Phyllachora maydis is a fungal pathogen that causes tar spot disease in corn, also known as maize. This fungus is an obligate parasite, meaning it requires a living corn plant to complete its life cycle. The disease is characterized by distinct black spots that can appear on various parts of the corn plant. While P. maydis has been recognized in other regions for over a century, its emergence in the United States in recent years has brought increased attention to its impact on corn production.
Understanding Tar Spot Disease
Tar spot disease manifests as small, raised black spots, referred to as stromata, on corn leaves, stalks, and husks. These spots are the fungal structures embedded within the plant tissue and have a rough or bumpy feel. Unlike other leaf spots or insect droppings, tar spot lesions cannot be easily rubbed or scraped off the plant surface. Initially, symptoms may appear as flecking and mild yellowing before the characteristic black spots develop.
Sometimes, these black stromata are surrounded by tan to brown, elliptical or circular necrotic halos, creating “fisheye lesions.” While common in Mexico and Central America, their formation in the United States may be influenced by corn hybrid genetics or environmental conditions, as the associated fungus Monographella maydis is not consistently detected here. When infections become severe, these lesions can merge, leading to extensive tissue death and premature drying of the leaves.
Life Cycle and Environmental Factors
The life cycle of Phyllachora maydis begins with the fungus overwintering in infected corn residue on the soil surface, which serves as the primary source of inoculum for the following growing season. When environmental conditions become favorable, ascospores, the infectious units of the fungus, are released from these overwintered structures. Wind and rain disperse these spores, which can travel considerable distances, potentially infecting new corn plants in the same field or neighboring ones.
Infection and disease development are favored by specific weather conditions. Moderate temperatures, typically ranging from 60 to 73°F (16 to 23°C), coupled with high relative humidity (greater than 75%) and prolonged periods of leaf wetness (at least 7 hours), create an ideal environment for the fungus to thrive. Fields with frequent cloudy days, those in proximity to large bodies of water, or irrigated areas tend to experience higher tar spot pressure due to these conducive conditions. After a corn plant becomes infected, new fungal structures can form within 12 to 15 days, leading to multiple cycles of spore production and infection within a single growing season. This polycyclic nature allows the disease to spread rapidly and intensify throughout the season when favorable conditions persist.
Management Approaches
Managing tar spot disease in corn involves a combination of strategies aimed at reducing inoculum and protecting the crop. Selecting corn hybrids with genetic tolerance to tar spot is a foundational approach, as some hybrids exhibit varying levels of susceptibility. While no hybrid offers complete resistance, choosing more tolerant varieties can reduce disease severity. Growers should consult with seed companies and review local hybrid trial data to identify suitable options for their specific region.
Cultural practices, such as crop rotation and tillage, can help reduce the amount of overwintering fungal inoculum in a field. Rotating corn with non-host crops like soybeans allows for the decomposition of infected corn residue, disrupting the pathogen’s life cycle. Tillage can bury residue, which also contributes to its breakdown, although spores can still be dispersed by wind from other sources. These practices can help, but they are generally considered to have a minor role compared to other management tools due to the long-distance dispersal capabilities of the spores.
Fungicide applications are often the most reliable management strategy, particularly when disease risk is high. The most effective timing for a single fungicide application is typically between the tasseling (VT) and milk (R3) growth stages of corn. Applying fungicides preventatively or at the first signs of disease, especially during periods of favorable weather, can provide good control. Products with multiple modes of action are often recommended for better efficacy and to help delay the development of fungicide resistance. Tools like the Tarspotter app can assist in assessing disease risk based on localized weather data, guiding timely scouting and fungicide decisions.
Significance for Corn Production
The emergence and spread of Phyllachora maydis impact corn production across various regions. Tar spot can cause yield losses, ranging from minimal to over 50 bushels per acre in severe cases, depending on disease onset, weather, and hybrid susceptibility. For instance, the United States experienced estimated losses of 241 million bushels between 2018 and 2020 due to tar spot. In 2024, tar spot was estimated to cause the greatest yield reduction in the northern U.S., accounting for 280 million bushels lost.
These yield reductions stem from the fungus’s interference with the corn plant’s photosynthetic capacity, which can lead to poor grain fill, reduced kernel weight, and even premature kernel germination on the cob. Beyond direct yield impacts, severe tar spot infections can also compromise stalk integrity, increasing the risk of lodging, which complicates harvest and further reduces recoverable yield. The disease can also negatively affect the quality of silage by reducing moisture and digestible components. Given its increasing prevalence and rapid spread, P. maydis represents an ongoing challenge for corn growers, requiring adaptive management strategies.