Frequent rainfall and cooler-than-normal temperatures across Missouri this May are creating favorable conditions for tar spot in cornfields in June, according to University of Missouri Extension.
University of Missouri Extension plant pathologist Mandy Bish said tar spot has been detected in June during each of the past three years and is expected to reappear during the 2026 growing season.
Tar spot is caused by a fungus that survives in corn residue and develops most readily when temperatures remain in the mid-60s to low 70s for extended periods. Since Missouri’s first confirmed case in 2018, the disease has spread through multiple counties across the top two-thirds of the state and continues expanding.
The disease produces small, raised black lesions, known as stromata, on both upper and lower leaf surfaces. In severe cases, lesions can also develop on husks and leaf sheaths. Corn is the fungus’s only known host.
Bish encouraged growers to begin scouting fields early and continue monitoring throughout the season. She advised producers to evaluate disease severity, symptom frequency, lesion location within the canopy and weather conditions before deciding whether to apply fungicides.
Monitoring whether symptoms remain confined to the lower canopy or begin progressing toward the ear leaf can help determine the level of risk and potential need for treatment.
Disease forecasting resources, including the Crop Protection Network’s Crop Risk Assessment tool, may help growers make management decisions. Warmer temperatures typically approaching July can slow fungal development and reduce disease progression.
Research cited by the university indicates fungicide applications made between the VT and R3 corn growth stages are most likely to provide a positive economic return when tar spot pressure is high. Applications made before VT or after R4 have not consistently delivered economic benefits.
Bish cautioned that fungicide applications made too early could lose residual effectiveness before southern rust pressure increases later in the season.
Tar spot symptoms can resemble insect frass, Physoderma brown spot or the overwintering structures of common and southern rust. Unlike those issues, tar spot lesions are embedded within leaf tissue and cannot be rubbed off. As the disease develops, lesions become visible on both sides of the leaf.
While no corn hybrids are fully resistant to tar spot, some exhibit partial tolerance. University of Missouri Extension recommends combining hybrid selection with crop rotation and timely field scouting to determine whether fungicide applications are warranted.
Source: University of Missouri Extension, "Conditions right for tar spot in Missouri corn crops"