Tar spot is a fungal disease affecting corn production across North America. First confirmed in the United States in 2015, this agricultural concern has since spread rapidly across many corn-producing states. The disease can quickly progress through a field, leading to substantial crop damage and economic loss for growers.
Identifying Visual Symptoms and the Pathogen
The disease is identified by the appearance of small, black, raised specks on the leaves, leaf sheaths, and husks of the corn plant. These specks are fungal structures known as stromata, which contain the reproductive spores of the pathogen. The spots resemble flecks of tar or black paint splattered onto the plant tissue. Unlike insect frass or other fungal diseases like rust, these structures are embedded in the leaf tissue and cannot be easily rubbed off with a finger.
In some instances, the black spots may be surrounded by a tan, necrotic ring or halo, creating a distinct feature referred to as a “fisheye” lesion. The organism responsible for the disease is the fungus Phyllachora maydis. This pathogen requires living host tissue to grow and reproduce. While Phyllachora maydis alone causes the black spots seen in North America, the secondary fungus (Monographella maydis) has not been widely confirmed in the United States.
How the Fungus Spreads and Survives
The fungus survives between growing seasons by overwintering in infected corn residue, such as leftover stalks and leaves, on the soil surface. Within these pieces of debris, the fungal stromata remain protected and viable, ready to serve as the initial source of infection for the next crop. As spring progresses, the fungus releases its primary infectious spores, called ascospores, from the overwintering structures.
The release and dispersal of these spores are highly dependent on specific environmental conditions. Infection is favored by moderate temperatures, ranging from 59 to 70 degrees Fahrenheit, in combination with prolonged periods of high humidity above 75 percent. The disease requires at least seven hours of continuous leaf wetness, often provided by rain, heavy dew, or fog, for the spores to germinate and penetrate the corn leaf.
Once the initial infection occurs, the fungus follows a polycyclic disease cycle, meaning it can produce multiple generations of spores and infect the crop repeatedly within a single growing season. Wind and rain splash are the main mechanisms that carry the spores, spreading the disease locally within a field and over long distances to adjacent areas. Under favorable conditions, new stromata can form within 12 to 15 days after infection, releasing more spores and rapidly accelerating the disease’s progression through the corn canopy.
Impact on Yield and Control Measures
The appearance of numerous tar spots on the corn leaves compromises the plant’s ability to photosynthesize, which is the process of converting sunlight into energy. A severe infection can lead to premature leaf senescence, or death, drastically reducing the effective leaf area available to fill the developing ear. This loss of photosynthetic capacity directly limits the sugar availability for grain fill, resulting in lower kernel weight and a significant overall reduction in crop yield.
In response to the energy deficit, the plant may begin to “cannibalize” sugars from its own stalk tissues to support ear development. This weakens the structural integrity of the plant, leading to poor stalk standability and increased susceptibility to lodging, where the stalk breaks or falls over before harvest. Severe outbreaks have been linked to significant yield losses, ranging from 20 to 60 bushels per acre in affected areas.
Managing the disease requires a comprehensive approach that integrates cultural practices, hybrid selection, and chemical control. Since the fungus overwinters in residue, management options like tillage can help bury infected debris, which may reduce the initial amount of infectious material. Planting corn hybrids with documented resistance or tolerance to the disease is another foundational step in minimizing risk. The most direct form of control involves the timely application of fungicides, which work to protect the plant tissue. Fungicide application is most effective when applied preventatively or early in the disease’s progression.