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Root rot can be diagnosed, but confirmation may come too late

Root rot caused by aphanomyces can cause yield problems in peas. There’s no easy treatment, but farmers can still take preventative steps

It was early in the 2012 growing season when Bernie McClean realized something was wrong with one of his pea fields.

The field had been seeded to peas several times over the years, but in 2012 the crop was “going backwards,” McClean, a Medstead-area farmer, said in an interview. Patches of plants were stunted. As the season progressed, they would yellow from lack of nitrogen, seemingly caused by root rot.

McClean and his agronomist, Errin Tollefson of Cavalier Agrow, scouted the field and pulled plants to send to a lab in Regina. The initial diagnosis was aphanomyces, a disease that causes root rot in peas. By fall, DNA analysis would confirm the pathogen.

But that diagnosis wasn’t enough to save McClean’s field.

“Nobody had answers for me,” said McClean. “Do I go out, do I spray with Headline or some type of fungicide to try to protect what I still have left?”

“But the root was gone. There was no saving it. Nodulation was gone. It was horrible.”

In the end, McClean still pulled an average of 20 bushels per acre from the infected field, partly because one area that hadn’t been seeded in the past yielded very well.

McClean said about “70 per cent of those acres were just a complete write-off.”

“I was happy to get anything out of that (poor) field. You know, in all honesty that summer I thought, ‘It’s going to be a wreck. I’m not going to get anything.’ But in the end, where it was good, it was very good.”

In comparison, McClean’s other green pea field, which saw peas for the first time in 2012, produced 67 bushels per acre, he said.

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2012: year of the fungus

Speaking at a recent farm conference, Peter Walsh, an agricultural instructor at Vermilion’s Lakeland College, pegged 2012 as “the year of the fungus.”

Aphanomyces zoospores have tails, known as flagella, which propel zoospores short distances through soil, allowing them to infect roots. The pathogen thrives when there’s plenty of moisture early in the growing season, and so 2012 was a rough year for farmers with infected fields.

Ed Seidle and his family have been in the pedigreed seed business for 70 years in the Medstead area. He likes to dig into problems not being investigated by other researchers, studying fields where farmers have kept good records and applying information from his research library.

“My laboratory is the whole country,” he said in an interview.

Seidle said the disease spread very rapidly in 2012. He marked diseased patches in each field he visited.

“And every time I went to them, they were bigger in every direction. And considerably bigger,” said Seidle.

The first symptoms of aphanomyces will be patches of stunted plants, with yellowing on the older leaves, said Seidle.

In a patch, “every plant is affected. Every plant. There isn’t anything missed,” he said. As the season wears on, the plants will yellow and die off. Nodulation might not occur at all, or nodules may be destroyed.

“In real severe cases the plants don’t get very big and they’ll be knocked right out before they ever get to the podding stage,” said Seidle. “And if it’s not as severe or comes on… later, there may be some pods formed. Sometimes only one pea in a pod.”

Seidle has found signs of it in most fields he’s surveyed, ranging from trace levels to severe infestations. Yields have plunged as low as one bushel per acre.

“Even some of the fields that were first-time peas, as the season went along, I could find little patches here and there which could very well be the aphanomyces,” said Seidle.

Dr. Sabine Banniza, researcher with the University of Saskatchewan, started getting calls from growers about root rot problems in 2012. Banniza and her colleagues collected samples and tried to isolate the pathogen.

They were able to detect aphanomyces in samples, including McClean’s, using molecular testing.

But success in isolating aphanomyces depends on how much of it is in the root compared to other stuff, said Banniza in an interview. “And obviously the longer you wait, the less is there.”

Once aphanomyces infects roots, it opens the door to other diseases, such as fusarium. Wait too long to send samples and secondary infections will have over-run the sample, making diagnosis tricky.

In 2013, Banniza and her colleagues collected contaminated soil, planted peas, and then tested for aphanomyces when the pea seedlings were very young.

“And even with our indoor tests we noticed that first you seem to have aphanomyces and then eventually fusarium comes in there as well. But it’s really secondary,” she said.

Managing aphanomyces

There are no seed treatments or other chemical controls available for aphanomyces right now.

But farmers shouldn’t strike peas from their rotations altogether, Seidle said. “Just because they had a bad crop on one field, why discontinue (entirely)? Only because they may think that this root rot, it’s something like a flu… It’s going to hit every field. Well, the pathogen’s got to be there.”

Seidle suggested tracking pea yields and avoiding fields that have yielded poorly in previous years. “If the last crop was a 45 bushel crop and you’ve had at least four years in between, might as well go ahead with it.”

McClean said he’s very careful in how he selects fields now. “I was fortunate to have some land that I’d just bought that never had peas before. So I can keep peas in my rotation at a smaller acreage.”

Because the pathogen thrives in soggy soil, farmers should avoid planting peas in flood-prone fields. Banniza’s research shows peas will do poorly in water-logged soil even if it’s been sterilized. But flooding and pathogens are a double whammy. “Both will contribute to poor establishment of pea plants,” said Banniza.

Aphanomyces also thrives in compacted soil, and Seidle said it tends to show up in wheel tracks and high-traffic areas such as field entrances. Banniza said growers with root rot issues have told her they’d seeded early, when the soil was too wet, likely worsening compaction.

Seidle suggested trying cultivation before seeding to reduce compaction. Banniza said cultivating would help if it improved drainage, but she advised farmers to talk to a specialist, as plowing “may not alleviate the problem. It has to be very site-specific, what you can and can’t do.”

Seidle also suggested avoiding rolling peas when soil is wet, as wet soil is more likely to stick to equipment, spreading the disease.

In general, farmers should also try to minimize plant stress as much as possible. “Cold soils and soil residual herbicides create stresses. And as soon as the plants are stressed, they’re subject to invasion,” said Seidle.

Starting out with “top-notch seed” also helps, said Walsh. He suggested cold vigour and germ tests, along with certified seed. “Start with a healthy, vigorous plant. And don’t put it into zero-degree soil.”

Research out of France and Germany has found aphanomyces spores can survive for up to 10 years in the soil without a host. But researchers don’t know whether, for example, a couple of dry Prairie years drop spore numbers, said Banniza.

Regardless of exactly how long spores last, longer rotations are an important management strategy. Seidle said anything short of a four-year rotation is bad, and he knows farmers using six or seven year rotations who have had fewer problems.

And farmers should keep in mind aphanomyces infects other legumes. Chickpeas are relatively tolerant, Banniza said. Some fababean varieties have resistance, she said, and researchers will be identifying resistant Western Canadian varieties this year.

Dr. Robert Conner of Agriculture and Agri-Food Canada has been researching aphanomyces tolerance in field peas and is leading a new research project, funded by the Pulse Science Cluster. Scientists from France and the U.S. have also discovered disease resistance in a garden pea variety, and Banniza said plant breeders are now working to bring that resistance into field pea varieties. “But that’s on the longer horizon,” she said.Seidle said the root rot problem isn’t going away. Whatever bit of management we can use is important, he added.

“This philosophy that, ‘well, I had a problem, I’m going to discontinue growing peas,’ that’s not a scientific approach at all.” †

About the author

Field Editor

Lisa Guenther is field editor for Grainews based at Livelong, Sask. You can follow her on Twitter @LtoG.

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