10 Years Of Aphanomyces

Without Proper Management, This Disease Can Devastate Your Pulse Crops.

By Stacy Berry

According to Pulse Canada, 8.8 million acres of pulses were grown nationally in 2021. But there is a crop disease present – at low levels – in most of the soil across Canada as well. That disease is Aphanomyces root rot.

How can a pulse crop disease be found in soils, when many of those soils haven’t seen a pulse crop before?

Dr. Syama Chatterton, a plant pathologist out of Lethbridge, Alta., explains: “Aphanomyces is not fungal or bacterial; it’s an oomycete that is more closely related to algae.

“Scientists suspect it’s naturally present in most soils that were historically under an ocean, so it’s often found in low levels in lots of soils. Not until there is a cropping history (of pulses) is there a concern.”

The first cases of Aphanomyces issues were found in the Prairies in 2012 (Saskatchewan) and in 2013 (Alberta).

The main risk of Aphanomyces is the yield loss that it can cause, to potentially devastating levels.

“This is a very concerning disease,” says Sherilyn Phelps, director of Research and Development at Saskatchewan Pulse Growers.

“The big impact is on the ability to grow peas and lentils in those affected fields, which would mean a drop in acres and our ability to produce and export pulses.”

Let’s take a step back: What are pulses? Simply put, “pulses are the dried seed of a legume plant, like chickpeas, lentils, peas, and dry beans,” explains Trent Richards, who farms near Assiniboia, Sask.

Pulse Crop Field
    Laura Schmidt photo

Laura Schmidt, production specialist for Western Manitoba with the Manitoba Pulse Growers, points out the agronomic value of a pulse crop. “They are a nitrogen-fixing legume, and a great disease and pest break in crop rotations.”

Phelps agrees that pulses are valuable. “They are high in protein, with good starch and fibre. It’s a good nutrition addition for both humans and livestock. There is a worldwide movement towards plant-based proteins, so the value of pulses is only increasing.”

Pulse Canada has numbers to back that. In 2021, pulse acres added $6.3 billion to Canada’s economy.

This means that a potentially devastating disease is a real concern when dealing with this valuable crop. “There are areas that cannot grow pulses already now; it is easy to lose acres,” says Richards. “Pulse acreage has reduced over time in some areas due to the long rotations already,” he adds.

What does Aphanomyces do?

“The Aphanomyces life cycle has two phases. The first phase is the oospore stage,” explains Chatterton. “This is a fairly large, single-celled resting spore, surrounded by a very thick cell wall. The oospore can survive in soil for six to 10 years, or more. When a suitable host is around, the oospores germinate, to produce zoospores.

An oospore under a microscope.
    An oospore – the first stage of Aphanomyces development. -Syama Chatterton photo

“These spores are mobile, with flagella (tails) so they can ‘swim’ through the soil water towards hosts. They infect the roots. Once the roots are depleted, they produce the resting oospore, to wait for more host plants in the coming years.”

Does that life cycle sound familiar?

It bears several “similarities to club-root,” says Chatterton. A crucial difference is, of course, the host plants – clubroot infects cruciferous crops, not pulses. However, another key difference is inoculum load. “You count clubroot spores in the millions and billions. You count Aphanomyces in the hundreds. At 100 spores per gram, you’re seeing disease.”

All of that happens at the microscopic level. What does this look like in the plant?

Unfortunately, the symptomology of Aphanomyces isn’t unique. “By June, above-ground you’ll see yellowing plants, that will start in the low spots, headlands, areas with poor drainage,” says Schmidt. “If the soils remain moist, it will spread throughout the field and with machinery.”

If you see that yellowing in your field, “pull some plants and test those roots,” urges Schmidt.

Phelps explains, “you will see caramel-coloured roots instead of the nice, healthy, white roots you expect to see. The root will pinch at the soil line. Run your fingers along the roots and they will feel almost slimy.” Late enough in the season – usually around July – the roots might be gone entirely.

Everyone agrees that testing for Aphanomyces is key since the symptoms aren’t always cut-and-dried.

“Your best results will be from testing the plant. When you see that yellowing, send plant samples in. Seed testing labs that offer this test can give you a reliable yes or no,” explains Chatterton.

Soil testing for Aphanomyces is an option, however, “some soil types will give you more false negatives. It’s difficult to test the soil directly for the oospores because of that hard cell wall. It’s difficult to extract the DNA.”

