In today’s bad news/good news ledger of pulse crop diseases, the bad news is there’s still no quick fix for controlling root rot in field peas and lentils.
On the good news side, there are management options that help reduce the risk, research is narrowing in on some treatments that may help control the disease, and plant breeding is making progress on resistance.
That’s the update from Michelle Hubbard, a research scientist at Agriculture and Agri-Food Canada’s Swift Current Research and Development Centre and a plant pathologist specializing in pulse crop diseases.
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As Hubbard told producers attending the recent Western Applied Research Corporation (WARC) crop conference, being aware of the potential risk of aphanomyces root rot is an important first step in dealing with the disease.
“Unfortunately there are no in-crop rescue treatments that work for controlling aphanomyces,” Hubbard says. “But the best approach is first to be aware that the disease is out there and avoid fields that are at high risk of disease development.”
She recommends having soil or pulse crop roots tested for signs of the disease. There is good information online about collecting samples; growers can also consult one of the provincial pulse crop associations or ask a crop consultant how to collect samples and where to have materials tested.
Aphanomyces euteiches is a root rot pathogen that thrives under wet conditions. It was first detected on the Prairies about a dozen years ago, but it’s suspected to have been around for much longer. Field peas and lentils are the pulses most affected by aphanomyces, while other crops such as chickpea, faba bean and soybean appear to have resistance.
Infected pea and lentil plants will appear wilted and yellow and symptoms are often first detected in low spots or the perimeter of drowned-out areas.
“At times aphanomyces can be very patchy in a field,” Hubbard says, “although whole fields can be affected” and infection levels can range from relatively mild to severe.
She says if producers suspect a production issue, they should have soil and/or root samples tested. Infected roots will become mushy and may have a caramel discolouration, but this can be masked by other root rots. The best way to confirm an aphanomyces infection is by submitting a soil or root sample to a lab.
If you’re planning to grow peas or lentils in fields where pulse crops have not been grown for several years, she says to have soil tested to make sure the field is disease free.
What doesn’t work
Hubbard says research at seven different sites across Western Canada over the past six years is helping to sort the wheat from the chaff in terms of which treatments don’t work and which show some promise.
In the what-doesn’t-work column, intercropping peas with canola, mustard, camelina or oats doesn’t appear to be of any value in controlling the disease. The thinking with intercropping peas with brassicas has been that the glucosinolates in oilseeds would break down in the soil, producing an antifungal or biofumigant compound that might check the aphanomyces pathogen — but so far they have seen no effect.
Hubbard cautions not to throw out the intercropping idea completely. While it didn’t have any effect on the root rot disease, they did observe the pea/mustard and pea/canola intercropping showed a yield benefit at some sites, some years.
Applying herbicides also didn’t reduce disease levels on the pulse crop — but again, there are pros and cons. Hubbard says on the “con” side, herbicides can actually be a stress on the crop and weaken plants, making the crop more susceptible to disease injury. On the “pro” side, however, a herbicide application can control weeds such as shepherd’s purse, chickweed, vetches and others that are also hosts for aphanomyces pathogens.
Other treatments that so far also appear ineffective in controlling aphanomyces are biocontrol and natural products, including arbuscular mycorrhizal fungi (AMF) which can be effective in controlling other pathogens.
Treatments that might work
Results have been a bit sporadic, but Hubbard says research has shown some benefit from some treatments at some sites, “so it is worth looking at those practices further.”
Practices which appeared to produce some benefit in some locations included liming and applying gypsum, both of which can increase soil calcium.
“There are certain liming products such as spent lime from sugar beet processing, which helps to increase soil pH and calcium levels, which may have a benefit,” Hubbard says.
In looking at liming and gypsum treatments at the Swift Current site in 2022, for example, gypsum didn’t reduce the disease load, but it did appear to have a real benefit in increasing yields.
“That was in 2022, and then to add to the confusion in 2023 at the Swift Current site, gypsum appeared to have an effect in reducing root rot infection, but it had no effect on yield,” Hubbard says. “So this is telling us that gypsum and liming can have an effect — we just need to figure out how and why.”
WATCH: AgGronomy TV: Avoiding root rot in pulse crops
While biocontrol products so far haven’t been effective, Hubbard says new products are still in development which could reduce disease load or strengthen crops against the disease.
One natural product being evaluated, is under development by MustGrow Biological Corp. in Saskatoon. The company uses extracted and concentrated molecules from mustard plants, with the goal of commercializing them as natural, organic biopesticides, biofumigants and bioherbcides.
Hubbard is also looking at whether increasing rates of nitrogen fertilizer with pulse crops might be an effective treatment in reducing yield losses due to root rot.
And, of course, plant breeding is another important tool in developing crops with disease resistance. She says the University of Saskatchewan is getting close to releasing a field pea variety with some measure of resistance to aphanomyces.
Developing a lentil variety with disease resistance, however, appears to be a longer-term breeding project.
Take-home message for 2024
With research rolling along in the background, producers are advised to follow the best management practices for producing pulse crops this coming year.
- Be watchful for signs of root rot disease and have soil and plant roots tested to determine if the disease is present.
- Assess your disease risk, avoid fields with known issues, or sites with high moisture which may be prone to disease development.
- Follow a proper crop rotation, avoiding back-to-back pulse crops on the same field, and, if possible, produce pulse crops in a four-year rotation, which helps reduce the risk of disease development.
- If the disease is detected it may require a seven- or eight-year break in rotation before peas and lentils are seeded again.
- Grow other types of pulse crops, such as chickpea, faba bean or soybean, which are resistant to aphanomyces.
- Use good-quality seed, which helps to produce seedlings with high vigour.
- Use seed treatments that provide some measure of protection against root rot and other diseases.
- Select varieties with disease resistance — such as a pea variety with moderate resistance to fusarium — again, to produce a healthy crop stand.
- Make sure the pulse crop has proper nutrition.
- Keep good records on how pea and lentil crops perform during the growing season.
“Research continues to evaluate best management practices and also to better understand the disease complex which affects peas and lentil crops,” Hubbard says. “And when we do have new varieties with disease resistance, we will need to take care of it. Pathogens are always looking to adapt to and overcome genetic resistance, so it is important to apply proper management to protect that resistance as best and for as long as we can.”
