Aphanomyces fight fed by rotational spread

The best tool to manage aphanomyces root rot is to extend pea and lentil rotations.

“We really need to get away from those lentils every two years, peas every three years, even peas every four years. We are seeing improvements of pea yields on fields where they get back to that eight-year, 10-year rotation,” said Sherrilyn Phelps, SaskPulse Agronomy Manager.

“I know it’s not something growers want to hear, but this is our new reality.”

She said there are places in the United States and France that can’t grow peas because of aphanomyces, so it’s critical spore levels in fields be brought down to a level crops can tolerate.

When it comes to managing root rot, particularly with aphanomyces, close attention has to be given to the how the fields handle moisture, because the disease thrives in saturated conditions.

“Choosing fields that are drier, have good drainage, are coarse-textured soils, have more sand versus clay, and again looking at the history of your fields and when there was root rot,” says Phelps.

“You want a field that has had no history of root rots in the past.”

The environmental history of when peas and lentils were last grown on a field should also be considered because if these crops were grown on a field during a wet year, there is a good chance those fields have a higher risk for root rot to appear the next time these pulse crops are grown.

“So if you’ve grown pea or lentil within those eight years, within any one of those times where there was higher moisture within a particular region and field, those are areas that are high risk,” Phelps said.

Fields or areas within fields where compaction is known to be an issue also have a higher risk for aphanomyces.

Compaction adds stress to roots because there is little air available to them, and these areas tend to be more saturated.

A survey of aphanomyces in Western Canada found the disease to be widespread without any hot spots.

“That leaves us to believe that it is just at background level there, and the more times you grow peas and lentils, the more we’re increasing that spore level,” Phelps said.

But if peas and lentils can only be grown on a field every eight years growers, will need to find other pulse crops to grow if they want the agronomic benefits of pulses.

“Peas and lentils and cicer milkvetch, are susceptible to aphanomyces. Dry beans and alfalfa, depending on the variety can be variable in terms of their sensitivity and susceptibility. Chickpeas and fababeans are more resistant, and soybeans and fenugreek are not hosts,” Phelps said.

She said a question that often comes up is whether an aphanomyces-resistant chickpea or fababean crop will cause the disease to build up in the soil, because these crops are still a bit of a host for aphanomyces.

A project was started in 2018 and will run until 2023 with six sites in Western Canada, which examines chickpeas, fababeans, and soybeans in a rotation with peas, to see how these crops impact the level of aphanomyces in the fields.

The team examined the disease severity of the crops and if root rot was present in the resistant pulse crops.

Then researchers examined what the specific pathogens were.

Phelps said fusarium was found in all of the samples in the first year of data.

Researchers also found aphanomyces in some of the more resistant crops and non-hosts.

“Soybeans did have a little bit of aphanomyces, it was detectable, same with the chickpeas there was a very small amount. But the amount was so small that they feel that it’s not increasing the level of oospores in the soil for the peas,” Phelps said.

“Right now, it’s looking promising that these alternative pulse crops could be a good fit in a rotation and not impact the oospore loads for the peas and lentils.”

She said another question that needs to be answered is how other crops in the rotation can influence spore loads.

“Brassica species such as mustards with high glucosinolates, are an example where they found that there can be some antimicrobial effects, that have under lab conditions reduced the levels of aphanomyces,” Phelps said.

“The other way the brassica may impact the health of the crop is just because the large biomass that it might return to the soil and provide a source of nutrients to improve the health of these crops.”

She said there has also been some research that suggests brassica species such as brown and oriental mustard has a suppressive effect on the root rots on aphanomyces affected soil on peas.

“Oats is another one in the literature, where they found oats can have a suppression effect on aphanomyces because of the antimicrobial properties, and there is also saponins in oats,” Phelps said.

“The other theory is the oat biomass could stimulate the spores to germinate, and if the oospores germinate when there is no host present maybe that would cause them to die.”

The research work that has been done to see how tillage practices affect the aphanomyces levels in soils does not show a direct impact on the diseases, however she said tillage can be a tool to help manage compaction and help manage water.

“Other impact with tillage, burying the residues, so if you have a high infestation of fusarium from your cereal crops, burying that residue can decrease that load a little bit, but the research has also suggested that environmental conditions have more of an impact then actually burying it does,” Phelps said.

However, she said growers need to be very careful in terms of the conditions they roll their pulse crops.

“Anytime you’re rolling those fields with those heavy rollers and it’s wet, you’re creating compaction and you cause a stress on the plant,” Phelps said.

“All it takes is a stress to allow those roots to be infected by fusarium (which can reduce a plant’s ability to resist aphanomyces). You add in a compaction and a higher level of water and you have aphanomyces. So make sure when guys are out there rolling that the soil is dry.”

Aphanomyces prefers acidic soil, from 4.5 to 6.5 pH.

“Pea and lentil don’t like acidic soils, they like more neutral soils. If you’re trying to grow them where it’s more acidic we’re putting them under stress, we’re also making it more favourable for the pathogens,” Phelps said.

Herbicides do not cause root rot, but they can add stress to the crop and make it more susceptible to the disease.

“The plant can be under stress before you put the herbicide on and it multiplies up the stresses.

Weeds are hosts for aphanomyces.

“Shepherd’s purse, chickweed and vetches are hosts. There is some indication in the literature that kochia and lambs-quarters can also be hosts for aphanomyces. So weed management becomes critical not only in your pulse year, but in your other crop year,” Phelps said.

There is only one seed treatment registered for aphanomyces in peas and lentils, Intego Solo , ethaboxam, which is for early season suppression.

Phelps said pea leaf weevil can amplify aphanomyces in pulses, particularly in the southwestern part of Saskatchewan.

“Just a quick crash course on pea leaf weevil, the problem with them is when they lay the eggs, the larvae go down into the roots and eat on the nodules. That’s where the most impact is,” she said.

If fusarium and the pea leaf weevil is present, then aphanomyces levels in crops can become even more severe.

There is currently no resistance to aphanomyces root rot in current pea or lentil varieties, but there is potential in peas. Lentils don’t currently appear as likely.

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