Tar spot is a fungal disease that has become a significant concern for corn growers across the Midwest. The disease, caused by the pathogen *Phyllachora maydis*, manifests as small, raised, black spots on the leaves and husks of the plant. While these spots might seem like a mere cosmetic issue, they represent a complex biological process that directly interferes with the plant's ability to photosynthesize and mature grain, ultimately threatening yield potential.
Understanding the Biology of the Pathogen
The trouble with tar spot begins long before the spots are visible to the naked eye. The fungus overwinters on infected corn residue left in the field, providing a constant source of inoculum year after year. When environmental conditions—specifically prolonged periods of leaf wetness and temperatures between 60°F and 80°F—align, the fungus releases spores. These spores are splashed by rain or carried by wind onto the leaves, where they germinate and establish infections within the leaf tissue.
How the Disease Physically Manifests
As the infection progresses, the fungus develops within the leaf, eventually pushing black, fruiting bodies (stromata) through the leaf surface. These structures are what give the disease its name, as they resemble specks of tar pressed onto the green tissue. Unlike some other corn diseases that cause general yellowing or necrosis, tar spot is characterized by these distinct, firm, and raised black spots. Importantly, the fungus grows between the top and bottom layers of the leaf, directly disrupting the critical cellular machinery responsible for photosynthesis.
The Direct Impact on Photosynthesis
The primary mechanism of damage is the destruction of leaf tissue. Photosynthesis occurs in the chloroplasts of the leaf cells, and as the fungus expands, it kills the surrounding cells. This death of tissue reduces the total leaf area available to capture sunlight, directly limiting the plant's ability to produce the sugars necessary for growth and grain fill. A corn plant relies on these sugars not only to fill the kernels but also to maintain overall health and defend against other stressors. When the leaf surface is compromised, the entire energy production system of the plant is weakened.
Progression to Husk and Ear Impact
The negative effects do not stop at the leaves. If the infection is severe, the spots can cover the husks, which play a vital role in protecting the developing ear and facilitating the drying process. Damage to the husk tissue can lead to premature drying, which might seem beneficial for harvest but actually leaves the ear vulnerable to insects, mold, and other mycotoxins. Furthermore, if the fungal growth extends into the ear leaf or impacts the ear directly, it can interfere with kernel development, leading to smaller, shriveled kernels that test low in test weight and market value.
