Tar spot in corn is a fungal disease that has become a significant concern for growers across the United States and parts of Canada. Caused by the pathogen *Phyllachora maydis*, this disease first appeared in the country in 2015 and has since spread rapidly. It manifests as distinct black, raised spots on the leaves, and while it might look like simple dirt splatter, it is a living organism that actively feeds on the plant's tissue.
Understanding the Fungal Pathogen
The culprit behind tar spot is *Phyllachora maydis*, a ascomycete fungus that survives exclusively in living or dead plant material. Unlike some other corn diseases that thrive in wet conditions, the fungus responsible for tar spot prefers cooler temperatures and high humidity. The black spots seen on the leaf surface are the fruiting bodies of the fungus, known as perithecia, which produce spores that can infect new leaves or move to the next growing season.
Identification and Scouting Techniques
Accurate identification is the first step in managing this disease. The spots are the primary diagnostic feature; they are firm and raised, often with a tan or brown halo surrounding the black center. These spots can merge over time, creating large, necrotic areas that give the leaf a scorched appearance. Scouting should begin at the early dough stage and continue through maturity, focusing on the lower leaves before moving up the stalk.
Distinguishing from Look-alikes
Tar spot is frequently confused with other common leaf spots or even chemical damage. Gray leaf spot, for instance, presents as rectangular lesions bounded by veins, whereas tar spot spots are more circular and raised. Dust or dirt might wash away, but tar spot lesions remain firmly attached to the leaf surface. Misidentification can lead to unnecessary pesticide applications or missed opportunities for effective disease control.
Disease Cycle and Environmental Triggers
Understanding how the disease spreads is vital for implementing control strategies. The fungus overwinters on infected corn residue left in the field. When temperatures drop below 50°F (10°C) and humidity remains high, spores are released and can be carried by wind to new, healthy foliage. Corn hybrids vary widely in their susceptibility, and specific environmental conditions can dramatically accelerate the severity of an outbreak.
Optimal temperature for infection ranges between 60°F and 80°F (15°C to 27°C).
Prolonged periods of leaf wetness, often caused to fog or dew, facilitate spore germination.
No-till or reduced tillage systems that leave residue on the surface increase the risk.
Planting susceptible hybrids in regions with a history of the disease is a primary risk factor.
Impact on Photosynthesis and Yield
The direct impact of tar spot on yield potential is linked to its interference with photosynthesis. When leaves are covered in spots, the plant's ability to capture sunlight is reduced, leading to premature leaf death. This is particularly damaging when the disease affects the upper canopy during the grain-fill period. Studies have shown that significant yield losses can occur when lesions are widespread, potentially reducing yields by 20 to 60 bushels per acre depending on the timing and severity.
Management and Resistance Strategies
Integrated Pest Management (IPM) is the most effective approach to controlling tar spot. Since the fungus survives on residue, crop rotation to a non-host crop like soybeans can reduce the initial inoculum load. Selecting hybrids with documented resistance is a long-term solution, although widespread resistance genes are still limited. Fungicide applications may be necessary in high-risk environments, but they must be timed correctly to protect the upper leaves during critical growth stages.
