“Probably 25 per cent of the field was impacted,” the production specialist told attendees at a Westman Agricultural Diversification Organization field day near Melita, Man. “One in four plants were dying off, and that was actually in the drought year 2021.”

Phytophthora root rot is a water mould disease and a relative newcomer to Manitoba, affecting both field peas and soybeans. As with other soybean diseases, it’s gained presence in the province as soy acres expand.

The rise of soy

Today, soybeans have a solid foothold in Manitoba’s top three crops, vying with wheat and canola. In late June, Statistics Canada reported about 1.4 million acres were seeded in Manitoba.

Twenty years ago, they were almost non-existent. At the turn of the millennium, soybeans in Manitoba were a novelty crop, taking up less than 50,000 acres. By the middle of that decade, the crop had clawed its way to several hundred thousand acres.

From there, popularity took off. By 2013, it topped one million acres and hit an all-time high in 2017, when farmers planted almost 2.3 million acres.

Behind the crop, though, came diseases and pests. In 2019, Manitoba clocked its first positive soil tests for soybean cyst nematode. In 2021, above-ground symptoms were noted in central Manitoba.

Phytophthora has a longer history in the province. In 2017, as soybean acres reached their peak, samples submitted by MPSG to Agriculture and Agri-Food Canada tested positive for phytophthora in 35 per cent of the 89 fields surveyed that year.

Then came the drought. The industry group found no cases through its monitoring program in 2020 and 2021. In 2022, it found it in 11 per cent of fields.

Spore taste

The disease starts with resting oospores within the soil. When the oospores detect a nearby soybean root, they swim to it through a thin film of soil moisture.

The obvious first sign of a problem is when leaves start to wilt.

“The leaves stay attached to the plant when they’re wilted and you’ll have a brown lesion up from the soil,” Schmidt said. “When you pull those plants, they’ll pull really easily out of the ground because those roots are all rotted.”

Infection has a similar look to northern stem canker, which also causes brown lesions, but the latter attacks the stem rather than the root.

“When you pull that plant (infected with northern stem canker) out of the ground, there will be some resistance,” Schmidt said. “That’s a quick way to tell one from the other.

“But at the end of the day, a lab test at the Manitoba crop diagnostic lab in Winnipeg is going to be the best way to tell them apart.”

The field that sticks in Schmidt’s memory in the Souris area, southwest of Brandon, was infected with both phytophthora and northern stem canker.

The new disease reality

Soy and pulse experts like Schmidt urge producers to be more vigilant against soybean disease. Phytophthora root rot is one pathogen of concern.

“We’re still in a bit of a honeymoon where we’re not facing lot of major soybean diseases.” Schmidt said. “But coming down the pipeline, phytophthora is the one that we do have up here and, having this much moisture, I’d expect to see these symptoms in your soybeans, especially if you’re on tight rotations.”

Manitoba has had good conditions for development of fungal disease. May and June were cold and wet, sparking fears of sclerotinia and blackleg for canola growers, while Manitoba Agriculture noted problems with ergot in cereals.

For phytophthora, MPSG does have recent surveillance initiatives in place, Schmidt noted. Soil testing last year found less than five per cent of fields with symptoms, but “83 per cent of soils had phytophthora in them,” she said.

“Either we weren’t seeing symptoms mostly because it was dry or because major gene resistance and partial resistance was giving us some protection.”

Starting in 2022, the industry group began an assessment program to independently gauge resistance in different soybean varieties.

Built-in defence

There are two types of soybean resistance against phytophthora root. The first is major gene resistance, which largely stops infection at any growth stage in the plant’s life.

“That’s the most common form of resistance and the way it works is complete resistance to phytophthora throughout the growing season,” Schmidt said. “If (varieties) have a major resistance gene versus phytophthora, it will be listed in Seed Manitoba.”

