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Managing root rot in pulses

Peas and lentils have been a financial and rotational blessing for producers, but aphanomyces in partnership with fusarium is raining on their parade

If you’d mentioned aphanomyces at a farm show five years ago, you would have drawn quite a few blank looks. But these days speakers with aphanomyces expertise fill the seats.

The reason is no mystery to Prairie pulse producers. Field surveys have found aphanomyces from Alberta to Manitoba, as far south as the U.S. border and as far north as the Peace, as Syama Chatterton told farmers at CropSphere in Saskatoon this January.

That widespread distribution means that although aphanomyces wasn’t identified in Western Canada until 2012, it’s “actually been here for a number of years,” said Chatterton, an Agriculture and Agri-Food Canada researcher specializing in plant disease.

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It was just that susceptible crops escaped severe infections until pulse production hit a critical point, she added. In 2012 Saskatchewan fields also saw a lot of moisture, which may have been part of the reason aphanomyces made its presence known.

The key question for growers

Part of the reason aphanomyces, a water mould, escaped detection is that it rarely travels alone. When it infects a pea or lentil root, other root rot pathogens, notably fusarium, are likely to follow. (Chatterton pointed out fusarium strains avenaceum and solani cause these root rots, not graminearum, the cause of head blight).

In samples tested using conventional methods, fusarium also tends to obscure the aphanomyces. Now labs use DNA tests to detect or rule out aphanomyces.

But do farmers even need to test for aphanomyces, or can they manage root rot without knowing which pathogens are causing the problem?

Probably not very well, unfortunately. Knowing which root rot pathogens are afflicting crops is important for any farmers who want to keep pulses in their rotations. Although lentils and peas are susceptible to aphanomyces, chickpeas, soybeans, and fababeans are much more resistant.

But lentil growers considering switching some acres to chickpeas need to know if fusarium is part of the root rot problem, Saskatchewan farmer Derek Tallon said at CropSphere. “Because if it’s a fusarium, you might not be gaining a whole bunch moving to a chickpea.”

Chickpeas aren’t the only crop susceptible to fusarium root rot. Field surveys done in Manitoba and Alberta found Fusarium avenaceum in fababeans, according to a report on the Manitoba Pulse and Soybean Grower website. Alberta Agriculture also notes that both fusarium and rhizoctonia are sometimes found in fababeans.

Choosing seed treatments

Knowing which pathogens are rotting roots also helps farmers choose seed treatments. Intego Solo, a seed treatment containing ethaboxam, is now registered to suppress aphanomyces root rot early in the season. Chatterton said that it must be used with other seed treatments.

Intego Solo effectively suppressed aphanomyces in peas during greenhouse studies, she said. Researchers also tried it in the field on peas, and found it was effective when combined with Apron early in the season. In fact, that combination worked better than the check, Apron alone, or any of the other treatments in the study.

But while fusarium tends to infect roots at the seedling stage, aphanomyces can hammer roots at any time in the growing season. Researchers measured disease severity three times: early in the season, during flowering, and during pod fill. During flowering and pod fill, there was little difference between any of the treatments, Chatterton said. But she noted they hadn’t yet finished crunching numbers to see if the seed treatments affected yield.

More research in the pipeline

Chatterton said her Lethbridge lab has become “exponentially busier” since aphanomyces was first found in Saskatchewan in 2012.

“Because it’s such a recent detection for Canada, there’s really been no research done in our prairie soils,” she said.

The pulse industry has responded by ramping up research. Sherrilyn Phelps, agronomy manager for Sask Pulse Growers, said that researchers from across the Prairies are studying the disease. Sask Pulse alone has invested $2.7 million to date on root rot research, on everything from developing resistant varieties to field surveys, she added.

Some of the research focuses on field-based solutions, such as the seed treatments. Another potential solution is Phostrol, a fungicide used in the U.S. to control water moulds. It’s not registered for pulses, Chatterton noted, but results south of the border are promising. Producers would likely apply it four to six weeks after seeding, if it’s registered one day. However, it won’t protect the crop all summer, so a rainy July would still be bad news for pea and lentil growers.

Researchers are also trying soil amendments. If they work, they’ll have long-term control benefits, Chatterton said.

