Tar spot is a fungal disease of corn caused by the pathogen Phyllachora maydis, and it has become one of the most damaging corn diseases in the United States since its first detection in 2015. The disease gets its name from the small, raised black spots it produces on corn leaves, which look as though someone flicked drops of tar across the foliage. Under the right weather conditions, tar spot can devastate a corn crop, with estimated U.S. losses totaling roughly 241 million bushels between 2018 and 2020 alone.
What Tar Spot Looks Like in the Field
The hallmark symptom is the stroma, a small, dark, slightly raised structure embedded in the leaf tissue. These stromata are firm and glossy, and unlike many other leaf spots, they do not wipe or rub off. They can appear on leaves, husks, and leaf sheaths. In mild cases you might see scattered black dots across the upper canopy; in severe cases, entire leaves become densely covered, turning tan and dying prematurely.
Some stromata are surrounded by a larger zone of dead, tan-colored tissue that researchers call a “fisheye” lesion. For decades, these fisheye symptoms were attributed to a second fungus, Microdochium maydis (previously called Monographella maydis), supposedly working alongside Phyllachora. But more recent work in both the U.S. and Mexico has cast serious doubt on that idea. A culture-based survey of necrotic lesions surrounding Phyllachora stromata found that the link between Microdochium and fisheye symptoms is not well documented outside of initial descriptions from the early 1980s.1PubMed. Is Microdochium maydis Associated with Necrotic Lesions in the Tar Spot Disease Complex? A Culture-Based Survey of Maize in Mexico and the Midwestern United States Molecular work in Michigan provided strong evidence that fisheye lesions do not require Microdochium at all and may instead result from secondary infections by other fungi, or from certain genotypes of Phyllachora maydis itself.2PubMed Central. Identification of Fungal Communities Within the Tar Spot Complex of Corn in Michigan via Next-Generation Sequencing The practical takeaway for growers is that seeing fisheye symptoms does not necessarily mean a different pathogen is at work; Phyllachora maydis is still the primary culprit driving the damage.
How Far and How Fast It Has Spread
Tar spot was first confirmed in the United States in Illinois and Indiana in 2015. Within just five years it had been found in Florida, Iowa, Michigan, Minnesota, Missouri, Ohio, Pennsylvania, and Wisconsin, plus southwest Ontario, Canada.3Plant Health Progress. Recovery Plan for Tar Spot of Corn, Caused by Phyllachora maydis The speed of that geographic expansion caught pathologists off guard. Based on herbarium specimens, the Phyllachora species that infect maize appear to have originated from Central America, Mexico, and the Caribbean, where the disease has been present for much longer.4PubMed Central. Phyllachora species infecting maize and other grass species in the Americas represents a complex of closely related species How exactly the pathogen arrived in the U.S. Midwest remains unclear. Phyllachora maydis is an obligate parasite, meaning it cannot be cultured on artificial media and grows only on living or dead corn tissue, which makes tracing its movement complicated.
Indiana data from 2015 through 2022 show a pattern that matters for growers in areas where tar spot has newly arrived: once it shows up in a locality, it tends to persist, and pockets of severe outbreaks become more likely in subsequent years when weather cooperates.5Plant Health Progress. Tracking the Distribution and Risk of Tar Spot of Corn in Indiana from 2015 to 2022 In other words, a single mild year of tar spot in your county does not mean the problem has passed. It means the fungus is established and waiting for the right conditions.
Weather That Fuels Outbreaks
Temperature and moisture interact in ways that help explain why some years produce devastating tar spot epidemics and others produce almost none. Research analyzing multi-year field data across the Midwest found that extended stretches of moderate mean temperatures, roughly 18 to 23 °C (about 64 to 73 °F), over a 30-day window were the most important factor driving tar spot development.6PubMed Central. Uncovering the environmental conditions required for Phyllachora maydis infection and tar spot development on corn in the United States for use as predictive models for future epidemics Hot, dry summers can actually suppress the disease.
Moisture plays a nuanced role. High relative humidity above about 90 percent during June, July, and August was positively correlated with tar spot severity in Indiana, and precipitation during June and August also tracked with worse disease.7Plant Health Progress. Tracking the Distribution and Risk of Tar Spot of Corn in Indiana from 2015 to 2022 Yet over shorter windows of two to three weeks, some moisture indicators were actually negatively correlated with tar spot, suggesting that the disease needs sustained moderate humidity rather than short drenching rains.8PubMed Central. Uncovering the environmental conditions required for Phyllachora maydis infection and tar spot development on corn in the United States for use as predictive models for future epidemics The classic recipe is a cool, humid summer with persistent dew, the kind of season that keeps leaf surfaces wet into mid-morning.