The second type is partial resistance or field tolerance. In this case, plants still get infected but are able to tolerate the infection and keep growing. Symptoms aren’t as severe and can be variety specific.

“We’ve actually developed a soil test where we can sample phytophthora and find, specifically, what pathotypes are living in the soil,” Schmidt said. “We only used to be able to say we know it’s phytophthora.”

Knowing the race in a specific field can help producers choose varieties and better manage the disease, she noted, as well as informing development of new resistant soybean varieties and new approaches to management.

In 2016, four races of phytophthora were identified, according to the MPSG website. Ensuing years saw the dominant race shift and, in 2018, a new one was detected.

“We expect more PRR (phytophthora root rot) races and pathotypes to be identified over time, as is the case in Ontario and the U.S. where soybeans have a longer history,” the MPSG website states.

“Race-specific resistance is still a beneficial management tool, but partial resistance will become more important over time to combat numerous PRR races.”

Last year, the association surveyed 70 soybean fields and sent soil samples to a Quebec lab for analysis.

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For example, if you introduce hydrochloric acid (common stomach acid) to lye (sodium hydroxide) the result is sodium chloride (table salt) and water. The sodium half of the molecule has a strong positive charge and is called the cation. On the other hand, the chloride is negative. Once the two are bolted together, the resulting sodium chloride molecule is neutral.

This is a relatively simple example, but it illustrates the point. Farmers face a number of different salts, some chemically simple and some more complex.

Another salt farmers know well is an isopropylammonium salt — an acid molecule bound to a salt for the purpose of packaging and handling. It’s more commonly known as glyphosate.

Magnesium sulfate, also known as Epsom salts, are fairly common in bath salts and are used in a number of medical treatments. It’s also one of the most common salts found in prairie salt clays and, since it dissolves easily in water, it’s one of the most serious causes of soil salinity.

Salt and water

Most of our crop species don’t tolerate soil salts particularly well. This is because salt interferes with the plant’s uptake of water. Water is precious in a dry environment and a plant that can’t get water is a dead plant. Saline groundwater messes with the “water potential” of the environment.

Simply put, water likes to move to where the salts are. If the water is pure and it’s sitting next to a plant cell with “solutes” dissolved in it, that water wants to move to where the solutes are. The “water potential” is greater in the cell so the water crosses the cell wall and moves in. If the water in the surrounding soil has more solutes than the cell, the water potential is the other way and it moves out of the cell and into the soil. The cell dies.

Oddly enough, magnesium sulfate may also be used as a fertilizer on land that’s deficient in either magnesium or sulfur and it may also be used with crops that are big users of these elements such as potatoes or tomatoes. Still, too much of a good thing is toxic. †

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Farmers who own land prone to salinity have to learn to cope with it.

“The first thing we have to recognize is that soil salinity is not a soil problem,” explains farmer, Grainews columnist and retired University of Saskatchewan professor Les Henry. “It’s a water problem, so we have to focus on water.”

There are two reasons for this. First, groundwater can move from one place to another, gathering dissolved salts as it goes. Second, it’s about the water table — the measured distance from the surface of the ground down to the level where the oozing water saturates the soil. The closer the water table is to the surface, the more serious the potential problem.

Salts in Prairie water

So what are the salts? Henry says, “In Western Canada, the major problem is magnesium sulfate. There are others of various solubilities but most of our trouble is caused by magnesium sulfate.”

In fact it’s the salts in the sulfate family that you usually find in Prairie water. Calcium sulfate (gypsum) and calcium carbonate (lime) are fairly common in Prairie subsoil but they’re relatively insoluble. Since they won’t dissolve easily into the groundwater they tend to stay where they are.

On the other hand, chloride salts of sodium, calcium and magnesium are soluble and love to travel with the groundwater. Fortunately they’re not common so they’re limited to a few small areas. It’s the sulfate salts that dominate the saline portions of the prairies.