“Calcium is the one important factor that has been found to prevent zoospores from germinating,” Chatterton said. She acknowledged adding calcium might prove to be impractical for farmers, but researchers want to see if it’s effective.

They’re also looking at brassica cover crops. Greenhouse studies show that as the green manure breaks down, it creates bio-fumigants in the soil, Chatterton explained. Those bio-fumigants suppress the aphanomyces oospores, which are the spores that can lie dormant in the soil for years. Trials funded by Sask Pulse will get underway in 2017 to determine whether brassica covers will be effective in the field and how feasible the idea is in Western Canada.

Soil zone effects

While lab tests can now tell farmers whether they have aphanomyces in a field, the tests don’t tell farmers how severe the infestation is. That makes it difficult to know the risks of seeding a susceptible crop in that field.

However, researchers have been studying how many spores are needed to cause disease. They’re also looking at whether that number varies between soil zones, and whether other soil-borne pathogens influence the disease severity.

They’ve found that dark brown soil zones are most conducive to aphanomyces disease development, probably because of the high clay content, Chatterton said. Dark brown soils only need 100 oospores per gram of soil, compared to 750 spores per gram in the brown and black soils.

But once you add fusarium to the brown or black soils, that disease threshold drops to about 100 aphanomyces spores per gram, she added.

Researchers also assessed root rot disease severity in lentils and peas by soil zone. Peas suffered the most severe root rot in the brown soil. They looked best in the grey soil zone (although Chatterton cautioned they had a small sample size in that zone), followed by the dark brown.

Lentils in the black soil zone were hit hardest by root rot, while those in the brown soil zone fared the best.

Sabine Banniza, a University of Saskatchewan researcher, looked at how aphanomyces is distributed through the soil profile. Banniza pulled samples from the dark brown, brown, and black soil zones at different depths.

Banniza’s project found the highest levels of aphanomyces inoculum in the top 20 cm of soil. But researchers also found aphanomyces in the 40- to 60-cm samples. That’s another indication that aphanomyces has been around for a while, Chatterton said.

Managing in the meantime

Some considerations for controlling root rot in peas and lentils

There are still several questions around how best to manage aphanomyces. But Syama Chatterton of Agriculture and Agri-Food Canada offers some tips:

  • Crops suffering from root rot should be tested for aphanomyces. Its presence or absence will dictate how farmers manage root rot.
  • If a field has aphanomyces, farmers should avoid planting peas or lentils for six to eight years. That longer rotation will drop inoculum levels.
  • Farmers should consider seed treatments that target the root rot pathogens found in their fields.
  • Sometimes aphanomyces is spread uniformly through the field, while other times it’s patchy. Using a drone to snap photos of the pea or lentil crop can help farmers and agronomists figure out from where to pull samples.
  • In-crop fungicide applications have no effect on root rots.

Risk factors

Chatterton adds that these risk factors may influence deciding in which fields to seed to lentils or peas:

  • Wet soils favour aphanomyces. The oospores can survive and germinate in dry soil. But it’s the mobile zoospores that infect roots, and they need moisture.
  • Aphanomyces also loves compacted soils. Keep in mind that soils with high clay content have a higher risk of compaction.
  • Researchers think that four to five cropping cycles with a susceptible crop might bump a field to the threshold level. That’s because every time a pea or lentil crop is planted, aphanomyces inoculum builds. The oospores can survive in soil for years, so even a four- or five-year rotation builds inoculum.
  • Other root rots, especially fusarium, make plants more vulnerable to aphanomyces. Pythium is also a problem.
  • Acidic soils and warmer soils (24 C) are also a little more vulnerable to aphanomyces, although Chatterton says these factors are less important than the others. But aphanomyces’ love of warm soils is probably why farmers tend to see the disease in late June and early July.

Researchers, farmers, and agronomists know a lot more about aphanomyces than they did five years ago, yet it’s hard to say when the pulse industry will really get a handle on the disease. It’s a safe bet that for the next few years, anyone speaking about aphanomyces will have a full house.

About the author

Field Editor

Lisa Guenther

Lisa Guenther is a field editor for Country Guide.

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