STACKED DECK: Root rot pathogen holds all the cards

You can’t spray aphanomyces or buy resistant varieties, so longer rotations are the only tool

The yellow patches indicate that aphanomyces root rot has reached an advanced stage. But even if detected earlier, there’s nothing that can be done save not planting peas in this field for several years.

Hard to spot, impossible to treat, and no resistant varieties.

So when aphanomyces root rot does show up, you know you’ve got a long-term problem.

“Once you see it, you kind of get scared and realize you’re stuck with it for a bit,” said Bow Island producer Will Müller.

Because it is a soil-borne pathogen that attacks roots, it can’t be sprayed. Seed treatments are only effective in the early stage of crop growth, and there are no resistant cultivars yet, although extensive work is being done at the University of Saskatchewan’s Crop Development Centre.

Even determining if a field is infected isn’t easy, said Syama Chatterton, a plant pathologist at AgCanada’s Lethbridge research station.

“It’s just been a really challenging pathogen to work with because it’s in the soil matrix, so it makes detection and quantification of DNA a little more challenging,” she said.

“It definitely has the potential to take out a whole field, and it’s very environment dependent.” – Syama Chatterton. photo: Supplied

The silver lining — such as it is — is that producers who are new to growing peas and lentils know that the pathogen is a major threat. Early adopters of the crop were side-swiped and it wasn’t until 2013 that aphanomyces was officially confirmed in the province (although it was likely around for a long time before that).

“Producers who had been growing lentils and peas for 20 years were hit with this quite unexpectedly and didn’t know what was going on,” said Chatterton. “We have more knowledge now. It’s good for new producers to know how to manage this going forward.”

However, the only effective tool is to spread out rotations and not grow peas or lentils too often. In fact, Chatterton recommends growers consider one-in-eight-year rotations for those crops (although she also recommends another pulse, such as chickpeas, in order to have a one-in-four-year pulse rotation).

The intensity of an infestation can vary, and usually depends on the weather. In a wet year, the pathogen can spread widely and reduce yields by as much as 70 per cent. And if there’s standing water, aphanomyces can spread through the whole field.

“In other areas, where you don’t get that moisture, you’ll see it in patches, and it really doesn’t affect the whole field,” she said. “It definitely has the potential to take out a whole field, and it’s very environment dependent.”

If a pea crop is affected by aphanomyces, it can show some yellowing.

“One of the challenges is they (infections) don’t always show shoot symptoms,” said Chatterton. “It’s not until the disease is quite progressed that you’ll start seeing the yellowing and stunting of the plant. I always recommend that people go and dig up roots and take a look at those roots, and get an idea of what is happening below ground before you see that yellowing.”

Aphanomyces root rot is caused by “a highly specialized pathogen of legumes,” according to a fact sheet put out by Saskatchewan Pulse Growers. Peas and lentils are most susceptible, fabas have some natural but partial resistance, and chickpeas are considered moderately resistant.

The fact sheet also notes that there is a lot that isn’t known about infections. Sometimes healthy plants are found right next to infected ones. Infections later in the growing season may not cause substantial yield loss but can delay maturity and cause severe lodging because of weakened stems. Soil moisture plays a critical role but it’s not known what moisture level causes peak disease or why major outbreaks can occur in dry conditions.

All three Prairie pulse commissions are funding research into these and other questions. A key tool would be to have a soil test to measure aphanomyces levels — something that Chatterton has been working on.

“That’s been my major research focus for the past five to six years,” she said.

This year she is also starting a three-year study to see whether different types and rates of calcium-based products (limestone, hydrated lime, and calcium sulphate) can be used to mitigate the impact of aphanomyces in infected fields.

About the author

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Alexis Kienlen

Alexis Kienlen lives in Edmonton and has been writing for Alberta Farmer since 2008. Originally from Saskatoon, Alexis is also the author of two collections of poetry, a biography, and a novel called "Mad Cow."

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