So we know it’s in the water and we know it moves. The next question is: how does it get to the surface? Simply put, it’s carried there by the groundwater but not because a flowing spring brought it there. This is where the water table — that critical distance from the water table to the soil surface — becomes important. The closer it is to the surface, the greater the potential problem.

Moving it to the surface

Water is drawn to the surface through a process called capillarity — the same process plants use to move water from the roots to the leaves. Water can move through microscopic pores, some of which are almost small enough that the individual molecules have to line up and move single file. The combination of surface tension (cohesion between water molecules) and adhesive forces between the water and the surrounding pore material actually causes the water to move upward against gravity. The water takes the salt along for the ride, but when they reach the surface, they part company.

The water evaporates into the air and leaves the salt on the ground where it can form a crust. Generally, if the water table is within two meters of the soil surface the capillary action on the water will bring it all the way to the surface.

“So soil salinity is caused by a high water table, near the surface and close enough that suction lift can bring it up to the surface concentrating the salts there.” Henry explained. “You need to have evaporation greater then precipitation so groundwater movement is required to maintain that water table during the dry periods.”

What keeps that capillary action from working is a ready supply of fresh water coming from the other direction, forcing it the other way. Any form of precipitation, either rain or spring snowmelt, helps to maintain a better quality of soil moisture. As fresh water percolates into the soil it forces the more saline groundwater lower in the profile.

As long as the upper levels of the soil have fresh water, crops will use it quite happily. But if it’s not replenished the salt-rich water from lower down will be drawn to the surface. The crops will take on a bluish tinge and fail.

This has been going on since the last glaciers retreated.

That is the other dimension we have to consider with moving groundwater: time.

“To think about salinity you have to think like a geologist and you have to think in four dimensions.” Henry said. “You’ve got ‘how wide,’ and ‘how broad’ and ‘how deep’ and ‘how long.’ Salinity is the net water movement at the soil surface over ten thousand years.”

So it’s a combination of geology and groundwater movement. There’s nothing new about this, it’s been going on since the last ice age ended.

Controlling salinity

The next question is, “What can we do about it?” Can we control soil salinity?

The answer is an unqualified maybe. These groundwater systems can be fairly complex so controlling soil salinity can be costly and risky. The best hope for reversing salinity is in areas where it’s a recent phenomena or where it may be seen to fluctuate.

The first thing to do is a detailed investigation of the local water quality in nearby wells. It helps to check local soil maps to see what they might tell you.

The next phase, if it’s worth the cost, is to drill into the ground to determine the basic geology and it’s capacity to carry water. At this point either piezometers or a water table well can be installed to monitor the water table and see when it’s nearest the surface. Or Henry says you can get a rough idea of that on the cheap.

“You need a Dutch auger with two extensions.” he says. “You go to the local hardware store and buy 10 feet of thin PVC pipe that they use for central vac, you put a plug on the bottom and you put it in the ground. I’m 72 years old and I can put one of those in in about 20 minutes.”

If you install slotted pipe the level of the water inside will be the actual level of the water table. If the pipe isn’t slotted the level of the water inside the pipe will indicate the “piezometric head,” — the level maintained by the groundwater pressure. The next step is reclamation.

“Reclaiming saline soils is draining plus leeching. Drainage or leeching alone won’t do it, so do the two of them together.” Henry suggests. “If it’s calcium and magnesium salts, you drain it and leech it, as simple as that. It’s very easily reclaimed. If you have saline sodium salts then you may need to add gypsum and then drain and leech, but for most of the situation that we have all you need to do is drain them and leech them and they reclaim very quickly.”

There are two other possibilities. Some native plants cope with salt better than crops so it may even be best to seed to grassland and use the land for grazing.

“The best way to manage saline soils is to sell it in the winter time.” Henry laughs. On a more serious note, he says, “To understand soil salinity you must understand the ground water movement and there’s no spray or spread solution. If you’ve got a serious problem the only thing you can do is to plant it down.” †

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