Tolerant hybrids are the first line of defense against tar spot

While fungicides may help reduce the impact of tar spot on yield, the best long-term solution to this devastating corn disease is selecting hybrids with genetic tolerance, experts reported at the recent Tar Spot Summit hosted by AgReliant Genetics.

″Fungicides can delay the onset of tar spot but can’t provide 100% control,″ said Damon Smith, Ph.D., University of Wisconsin professor and Extension plant pathologist. ″Although we can try to manage our way out of it, I think the real solution to tar spot is going to come with hybrids. Finding genetic disease resistance is really the Holy Grail of plant pathology.″

Sneaky spread of tar spot

Tar spot’s growth curve sets it apart from other corn diseases, making it frustrating for farmers to manage, Smith said.

″I call tar spot a sneaky disease. It starts out slowly and then grows really fast and ravages later in the season.″

Smith explained that tar spot has an incubation period 21- to 30-days after planting, followed by a 40- to 50-day latent period. The first tar spot symptoms – small black specks on leaves – typically show up during this latent period, about the VT growth stage. ″Tar spot then spreads exponentially during the reproduction stage that follows,″ Smith said. ″That’s why you can’t control it only by scouting.″

Tolerant hybrid results

Tar spot can reduce yield by hampering kernel development during critical late-season grain fill, said Missy Bauer, agronomist with B&M Crop Consulting in Coldwater, Michigan, who also spoke at the Tar Spot Summit.

Field trials by B&M Crop Consulting in 2021 demonstrated the value of hybrid selection in a tar spot management program. Plots compared performance of AgriGold hybrids that were rated either tolerant or susceptible to tar spot, both with and without fungicide treatment.

″We saw the best yield with a highly tar spot tolerant hybrid combined with a fungicide,″ Bauer said. That management combination yielded 52 more bushels per acre compared to a tar spot susceptible hybrid treated with a fungicide. Even without fungicide treatment, the tar spot tolerant hybrid yielded 27 more bushels per acre than the susceptible hybrid that received a fungicide.

Although tar spot risk varies from year to year, Bauer said: ″We believe the hybrid is going to be the number one defense.″ Bauer said farmers will need to carefully weigh hybrid selection, considering not just tar spot tolerance, but also yield and agronomic characteristics.

Expanding genetic solutions

AgReliant Genetics corn breeders attending the Tar Spot Summit emphasized their ongoing work to expand understanding of tar spot and develop AgriGold and LG Seeds hybrids to combat it. Research focuses both in the field and in the lab, said Benzon Lorenzana, Ph.D., director of corn inbred line breeding.

″We characterize our experimental hybrids based on their reaction to tar spot pressure under field conditions. That allows us to rate the genetic tolerance that already exists,″ Lorenzana said. ″We also use DNA fingerprinting to study our vast genetic library of corn inbreds, looking for regions in the genome that carry tar spot tolerance. That information leads us to new, one-of-a-kind genetic solutions.″

In 2022, AgReliant Genetics expanded its disease research capabilities, adding two new field locations dedicated to the study of tar spot tolerance. The goal is to discover new genetic hosts that bring disease tolerance, said Mike Kavanaugh, director of product development.

″For a disease like tar spot to thrive, you need to have the pathogen present, the right environment for the pathogen to grow, and also a susceptible host,″ Kavanaugh said. ″The best way to break the disease triangle is by bringing in a new genetic host.

″Although no hybrid is completely resistant to the disease, starting with a tar spot tolerant hybrid is a foundational recommendation for a systems approach to tar spot.″

Don’t abandon conservation tillage

While emphasizing the role of hybrid selection, speakers at the Tar Spot Summit cautioned against putting too much stake in residue management and crop rotation to control the disease.

″We found the tar spot inoculum moves a fair distance. You’d need a 10-mile radius of tilled ground to truly eliminate the inoculum load,″ Smith says. In addition, although some tar spot spores die over the winter, the pathogen doesn’t fully clear from fields.

University of Wisconsin researchers studied the impact of corn and soybean residue in fields with a history of tar spot. ″Even in a heavy tilled, rotated system we still see tar spot developing from the bottom and going up into the crop canopy,″ Smith said.

In seven field trial sites in Wisconsin, Iowa, Indiana, and Michigan, fields planted with tar spot tolerant hybrids had a significant reduction in disease severity, regardless of the level of crop residue, he reported. Although lower residue levels reduce the severity of tar spot to some extent, fields planted with tar spot tolerant hybrids had a mean 50% reduction in crop severity score across the seven sites.

Smith’s advice: ″Don’t throw conservation tillage out the window″ in an attempt to control tar spot.