Seed Companies React When Crop Threats Loom

Michael Chandler vividly remembers flying into Chicago O'Hare airport the first week of September 2018 and looking down to see a corn crop that had literally died on the stalk.

"I'd been gone 10 days, and, in that time, the crop had completely senesced. It was eye-popping," says Chandler, Corteva Agriscience corn research lead for the northern U.S. Corn Belt and Canada.

By mid-September of that year, tar spot, a relatively unknown disease at the time, had ravaged corn throughout northern Illinois, pockets of Indiana and southern Wisconsin. Chandler, who also runs a corn-breeding program out of Janesville, Wisconsin, recalls that by early fall, infested corn fields looked as though the calendar had flipped to Thanksgiving. Bleached leaf remnants and dry-husked ears drooped from brittle stalks.

While the disease hasn't been as yield-limiting in years since, its range has expanded across the Corn Belt as far west as Kansas, Nebraska and South Dakota, and as far east as Maryland, New York and Pennsylvania. Southern states like Florida, Georgia and Kentucky have also confirmed this disease. Survey work by the Crop Protection Network (CPN) estimates tar spot reduced yields by 116.8 million bushels in the United States in 2022, more than twice the amount caused by any other disease.

Tar spot has also become the poster child for how an emerging crop threat can explode seemingly overnight and send seed companies seeking solutions. Prior to the 2018 epidemic, little field screening had been done in the U.S. for corn germplasm tolerance to tar spot.

"We instantly saw genetic variation in terms of tolerance versus susceptibility for that disease," Chandler says. "At Corteva, a rich history of breeding and understanding the differences in genetic families allowed breeders to recombine within the breeding pipeline and expand upon those tolerances."

Land-grant university trials have also been integral to discovering that no hybrid is resistant to tar spot, but some are less susceptible.

"Our quick answer for tar spot right now is hybrid selection and a timely fungicide application," says Darcy Telenko, Purdue University plant pathologist. "Maybe 20 years from now, tar spot will be like a lot of diseases, and we'll have bred hybrids to the point where it is a minor issue," she says. "But, that takes time and multiple levels of support."

CHECK HYBRID RATINGS

Chandler sees progress. "We're seeing heritable data that repeats year on year, and we're increasingly confident that the tar spot ratings we're putting on commercial hybrids and marketing to growers are accurate," he says. Applying expertise gained through characterizing other diseases such as Northern corn leaf blight and gray leafspot has afforded Corteva researchers the ability to leverage well-established best practices to get a jump on the breeding strategy for tar spot, he adds.

Mike Kavanaugh, director of product development for AgReliant Genetics, says developing seed isn't quite like other crop inputs that fit into a jug. "Producing pesticides are challenging enough, but creating a living thing is another level," he says.

"First, you must make sure the plant disease or threat you are screening for is present when making disease characterizations. Even if you find a population that is tolerant to those threats, there may be some other issue that knocks the hybrid out of contention for commercialization. It may not have standability or have low grain quality, or yield 10 bushels off the hybrid leaders," Kavanaugh says.

"A hybrid can be bulletproof for one problem, but farmers won't plant it if it has another weakness. Add geographical differences, too -- you might address tar spot but not Goss's wilt or green snap. That's a lot of complexity to come together in one little seed," he adds.

ROBUST RESEARCH PIPELINES

Every seed company has its own secret sauce for bringing innovations to market. At AgReliant Genetics, hundreds of thousands of hybrid candidates are evaluated each year in the research pipeline for AgriGold, LG Seeds and Pride Seeds. Some 200 pre-commercial research (PCR) locations spread across different yield environments test products that align with challenges important to that area. Kavanaugh says about 120 hybrids advance to final PCR trials, and about 30 hybrids eventually meet the standards for introduction.

AgReliant Genetics corn breeder Mike Popelka believes access to a global germplasm pool through collaboration with parent companies KWS and Groupe Limagrain diversifies genetics, which broadens solutions.

"We know there's some variability between susceptible and tolerant with tar spot. As hybrids go through testing, we keep adding more phenotyping (visual information) as to where hybrids rank in that spread," he explains. "Ultimately, DNA fingerprinting and genotyping will help better understand the mechanisms behind the disease."

GETTING A GRIP ON SOYBEAN CYST

It's been nearly 70 years since soybean cyst nematode was discovered in North Carolina, and the first varietal resistance was introduced in 1965. However, SCN is still the leading cause of soybean yield loss.

Heavy reliance on a single source of native genetic resistance with the PI88788 gene in commercial soybean lines is resulting in nematode populations that can overcome or partially overcome the resistance. There's urgent need for new answers to this old problem.

"The primary reason PI88788 was used for so long is that as a single gene, it was easy to breed with," says Tim Pruski, BASF soybean breeder. "And, it was really effective for a long time. But, nothing lasts forever. Nature finds a way."

Another source of resistance called Peking is becoming more available in commercial varieties, but it requires a minimum of two genes to have base resistance. Making sure other attributes, such as yield, aren't compromised in the breeding process becomes more complicated, Pruski notes.

BASF is currently developing a Bt trait, a Cry14Ab protein, as the first genetically modified solution for nematode control. The nematode-resistant soybean (NRS) will be stacked with PI88788 initially. Pruski says BASF also hopes to have a Peking option available.

"Five years of internal field trials within our trait development group indicate there's an 8% yield improvement over today's SCN resistant varieties," Pruski says. Last year, the company evaluated 30,000 Enlist E3/NRS lines, and slightly more than 4,000 made it to 2023 yield trial testing. The NRS lines are expected in commercial lineups by end of decade, pending regulatory approvals.

BASF has entered into trait licensing agreements with other leading soybean-breeding companies to ensure that varieties will be broadly available with the NRS trait in a stack with Enlist E3.

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