The Danger of Tar Spot Tunnel Vision

According to the Crop Protection Network, tar spot was the leading cause of corn yield loss in 2024. This disease has dominated recent headlines — and for good reason — but it’s not the only disease threat to watch for. Southern rust, northern corn leaf blight and frogeye leaf spot each caused millions of lost bushels in 2024. To protect your soybean and corn crops, know the signs to look for because destructive diseases could be lurking in the canopy.

Septoria brown spot (Septoria glycines) starts with small, brown spots on lower leaves approximately two to three weeks after planting. These spots expand and merge together, ultimately reducing the plant’s capacity for photosynthesis. Without intervention, leaves will turn yellow and drop off due to the lack of chlorophyl.

“Septoria brown spot has become more aggressive,” Tyler Harp, technical product lead for fungicides at Syngenta, says. “It used to stay at the very bottom of the canopy where it wasn’t considered yield-limiting, but that’s changed in recent years.”

Harp says some growers attribute the change to new soybean varieties while others think the disease evolved into a more aggressive pathogen. Either way, Septoria brown spot can now move all the way to the top of the plant and potentially defoliate the entire crop. Active disease in early to mid-pod fill is the most likely period for brown spot to result in yield loss, so scouting the crop’s lower leaves and treating early are key to minimizing its impact in the field.

Cercospora Leaf Blight (Cercospora kikuchii) begins at the top of the plant and works its way down, making scouting theoretically a little easier. Unfortunately, its tiny, pale purple spots are tough to see early on. Appearance of the disease typically coincides with seed set, and as the lesions expand and coalesce, leaves turn darker purple, bronze or red and develop a leathery texture, sometimes confused with abiotic stress such as sunburn.

Planting resistant seed, choosing later-maturing varieties and applying fungicides with different modes of action can help reduce damage or yield loss. Under severe disease pressure, losses of up to 30 percent have been reported.

Frogeye Leaf Spot (FLS) (Cercospora sojina) presents as small light brown or tan lesions surrounded by a darker margin, typically purple or brown. When first reported in the U.S. in 1924, it was primarily an issue in the southern states but is now well entrenched in the Midwest.

“Frogeye is not the most common disease, but it’s probably the most yield limiting and the most destructive where it occurs,” Harp says.

Blake Miller, Syngenta ASR in west central Illinois, calls FLS his region’s number one soybean disease threat, and says scouting for the disease should focus on the expanding leaves. “It affects the newest leaves because they’re easier for the spores to penetrate,” he says.

Genetic resistance to FLS is the best option for managing the disease, Miller says. He advises using soybean varieties with the Rcs3 gene, which is shown to be resistant to the different strains of FLS. In conjunction with seed varieties, Miller recommends an R3 to R4 fungicide application to protect grain fill.

Southern Rust (Puccinia polysora) spreads differently than other corn diseases. The inoculum for Southern rust doesn’t live in the field; its spores are carried on the wind from warmer climates and deposited on crops as far north as Canada. The fungus needs both a wet leaf surface and warm temperatures to catalyze infection, and as little as six hours of dew can be enough for it to take hold. Southern rust appears as orange pustules on leaves, which release spores to spread the disease. The infection cycle is short but repetitive, so the infection can quickly reach epidemic levels.

Aaron Kromann, ASR in South Dakota, says his state usually sees minimal levels of Southern rust; however, last year the infection came earlier and was reported in roughly a third of the state.

“It takes a lot of wind and a lot of weather patterns to get it up here, and it’s usually not super impactful just because it sets in relatively late,” Kromann says. “But last year, the southern half of North Dakota saw some Southern rust as well, which is very atypical.”

Management practices for Southern rust include planting resistant hybrids and utilizing early application fungicides when the first pustules appear.

Northern Corn Leaf Blight (NCLB) (Exserohilum turcicum) presents as cigar-shaped lesions of varying length on leaves, which may also contain visible black or gray spores. The disease reduces plants’ photosynthesis, compromising their ability to produce kernels.

Contamination with inoculum in the field through wind, rain-splash, heavy dew or overhead irrigation can kick off the infection; continued moisture and 60- to 80-degree temperatures drives the spread.

Lesions on or above the ear leaf two weeks on either side of tassel signals potential yield loss of up to 30 percent in hybrid corn, and greater loss potential in specialty varieties. Loss will usually be lower if lesions don’t appear on the upper leaves until late in the season.

Management practices for NCLB include selecting moderate-resistance hybrids to delay progression enough to protect yield, managing residue to reduce field inoculum and a one-year rotation away from corn followed by tillage to protect the following year’s crop. “Generally, rotation’s going to be your friend,” Kromann says. “Waiting a year, you’ll likely get some depletion of that residue in the soil.”

Gray Leaf Spot (GLS) (Cercospora zeae-maydis) Is one of the most widespread and tenacious fungal diseases in corn. It attacks the plant’s leaves, first showing up as small brown spots with yellow halos. Lesions take on a rectangular shape as they grow and turn gray as the disease develops, destroying photosynthetic leaf surface.

GLS is typically first seen on bottom leaves after inoculum has splashed up onto new plants and thrives in temperatures in the mid-70s to high 80s with humidity at or exceeding 90 percent.

Managing residue and planting resistant hybrids is recommended, especially in corn-on-corn operations.

“There are hybrids that are tolerant of gray leaf spot,” Harp says. “You might get a bit of a yield penalty as a result, but the penalty will outweigh the disease issues if you have a real susceptible area.”

For more information on recognizing and defending against corn and soybean diseases, visit BoostYourBushels.com.