Soybean School: Fighting phytophthora with genetic resistance

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Rainfall during the growing season is often a major determinant in how severe phytophthora root rot is in soybeans, but there are several tools growers can deploy to build a strong defence against the underground yield robber, including genetic resistance.

As soybean harvest winds up in Western Canada, and attention shifts to choosing varieties to grow next year, this Soybean School episode delves into how growers can stay ahead of phytophthora using a combination of race-specific resistance and general field tolerance.

As with any race-focused genetic resistance, whether we’re talking about blackleg or clubroot in canola, or phytophthora in soybeans, it’s about using genes that are resistant to the disease pathotypes that are present in a field, explains Allan Froese, product selection lead with Syngenta, based in Carman, Manitoba.

Research funded by Manitoba Pulse and Soybean Growers (MPSG) in 2016 found phytophtora races 3, 4, 25, and 28 were present in the province. In 2016, 4 was the most common, but in 2017 race 25 was the most prevalent. Since then, MPSG has also reported the discovery of a new pathotype with 1d resistance.

As it turns out, one of the genes used for phytophthora resistance in soybean varieties in Canada and the U.S. — the Rps1A gene — isn’t effective against any of those major races found in Manitoba, explains Froese. “So even if you have a gene, it may not be acting on the major races that you have.”

“Genes like (Rps) 1C, 3A, 1K — all of those have more effectiveness on the races that we have here,” he says.

MPSG’s research shows the Rps 6 gene also provides resistance to the races that have been detected in Manitoba, including the new pathotype. (Information on the Rps genes that soybean varieties currently available in Western Canada carry, if any, can be found in MPSG’s variety guide and the annual Seed Manitoba guide.)

As the pathogen evolves and the dominant race changes, growers and agronomists should closely monitor how well the genetics in a variety perform.

“With any pathogen, like clubroot, like blackleg, you can throw as many genes at it as you want, but it will evolve over time. We’re going to have to stay ahead of this,” explains Froese. “We’re not in a bad situation right now, but in the future this is something we’re going to want to monitor through rotations, and even monitoring which resistance gene you’re using on that field.”

Breeders are also starting to stack multiple phytophthora resistance genes in a single variety, similar to what’s been done in canola for blackleg and clubroot.

“We’ve been focusing on that for several years and will be bringing products forward very soon with stacked resistance to help with all the phytophthora issues not only here, but across Canada,” says Froese.

On top of race-specific resistance, different varieties also have different field tolerance — the term used to describe when multiple genes in combination help the plant resist infection.

“This is the general tolerance of the soybean plant to the races of phytophthora that you have in a field,” notes Froese. “And when you get into gene stacks…combine that with good field tolerance, and you’re going to have a really good phytophthora product to withstand almost anything that you see in your field.”

Check out the video below for this school episode on where things are headed with genetic resistance to phytophthora in soybeans grown on the prairies:

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