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Phytophthora a growing risk to soybeans

The once-rare infection is now the No. 2 cause of soybean root rot in Manitoba

The threat of phytophthora is on the rise in Manitoba. Results from a 2017 disease survey show it’s becoming common enough soybean growers will need to keep it top of mind as they plan their rotations this year.

About 35 per cent of tested soybean crops in Manitoba were positive for phytophthora, although fusarium was still the most common root rot in the province, draft data from the most recent Canadian Disease Survey reports.

The survey noted that all 106 tested soybean fields showed some sort of root rot in July last year, although severity ranged widely from 1.2 to eight out of nine. No severity rates were reported for phytophthora, although fusarium cases ranged between 3.3 and 5.9.

Holly Derksen, field crop pathologist with Manitoba Agriculture, said she was busy with suspected phytophthora cases last year. As well as central Manitoba, where the disease has previously been found, Manitoba Agriculture confirmed phytophthora farther west, north and east than previous years. Among those, the municipalities of Lansdowne and Whitemouth, municipalities around Brandon and as far north as the RM of Dauphin and RM of Bifrost had confirmed cases in 2017.

Some of that spread may be due to changes in testing rather than actual spread of the disease, Derksen said. Surveys and tests have expanded alongside soybean acres.

“There’s two things going on here,” she said. “One of them is that this might have been the first year that we actually collected samples that were symptomatic in the Dauphin RM, so it could have been there for a number of years. This is just the first time that we’ve collected those samples and then had them tested… also, it’s a soil-based pathogen, so it moves anywhere soil moves. It could have moved up there more recently or it could have moved up there a number of years ago, but with the increasing soybean production, it presents a host there and then we saw more of it.”

Back to reality

The news comes at the same time, more and more industry and producer groups are warning that the honeymoon period for soybeans might be drawing to a close. The comparatively new pulse has so far dodged many of the pathogens specific to soybeans.

Last year was particularly friendly to phytophthora, Derksen added, and more phytophthora was reported in general than previous years. Derksen noted that 2017’s wet spring, followed by dry conditions through the growing season, made for an ideal environment for infection.

“With newer soybean production areas, guys are newer to growing soybeans, so there’s a lot of things that they have to learn. One of those things is obviously pest management, so figuring out which pests to be concerned about in soybeans,” she said. “For the most part, we aren’t concerned with foliar diseases the way we are with other traditional crops like canola and wheat. A root rot pathogen is something that’s a little harder to wrap your head around. It’s not something that you can physically see. Often there are years where you might have it at really low levels and not even know that you have an issue.”

Soil movement also becomes a concern with the fungus, much like clubroot in canola. Both pathogens can live for long periods in the soil without a host.

Derksen advised farmers to be aware of the risk on their own farm, such as any previous cases in their region, and choose varieties accordingly.

Diagnosing

Manitoba Agriculture is encouraging producers to send in samples of suspected phytophthora cases, since the disease may be hard to diagnose in the field.

“It’s not something that we can eradicate, for one,” Derksen said. “It’s not something that we can really even prevent because it’s so mobile in the soil. The No. 1 thing is education to be able to recognize when you are having an issue with it.”

Phytophthora is easier to diagnose later in the season, the pathologist added. A field may have a different problem if stem discolouration does not reach all the way to the ground. Since phytophthora is a root rot, discolouration will stem from the bottom up, and any break of healthy colour may indicate a different concern.

Likewise, the infection will impact all the way through the stem, and a plant with a healthy stem interior may not be suffering from phytophthora at all.

Manitoba Agriculture suggests that any samples be noted as a suspected phytophthora case and that farmers also send in a healthy plant, if possible, for comparison.

Full disease survey results will be published through the Canadian Phytopathological Society this year.

About the author

Reporter

Alexis Stockford is a journalist and photographer with the Manitoba Co-operator. She previously reported with the Morden Times and was news editor of  campus newspaper, The Omega, at Thompson Rivers University in Kamloops, BC. She grew up on a mixed farm near Miami, Man.

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