Agronomists have confirmed the presence of Tar Spot (Phyllachora maydis) in two Midwest states this week, reigniting concerns across the Corn Belt. This early detection in June-amid favorable conditions for fungal growth-raises the stakes for crop protection strategies, yield risk management, and the agricultural supply chain. Could 2025 echo the damage seen in previous outbreaks?

Early Emergence of a Dangerous Fungus

The Tar Spot pathogen, first reported in U.S. corn in 2015 in Indiana and Illinois, has rapidly expanded its geographic footprint. It thrives in cool (60-70°F / 15-21°C), humid environments and is characterized by small, raised black stromata-which do not rub off and often spread quickly if conditions persist. While initially considered cosmetic, the fungus has proven to be much more than a visual nuisance. During severe outbreaks, such as those in 2018 and 2021, producers experienced yield losses up to 50 bushels per acre (3.1 metric tons per hectare), particularly in fields with poor airflow and dense canopy structure.

This season, Tar Spot was confirmed in fields across northern Indiana and parts of Iowa, marking one of the earliest seasonal appearances in recent memory. The confirmation came as growers entered early vegetative stages (V6 to V8), heightening the urgency for scouting and fungicide planning.

Why Tar Spot Is a Rising Threat

Tar Spot's rapid development is driven by environmental factors and field history. The pathogen overwinters on infected corn residue, making no-till and continuous corn systems especially vulnerable. Fields with seven or more hours of leaf wetness per day, combined with cooler temperatures, provide an ideal setting for infection. The fungus not only reduces photosynthetic area but can cause premature leaf senescence, accelerating plant decline and limiting kernel fill.

Additionally, its ability to spread rapidly under high humidity conditions-often after consecutive rain events-makes it a moving target. Regions such as Illinois, Michigan, Ohio, Wisconsin, Minnesota, Missouri, and now Kansas have seen a sharp uptick in cases over the past five years. With Tar Spot being a polycyclic disease, it can cycle multiple times per season, compounding pressure on the crop.

Scouting and Detection: What to Look For

Growers are advised to begin scouting in earnest from V6 through VT stages, especially in fields with previous Tar Spot history. Initial symptoms may appear on lower leaves, manifesting as tiny, black, irregular spots that become more visible under magnification. Infected tissue does not easily detach or rub off-one of the key diagnostic differences from other foliar diseases.

Scouting efforts should prioritize humid microclimates, sheltered fields, and those with dense canopies, where airflow is reduced. Agronomists also stress the value of regular monitoring, particularly after three to five consecutive days of high moisture conditions.

Fungicide Strategies and Resistance Management

Research from land-grant universities and extension services suggests that timely fungicide applications can significantly reduce disease severity. Products such as Veltyma, Revytek, Delaro Complete, and others containing multiple modes of action have shown consistent results when applied at tasseling (VT) or early silking (R1).

However, application timing is critical. Early sprays during vegetative growth often lack longevity, while delayed applications may miss the window for effective control. Producers are urged to coordinate closely with crop advisors and monitor regional disease progression using local alerts and weather-based risk models.

Rotating fungicide chemistries and avoiding back-to-back use of the same active ingredients also helps prevent resistance development-a growing concern as the pathogen becomes more widespread.

Integrated Disease Management: More Than Just Fungicides

While chemical tools are essential, loterm management of Tar Spot will depend on integrated approaches. This includes:

Crop rotation: Introducing soybeans or small grains into corn-heavy rotations can break the inoculum cycle.

Residue management: Burying infected residue through tillage reduces overwintering inoculum.

Hybrid selection: Some corn hybrids exhibit partial resistance; choosing these can buffer against severe outbreaks.

Canopy management: Adjusting plant populations or row orientation may improve air movement, reducing leaf wetness duration.

These strategies not only curb Tar Spot but align with broader sustainable agriculture and soil health objectives-critical in the era of climate-smart farming.

Final Takeaway: A Season to Stay Vigilant

As of mid-June 2025, the reappearance of Tar Spot in key Midwest states serves as a sharp reminder of the increasing disease pressure facing corn producers. While early detection allows time to act, the window for preventative fungicide use, scouting, and adaptive planning is narrow.

The fungus's persistence underscores the importance of precision agriculture, real-time weather monitoring, and collaboration with land-grant universities and USDA programs supporting conservation practices and farm risk mitigation.

In a season where yields, commodity markets, and food security remain closely linked, every black spot on a corn leaf could have broader implications for the U.S. agricultural economy.