to Latin America, where cool, humid conditions in high-elevation corn regions kept it endemic but contained. That changed when it appeared in the continental United States. Within five years of the 2015 detection, it had reached major corn states including Iowa, Wisconsin, Minnesota, Michigan, Ohio and Missouri.

The pathogen, Phyllachora maydis, thrives on prolonged leaf wetness and moderate temperatures. Under those conditions it can cycle rapidly, producing the characteristic black stromata that dot leaves and shut down photosynthesis. Severe infection drives stalk lodging and significant test-weight losses.

Economic damage follows fast. University trials have documented yield losses ranging from modest single digits to more than 40 bushels per acre where infection hit early and fungicide timing missed. The US corn industry responded with an aggressive research push — sentinel plots, disease forecasting models such as Tarspotter, and fungicide trials — but the pathogen still forces routine fungicide applications across millions of acres.

A second disease with comparable potential would multiply that management burden: more scouting, more spray passes, tighter timing windows and added cost per acre in an already thin-margin year.

What should growers watch next?

The immediate practical step for farmers is vigilance. Historically, new corn pathogens in North America have been caught first by scouts and agronomists in sentinel plots and university trial fields, not in laboratory surveillance. Growers who find unfamiliar lesions, leaf spotting or unusual canopy decline should flag it quickly and contact their state extension plant pathologist, since early identification shapes both quarantine response and research priority.

Fungicide strategy also matters. Where tar spot is established, extension guidance stresses application timing tied to disease development rather than calendar dates, typically around VT to R2 growth stages when weather models indicate risk. Any new pathogen with similar epidemiology would likely reward the same discipline: timely application, full coverage and product selection based on university trial results.

Seed companies and fungicide manufacturers will be watching too. Tar spot prompted rapid development of tolerant hybrids and multi-mode-of-action fungicide programmes; a comparable threat would trigger a similar race among suppliers, with hybrid ratings and trial data becoming a central buying consideration within a couple of seasons.

The full details of the Farm Progress report — including the identity of the disease, affected crops and affected regions — merit close reading by corn growers this season. Watch extension bulletins and crop-disease monitoring networks in the coming months for confirmed sightings, and treat any unfamiliar foliar symptoms in corn as a reason to sample early rather than spray blind.