Black Spots in Corn? Southern Rust, Tar Spot and More

By Tamra Jackson and Kyle Broderick

While walking through corn fields in Nebraska this year, you may find black spots are more common, but they may not be what you expect. Wetter conditions in many areas of Nebraska in 2024 have generally been more favorable for diseases. Tar spot, late (teliospore) stages of southern rust, Physoderma brown spot, and others can cause dark or black spots that are often difficult to differentiate. Although it’s too late to treat with fungicides and expect economic returns this season, it’s important to note which diseases are impacting fields because most pathogens overwinter and will cause disease again in future years when conditions are favorable again. Accurately identifying diseases during the season NOW is your first step toward effective management, whether it’s selection of disease resistant corn hybrids this winter, crop rotation with non-host crops, budgeting for a possible fungicide application next year(s), or more.

Tar Spot

Tar spot (Figure 1a and 1b), caused by the fungus Phyllachora maydis, is a relatively new corn disease in Nebraska — first confirmed in 2021. Since then, the fungus has overwintered and continued to cause tar spot under favorable conditions and is spreading further west. Distribution of tar spot is documented at the Tar Spot IPMPiPE website. To date, tar spot has been confirmed in corn fields in counties about halfway (east to west) across the state in 2023, roughly along the Highway 281 corridor from about Hastings to O’Neill (counties shaded gray). As expected, tar spot has redeveloped in some of those same counties this growing season (counties shaded gold).

Figure 1a. The classic symptom (sign) of tar spot is the presence of black, raised fungal structures on the leaf or husk surface, called ascomata or stromata. Young, smaller spots are very small and may only be on the upper leaf surface.

Figure 1b. Over time, spots lengthen and may appear pointed and grow through the plant so they also visible on the underside of the leaf. These spots will not rub off the leaf surface (like insect frass).

Tar spot produces small, raised black spots across the upper and lower leaf surfaces that cannot be scraped off the leaf surface. These black structures are called stromata or ascomata (a type of fungal fruiting structure) and may be circular or elongated. A tan to brown halo may surround the ascomatum, giving it a “fisheye” lesion. The fungus overwinters in our fields once disease develops. Tar spot will continue to develop on corn plants in the same areas this year and will likely continue spreading to other locations, possibly continuing westward.

Southern Rust and Common Rust

Rust diseases are common in Nebraska corn this year. Common rust (Puccinia sorghi) prefers cooler temperatures (especially averaging about 70°F) and developed earlier in the 2024 growing season and to a greater extent than normal. Dark red to brown spores can be produced on the top or bottom of leaves.

In addition, southern rust later developed and became important in some areas of the state. Southern rust (Puccinia polysora) is favored by warmer temperatures (especially averages about 80°F) and developed in mid-July in Nebraska to a wider distribution than is normal.

Southern rust pustules are generally smaller than common rust, occur mostly on the upper side of the leaf and produce orange to tan spores. Rust spores can be rubbed off pustules with a fingernail and often leave a visible mark on your finger.

It's important to note that BOTH rust fungi produce black spores later in the season (now) alongside existing rust pustules and drastically change their appearance on leaves. Both common rust and southern rust produce black, thick-walled teliospores (Figure 2a and 2b) late in the season that can easily be confused with tar spot.

Figure 2a. Southern rust and common rust fungi can produce black spores late in the season. Southern rust was common in some Nebraska fields in 2024 and the presence of black telia and teliospores will make identifying tar spot stromata on leaves more difficult. Remember that black rust telia are usually produced adjacent to or around old rust pustules in rings or horseshoe shapes.

Figure 2b. A closeup of black rust telia on a corn leaf.

Neither fungus causing the rust diseases can overwinter in Nebraska, so having disease will not impact its development in your fields in the next growing season(s). However, rust diseases develop on corn in the subtropical areas of Mexico and extreme southern U.S. states and the spores are blown further north by south winds. Southern rust tends to be a bigger threat to dent corn (than common rust) and its distribution is tracked on the Southern Rust IPMPiPE website.

Physoderma Brown Spot

Physoderma brown spot (Figures 3a-c) (Physoderma maydis) lesions may appear black in the midrib, leaf collars and sheaths especially, but are often brown to tan on the leaf blades. Spores are inside lesions and won’t rub off. Infection occurs earlier in vegetative stages. Lesions often appear in banding patterns across the leaves, but their appearance is different depending upon whether they’re on the leaf blade versus in the midrib, where they may appear black and larger.

Saprophytic Fungi

Beneficial fungi colonize dead or senesced leaf tissue (not green leaves) and may also create black spots that can be confused with tar spot. The margins of their spots are not as clearly defined as those of disease lesions. These fungi help to break down plant residue to recycle it in the soil for nutrient release. These saprophytes usually have a dusty appearance on leaves and may not produce raised bumps seen with tar spot or rust pustules.