Another problem with Aphanomyces is that, often, it’s not travelling alone. “Rarely are we dealing with just Aphanomyces – it is often found in combination with other root pathogens like Fusarium. This can cause what we call the ‘dynamite effect’ – you can really see crops go backwards quickly,” says Phelps.

Saskatchewan grower Richards has been farming pulses for over 30 years, and only recently started to see small signs of Aphanomyces. “Soil testing is not as reliable as plant samples,” Richards agrees, “but it can still give you an idea of risk factor if you plan to grow pulses somewhere.”

As is often the case, prevention is the best medicine. A chorus of “extend your crop rotations” can be heard across the Prairies when asked how to prevent and manage Aphanomyces. Chatterton reminds farmers, “even on virgin land, follow a good crop rotation to start, like four or five years between susceptible hosts.”

“Peas and lentils are extremely susceptible to Aphanomyces,” says Schmidt. Other legume plants and weeds can be hosts, so watch for those as well. “Some varieties of alfalfa, dry beans, shepherd's purse, clovers, and vetches are also hosts, although not as susceptible.”

This means that planting alfalfa might not count as a disease break in the rotation when it comes to Aphanomyces. However, farmers don’t need to be concerned about all legume plants being susceptible to Aphanomyces.

“There are legume plants that are not hosts – soybeans, faba beans, and lupins, for instance, can be safely grown despite the presence of the pathogen.”

“Aphanomyces is ubiquitous in soils – it’s more common than we think,” adds Schmidt. That means that all the prevention a farmer does is simply preventing inoculum levels from getting too high.

What do you do if that happens?

“You have to extend your rotation as long as you can – like six to 10 years,” says Phelps. “It is a long time and it is hard on the industry, but when disease pressure is high enough, nothing else helps.”

As Phelps says, Aphanomyces is often found in the presence of other root rot pathogens that work synergistically – one pathogen may weaken the plant, and make it easier for another pathogen to infect as well.

Root rot
    Late season root rot. -Syama Chatterton photo

Richards: “There are a few options coming down the line, like seed treatments that can suppress the damage. Groups are looking into breeding resistance to root rot, but it can be tough to resist a complex of pathogens.”

Schmidt: “Establish a strong crop. Think about seeding dates and rates – the stronger they start, the more resilient the crop will be to other diseases. Use quality seed and balanced nutrition. And keep crop records! Think about the last time peas or lentils were grown and what were the moisture conditions so you can use a long-term management strategy.”

Besides the Aphanomyces concerns, there is another reason why some farmers are put off pulses.

“There are some challenges with harvestability, but better equipment and better agronomy are getting more product to the bin,” says Phelps.

Similarly, Richards notes, “there is a cost to equipment – special headers to cut closer to the ground, and rollers to smooth the rocks after seeding since we’re so rocky here.”

Despite the challenges, agronomists and farmers alike urge others to give pulses a chance.

“There is no ‘worst’ part to pulses; I love pulses,” jokes Richards.

“The best part of pulses is the economics and good returns. We can see the soil health improving year over year. In some areas, our organic matter started at 1.8 per cent, and have now increased to as high as 4.2 per cent.”

Phelps stresses the agronomic benefits of pulses.

“I love the sustainability. The nitrogen fixing means you do not need to rely purely on nitrogen fertilizers. Pulses can scavenge and utilize less available nutrients better than other crops.

They are a good fit for rotations, with their different rooting depths, breaking pest cycles, stimulating soil biology.”

Soil health benefits aren’t the only thing that Richards and Phelps appreciate about pulses.

Richards: “The health benefit for the consumer is huge – plant proteins and plant-based foods are on the rise and a top market globally.”

Disease management is a consideration around the globe as well. Other parts of the world have been researching Aphanomyces “for more than 25 years. Here in Canada, we have just been at it for 10 years. Time is of the essence to find a solution,” explains Richards.

Phelps adds that “in the United States, they have had to move pea processing plants because they could not grow peas close enough to the factories.

“Our problems with Aphanomyces are not new worldwide – just new to Canada. Likely because we just started getting into substantial pulse acres.

“Aphanomyces is a worldwide problem with no silver bullet solution, so hopefully an integrated solution will be found.”

Chatterton says encouragingly that despite the Aphanomyces risk, “people should grow more pulses! Just understand and embrace a good crop rotation from the beginning.” BF

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