How the Fungus Survives Between Seasons
Phyllachora maydis overwinters in infected corn residue left on the soil surface. Researchers recovered heavily infected corn foliage from fields in northern Illinois in March 2019, extracted viable ascospores from the residue, applied them to corn seedlings in a greenhouse, and observed tar spot symptoms 17 days later. That was the first conclusive proof that the pathogen can survive winter in the northern U.S.9Plant Health Progress. Phyllachora maydis, Causal Agent of Tar Spot on Corn, Can Overwinter in Northern Illinois Additional work on ascospore release and germination from overwintered residue has expanded our understanding of when and how those spores become available in the spring.10Plant Health Progress. Phyllachora maydis Ascospore Release and Germination from Overwintered Corn Residue
This overwintering biology has direct management implications. Fields planted continuously to corn, or those under reduced tillage where residue remains on the surface, face a higher inoculum load going into the next season. Rotating away from corn and incorporating residue through tillage can reduce the amount of overwintering fungus, though neither practice eliminates risk entirely since spores can travel on wind from nearby infected fields.
Yield Losses and Economic Stakes
Under favorable conditions for infection, yield losses from tar spot can reach up to 100 percent, and across the U.S. from 2018 to 2020 the cumulative toll was an estimated 241 million bushels.11Plant Health Progress. Recovery Plan for Tar Spot of Corn, Caused by Phyllachora maydis The disease is now considered one of the most significant yield-limiting diseases of maize in the United States.12PubMed. Uncovering the Infection Strategy of Phyllachora maydis During Maize Colonization: A Comprehensive Analysis
The damage goes beyond direct grain loss. Severe tar spot accelerates leaf death, which weakens stalk integrity. Stalk rot and lodging tend to increase when tar spot severity is high, creating harvest difficulties and additional field losses.13Crop Protection Network. Tar Spot of Corn Growers dealing with heavy tar spot in a field sometimes need to prioritize that field for early harvest before stalks deteriorate, which complicates logistics across the whole farm.
How Tar Spot Affects Silage Operations
Grain growers get most of the attention in tar spot discussions, but livestock producers who grow corn for silage face their own set of problems. Tar spot reduces both forage yield and forage nutritive value.14Crop, Forage & Turfgrass Management. Tar spot impacts silage corn yield and forage nutritive value The disease rapidly dries down the plant, lowering whole-plant moisture. That creates trouble at the bunker: drier material is harder to pack tightly, which leads to poor fermentation. During prolonged periods of poor fermentation, other fungi can colonize the silage, including species that produce mycotoxins. No mycotoxins have been directly attributed to Phyllachora maydis itself, but the cascade from poor packing to secondary fungal growth is a real concern.15Crop Protection Network. An Overview of Tar Spot The recommendation for silage growers is to prioritize fields with high tar spot levels for earlier harvest at optimum moisture before the disease drives whole-plant moisture too low.
Fungicide Management and Timing
Fungicides are currently the primary in-season tool for controlling tar spot, but not all products are equally effective, and timing matters as much as chemistry. Multi-state trials across the Midwest have consistently shown that fungicides containing two or three different modes of action provide greater suppression than products with just a single mode of action.16Crop Protection Network. Multi-state Fungicide Efficacy Trials to Manage Tar Spot and Improve Economic Returns in Corn in the United States and Canada In separate Midwest field trials, only products combining multiple active ingredients significantly increased yield over untreated controls; single-mode products often reduced disease severity somewhat but did not translate that into meaningful yield protection.17Plant Health Progress. Fungicide Efficacy on Tar Spot and Yield of Corn in the Midwestern United States
Timing is equally critical. Applications made between the tassel (VT/R1) and dough (R4) growth stages were most effective at reducing disease severity, protecting yield, and generating a positive economic return. Sprays applied before tasseling were too early to provide lasting disease suppression.18Crop Protection Network. Multi-state Fungicide Efficacy Trials to Manage Tar Spot and Improve Economic Returns in Corn in the United States and Canada Indiana trials examining both products and timing found that under high disease pressure, the yield increase from well-timed fungicide applications averaged more than 1,000 kg per hectare, with partial returns exceeding $170 per hectare.19PubMed. Investigations into Economic Returns Resulting from Foliar Fungicides and Application Timing on Management of Tar Spot in Indiana Hybrid Corn When disease pressure was light, however, the return on that spray investment was much smaller, which is why forecasting tools have become so important for spray decisions.
