Root Rots on the Prairies - PulsePoint
December 17, 2018
Latest survey results show root rots still causing problems for pulses
by Megan Madden
Seeing poor emergence in patches of your pulses? Maybe some yellowing of leaf tissue? Decayed, brown roots? You might be looking at root rot.
Root rot is a soil-borne disease that can affect the underground portion of peas and lentils at any stage. Understanding this disease and the pathogens that cause it is essential, because once root rot sets in, there is no treatment available.
This is where Dr. Syama Chatterton comes in. Dr. Chatterton is a plant pathologist at the Lethbridge Research Centre with Agriculture and Agri-Food Canada who conducted a root rot survey across Saskatchewan in the summer of 2018.
“We performed a lower number of surveys across the Prairies this year compared to previous years because we have already established that Aphanomyces euteiches and a couple of Fusarium species are the predominant pathogens. Research efforts and time are now better spent on looking at management options,” she explains. “Having said that, we did survey approximately 26 fields in Saskatchewan so that we can use the data to start validating a quantitative soil assay.”
Chatterton discovered that all of the fields surveyed had root rot present. Within a field there were 10 sampling sites and on average 85 per cent of the sampled sites came up positive for root rots. This shows that root rots are being found across the majority of a field rather than in isolated areas. The average severity rating across all 26 fields was fairly low at 2.8 out of 7 where 7 is highest severity. “We have not tested pathogen composition yet, but based on the root symptoms only, Aphanomyces seemed to be present in about 40 per cent of fields,” says Chatterton. It is difficult to identify the composition of root rot pathogens once plants are already damaged or dead because other organisms start to feed on decaying tissue. Also, there is often more than one pathogen present in root rot, making it harder to determine which pathogen was the primary cause of the disease.
These different pathogens thrive under different conditions. Weather and precipitation have a significant impact on infection, depending on moisture levels. Aphanomyces euteiches belongs to a group of root pathogens commonly referred to as “water moulds.” Chatterton
explains that water-saturated fields are the ideal conditions for Aphanomyces to infect and spread across a field with water movement. “Dry conditions inhibit infection by Aphanomyces, as it requires water for infection,” she continues. “But some of the other pathogens in the root rot complex, like the Fusarium spp., can still infect under dry conditions.”
The above average snowpack melted rapidly in the spring of 2018, causing wet soil conditions which provided excess spring moisture for early infection. “The effect of these early infections was then exacerbated by the dry conditions in July and August, as plants already starved for water would have a very diminished root system due to early root rot,” Chatterton says.
Diminished root systems can reduce stand establishment, impact nitrogen fixation, and have negative effects on root distribution and vigour. Those factors can then lead to uneven plant stand, which can cause subsequent difficulty in managing weeds in those areas.
Prevention of Aphanomyces root rot through field selection is currently the only tool to avoid a scenario like this repeating itself. As of today, there are no resistant cultivars, and seed treatments provide only early season suppression, and even then, only for about two weeks.
“It is important to know the history of your field, how often has a pea or lentil crop been planted in that field, and how recently?” advises Chatterton. “If you have not had a long history of growing these susceptible pulse crops, have been rotating out of peas and lentils for four or five years, and the last pea or lentil crop you grew did not show any signs of root rot, then it is likely safe to plant peas or lentils in that field.”
Growers are encouraged to watch their fields in the upcoming season for small patches of yellowing plants in water tracks or low spots in the field, as an indicator that a root rot problem may be developing. If a field has a long history of pulse production, has been on a tight rotation (peas or lentils every two to three years for a number of years), and the grower has noticed yellowing patches in previous crops, then Chatterton recommends choosing another non-host pulse crop (soybeans, chickpeas, or faba beans), or other non-pulse crop for the field.
While conducting your regular soil tests, try to choose some samples from areas that are prone to flooding, or that have yielded poorly in the past, to test for Aphanomyces. “Remove the stubble layer and take a soil sample from the top 15 to 20 centimetres of soil,” explains Chatterton. If Aphanomyces is detected, it is important not to grow a susceptible host for a minimum of six years, as it can infect peas, lentils, alfalfa, and more.
Rotation, soil testing, and strategic use of inoculants and seed treatments can all help mitigate the risk of infection. Moving forward, managing fields for root rot, in order to keep cropping options available, can help keep growers profitable.