There were ideal conditions for aphanomyces in large swaths of the prairies last summer.
But exactly how prevalent the disease was is largely unknown because Ag Canada researchers weren’t allowed to do in-field surveys due to COVID-19.
“We didn’t actually get to go out to do surveys, partly because of the COVID restrictions over what we were allowed to do over the summer. So, I don’t have hard numbers of what we saw,” said Agriculture Canada researcher Syama Chatterton.
She said her informal aphanomyces survey was largely from pictures she took when she was traveling for hikes or on vacation, conversations and from social media posts.
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“So from what I gathered from talking to people, from some of the phone calls I fielded and looking a lot on twitter, it was a really rough year for aphanomyces,” Chatterton said.
She said a colleague from Alberta Agriculture shared with her some of the data from field surveys the agency did over the summer, and 100 percent of pea fields had root rot with about a 67 percent incidence.
Disease incidence was obtained by stopping at ten spots per field to look for the disease.
“A 67 percent incidence would mean that on average 6.7 out of the ten spots, in every (pea) field in Alberta showed some root rot,” Chatterton said.
“Lentils fared a little better in Alberta. We saw a 90 percent prevalence, but only 14 percent incidence. And that is often what we see for lentils, in some of the lentil field we don’t see it as widespread throughout.”
This doesn’t bode well for future crops because how your pea and lentil crop performed in 2020 is going to determine the cropping interval, and effect decisions for that field for four to possibly over eight years down the road.
“Crop history is one of the main drivers of this disease. So if you start to take note of how your pea and lentil crops performed, take note of where some of those small spots might be, that’s going to be your best bet for making decisions going forward,” Chatterton said.
“It is very, very important to start recording how your pea and lentil crop performed every time it’s grown.”
She said its important to keep track of exactly where the crop has run into problems.
“Was there yellowing along water tracks or side hill seepage areas,” She said.
“Was there some spots that yielded poorly compared to the rest of the field. That’s the information that they want to start paying attention to.”
When it comes to a forecast for 2021, she said that because moisture levels and field history are the main drivers of the disease, a disease risk for a specific field depends on the prevalence of the disease in the rotational history.
For instance, if a pea field looked good this year then another pea or lentil crop could be planted on the field as soon as 2024.
But if there was a lot of disease in a crop, then it likely will need to be taken out for six to eight years, maybe longer.
Beyond keeping a close eye for problem spots, Chatterton said producers should also examine roots of crops during the growing season to help keep track of the disease, and use either a fall or spring soil test.
“If you had some yellowing spots and you did some soil tests and you found they were aphanomyces positive, then it’s definitely time to pause and reflect on when you can plant peas or lentils again in that field,” Chatterton said.
Historical precipitation patterns of when peas and lentils were last grown on a field can also help farmers better understand their risk for aphanomyces on specific fields.
So if your pea or lentil crop was in a high moisture area on the Prairies this summer, growers many want to consider expanding the interval until they rotate back into the crop beyond 2024.
The reason aphanomyces is so difficult to manage is because oospores, the thick-walled resting spores sit in the soil until both host roots and water are present.
“They do naturally decay in the soil, research about 25 years old now, theorize they have a half life of approximately one year,” Chatterton said.
“Under dry conditions the oospores can infect the roots directly, but it’s not the most efficient method of infection, so you’ll see a lot lower infection.”
She said oospores can multiply up 100 fold when they produce zoospores, the multiplayer stage, that infects plant roots.
The zoospores require saturated or wet soils to swim in to find the roots, which is why the disease is much worse under wet conditions.
It takes a very low threshold of inoculum for disease to develop, of approximately 100 oospores per gram of soil.
The more times pea or lentils have been grown in a field the more likely they are to have high inoculum levels, but each field is different in terms of how many times you can grow pea or lentil until you’re going to run into problems.
“In general what we’ve kind of seen what is happening in Alberta, Saskatchewan and Manitoba, its about that 20 year history with a pea or lentil that you’ve had every four years, it gives you about five or six times that a pea or lentil has been in a field that you’ll potentially start running into a problem,” Chatterton said.
However, she said this is not a hard and fast rule.
For instance, she said when peas and lentils are grown in wet years this seem to have the biggest impact on the next time you grow the crop, because small patches can spread to the entire field.
At the Ag Canada research station at Lethbridge, Alberta researchers noticed some small yellow patches in a 2011 pea crop.
The next time peas were grown on the field the entire field was infected with aphanomyces, and in 2020 almost no yield came from a pea crop on the same field.
“It just took that one flooding event to move it (aphanomyces) from small patches to an entire field. 2011, 2016, 2020 were all very wet years that had peas, and that really seems to be the key recipe for these wrecks that we’ve seen,” Chatterton said.
The interval needed between successful pea or lentil crops is field specific, but if the one-year half life of oospores is considered as well as the limit of ten oospores per gram of soil, growers can come up with a rough calculation.
“If you’re sitting at about 100 oospores per gram of soil and you start decreasing that amount every year your at that safe level, below 10 oospores per gram of soil, in about four or five years. If you’re at 1000 it takes you six to eight years to get below that level,” Chatterton said.
“If you’re really high and we did see a few fields that had this high 10,000 oospores per gram, then you can see that even after 10 years we still haven’t hit that (10 oospores per gram of soil).”
Sask Pulse recently published a new risk-analysis fact sheet for aphanomyces that can be found here.