Forecasting and Surveillance Tools
Because the economics of a fungicide spray depend heavily on whether an epidemic actually develops, researchers have put substantial effort into predicting tar spot risk before symptoms become obvious. The weather-driven predictive models built from multi-year data on temperature and humidity patterns have performed well: the most accurate model was validated at roughly 90 percent accuracy.20Crop Protection Network. Tar Spot Prediction in Corn: The Weather Matters These models have been integrated into the publicly available Crop Disease Forecasting web tool, giving growers and agronomists a way to check their local tar spot risk during the season.
Alongside weather-based forecasting, researchers have developed frameworks for tracking how tar spot epidemics unfold within fields over time. A data-driven model built from 2021 to 2023 epidemic data was able to accurately estimate how the 2024 epidemic progressed vertically through the canopy and temporally through the season.21PubMed. Unraveling the Dynamics of Tar Spot Epidemics in Corn Fields: A Data-Driven Framework for Disease Development and Surveillance Aerial remote sensing is adding another layer of precision. Machine-learning models trained on multispectral drone imagery have shown strong performance in estimating tar spot severity across different canopy levels, with accuracy reaching high levels of agreement between predicted and actual disease scores.22PubMed Central. Monitoring tar spot disease in corn at different canopy and temporal levels using aerial multispectral imaging and machine learning For large-scale operations, drone-based scouting could eventually replace or supplement time-consuming manual leaf inspections.
Breeding for Genetic Resistance
Fungicides work, but they are expensive, timing-dependent, and contribute to selection pressure for resistant fungal strains. The long-term solution for any crop disease is genetic resistance bred into the plant itself, and tar spot resistance research is gaining momentum. Mapping studies have identified several regions of the corn genome linked to tar spot resistance. One of the most consistently detected is a major quantitative trait locus on chromosome 8, designated qRtsc8-1, which has been confirmed across multiple independent populations and accounts for a meaningful share of the variation in disease severity among corn lines.23PubMed. Combined linkage and association mapping identifies a major QTL (qRtsc8-1), conferring tar spot complex resistance in maize Fine-mapping work has narrowed this region on chromosome 8 and identified candidate genes that encode proteins typically involved in plant defense signaling.24PubMed Central. Identification and fine mapping of a major QTL (qRtsc8-1) conferring resistance to maize tar spot complex and validation of production markers in breeding lines
More recently, additional resistance regions have been found on chromosome 1, comprising a cluster of five distinct spots that exceeded significance thresholds in two consecutive years. Gene annotation in those regions revealed a concentration of defense-related proteins, including wall-associated kinases and receptor-like kinases that plants commonly use to detect and respond to pathogens.25bioRxiv. Mapping and Genetic Dissection of Novel Tar Spot Resistance QTL on Maize Chromosome 1 These findings are still working their way into practical breeding pipelines, but the identification of stable resistance markers means seed companies now have tools to select for tar spot tolerance alongside other traits. No commercial hybrids are fully immune, but differences in susceptibility among hybrids are already large enough that hybrid selection is a meaningful layer of defense, particularly in areas where tar spot has become established.
Biological Control Research
A newer and still experimental avenue is biocontrol. Because Phyllachora maydis cannot be grown on artificial media, finding organisms that attack the pathogen in the field requires creative approaches. Researchers have isolated fungi from overwintered tar spot stromata, essentially looking for microbes that naturally colonize and degrade the structures where the pathogen survives. Several of those fungi, including species of Alternaria, Cladosporium, and the commercially available biocontrol agent Gliocladium catenulatum, reduced the number of stromata that successfully germinated compared to untreated controls.26PubMed Central. Potential Biocontrol Agents of Corn Tar Spot Disease Isolated from Overwintered Phyllachora maydis Stromata The idea is appealing: if you can knock down overwintering inoculum before the growing season starts, you reduce the spore load hitting the next crop. This work is still at the proof-of-concept stage, and no biocontrol products are specifically labeled for tar spot management in corn. But the existence of naturally occurring fungal antagonists is encouraging for growers looking beyond synthetic chemistry.
An Integrated Approach on the Farm
No single practice is a silver bullet for tar spot. The growers who manage it most successfully tend to stack several strategies together. Crop rotation away from continuous corn reduces the local inoculum reservoir. Tillage to bury residue accelerates decomposition of infected tissue, though this needs to be balanced against soil health and erosion concerns. Hybrid selection favoring lines with better tolerance can reduce the baseline level of infection. Fungicide applications timed around tasseling through dough provide the strongest in-season protection when weather forecasts or disease models indicate high risk. And monitoring tools, whether they are web-based forecasting models, drone imagery, or good old-fashioned scouting, help growers decide whether the cost of a spray is justified in a given season. None of these layers alone eliminates tar spot, but together they can keep losses manageable even in years when the weather favors the fungus.