Beware of tar spot in corn fields

This year’s corn crop looks good in most areas across the Midwest. Much of it has tasseled and progressed to the reproductive stage of pollination. This is a critical time for the plant both in terms of yield potential and disease exposure.

Although the current hot and dry weather pattern is not conducive to tar spot development, it has been identified in Minnesota, Iowa, Wisconsin, Indiana, Illinois, Nebraska, Kansas and Missouri at this point in the 2026 growing season.

What is tar spot?

Tar spot is a foliar disease of corn caused by the fungus Phyllachora maydis, which first appeared in the U.S. in 2015 and subsequently spread throughout much of the Corn Belt. The tar spot fungus requires living corn tissue to grow and reproduce. The pathogen produces stromata, which are slightly raised black lesions that look like spots of tar on the corn leaf. In many cases, stromata develop on both the upper and lower surfaces and are embedded in the tissue, giving the leaves a rough or bumpy feel. Stromata cannot be wiped off the surface of the leaf. Under favorable conditions, tar spot spreads from the lowest leaves to the upper leaves, leaf sheathes and, eventually, the husks of developing ears.

Once a plant becomes infected, stromata can develop within 2-3 weeks, depending on environmental conditions and host susceptibility. These stromata then produce additional spores, enabling multiple infection cycles within a single growing season when conditions remain favorable.

Favorable conditions for tar spot

Favorable conditions for tar spot include cool temperatures (60-75 degrees), high relative humidity, frequent cloudy days and extended periods of dew at night. Research has demonstrated a complex relationship between moisture and disease development, with prolonged periods of high humidity or excessive rainfall negatively associated with disease development or spore release.

Duration of leaf surface wetness appears to be a key factor in the development and spread of tar spot. In areas affected by tar spot, farmers with irrigated corn have experimented with irrigating at night to reduce the duration of leaf wetness.

Spores are dispersed primarily by rain splash and short-distance wind movement, making local field-level inoculum an important factor in early infections. Longer-distance spore dispersal between fields also appears to occur.

Research has confirmed the pathogen that causes tar spot can overwinter in corn residue, allowing stromata to persist and inoculate the corn in the following growing season. However, the correlation between the amount of residue on a field’s soil surface and disease severity the following year is unknown. Spore viability varies depending on environmental conditions and residue degradation.

Impact on the corn plant

Corn is susceptible to infection at all growth stages; however, tar spot is most commonly observed during the reproductive stages from silking through to late grain fill or black layer. Research indicates the greatest losses occur when disease develops before or during early reproductive stages from tasseling through kernel blistering.

Early infections may be observed in the mid-vegetative stages but may not cause significant losses if environmental conditions are not favorable for repeated spore dispersal and disease progress. When disease develops late, especially after the dough stage, yield impacts are often minimal.

Disease severity can increase rapidly under cool and wet conditions. Premature leaf death may occur once severity reaches approximately 30%, reducing photosynthesis capacity during grain fill. On susceptible hybrids, severity on ear leaves can exceed 50% by the time kernels start to dent. Yield losses result from reduced photosynthetic leaf area, poor kernel fill, reduced ear weight and premature plant deterioration of physical and cellular functions. Moderate tar spot infestation typically results in approximately 10% to 30% yield loss, while severe cases can result in 50% yield loss.

Severe tar spot can reduce silage corn feed quality by rapidly reducing moisture, decreasing digestible components and reducing available energy. High levels of tar spot can reduce whole plant moisture, making silo bunker management difficult and increasing the length of time to complete fermentation. High disease severity may also increase the risk of stalk rot and lodging, particularly when plants age prematurely. Care should be taken to harvest silage at optimum moisture, prioritizing fields with high levels of tar spot first.

To date, no mycotoxins have been associated with tar spot directly. However, during prolonged periods of poor fermentation, additional fungi can continue to grow, including those that produce mycotoxins.

Reducing risk for tar spot

Hybrid selection for genetic resistance to tar spot and foliar fungicide treatments are key management tools. Several foliar fungicides are labeled for control of tar spot in corn. Research suggests that tar spot may be challenging to control with a single fungicide application due to its rapid reinfection cycle, particularly in irrigated corn.

Work with your agronomist and crop advisory team to monitor tar spot development and devise a proactive plan to combat it.

Barry Visser is a nutritionist for Vita Plus.