Keeping Phytophthora Out

March 11, 2019

With soybean acreage growth in Western Canada, the prevalence of soybean pests is also on the rise. Phytophthora is an aggressive fungal disease that can significantly affect soybean production. “It’s definitely spread through most of the province,” says Brett Graham, Seedcare specialist with Syngenta based in Manitoba. “I’ve seen it up by Dauphin, even on the west side of the province, when typically it was mostly in the Red River Valley.”

As Graham explains, Phytophthora is a water mould that thrives in wet conditions. Saturated soils allow the zoospores produced by the pathogen to swim to nearby plants and infect them, causing the disease to spread in moist areas of a field. An early-season Phytophthora infection causes damping off or seedling blight that can result in reduced stands – something that can be easily misdiagnosed as seeding misses or other diseases. Graham suggests an easy way to test for the damaging disease: “If you’re looking for Phytophthora, pull a dead plant out of the ground. An infected plant will typically come out quite easily and you'll notice that the root below the surface is pinched off, but the best way to identify is with a lab test.”

Race-specific resistance

According to Graham, “there’s nothing we can do to eliminate Phytophthora once it has set in.” Instead, he suggests that prevention is the best approach.

“The biggest thing we can do is try to reduce the pressure in our fields. There’s a number of different ways to do that. The genetics we use is one of the main ones,” he says.

Graham advises planting resistant varieties. Many soybean varieties have race-specific resistance genes (Rps genes), which provide resistance to specific strains of Phytophthora. “The toughest thing about Phytophthora is that we don’t know what strains are in our field, so in order to try and select a certain genetic resistance, we’re really just playing the guessing game,” he says. For this reason, growers should keep good records.

“When you see Phytophthora show up, take note of what Rps gene your variety has, if any, and keep that written down. Next time you’re on that field pick a variety that has a different gene resistant to different strains than the original,” he says.

Field tolerance ratings

Most soybean varieties are also rated for their level of partial resistance or field tolerance. These ratings are determined by entering the varieties into Phytophthora ponds.

“Basically, breeders know there are certain strains present in each pond, so they’ll plant different varieties in rows and flood the ponds to induce high pressure Phytophthora. Then the breeders come back and rate how the varieties survive on heavily-infected ground,” says Graham.

Seedling protection

Aside from genetics, seeds treatments play a major role in helping prevent early-season Phytophthora. “If you think of a small plant like a small child, their immune system isn’t as strong – it’s the same thing with a plant. The field tolerance isn’t expressed as strongly in that small plant as it starts growing. That’s where seed treatments really come in to help protect that seedling.”

Graham concludes with a prevention message. “There are a lot of different options to help combat Phytophthora, but it’s really just understanding what’s going on in that field and working to prevent the disease from thriving rather than just letting it happen.”

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