Getting to the root of the rot

Late season root rot is creeping into pea fields. Sarah Weigum 
investigates the symptoms, and the situation

It sounds a bit dramatic, but the late season root rot that is creeping into Prairie producers’ pea fields feels like a betrayal. Besides being a profitable to grow, peas seemed like the “right thing to do.”

I tend to feel smug when I hear of clubroot in a distant county, because I assume growers pushed canola rotations. But it stings when I hear about farmers trying to avoid nasty canola and cereal disease being hit with a yield-devastating and (at this point) untreatable case of root rot. Planting peas and other pulses lengthens crop rotation from a short cereal-canola cycle and with their nitrogen-fixing capabilities, they are touted as an environmentally-friendly.

Like most farmers who grow peas, we typically plant peas one in every four years. While we have not seen noticeable root rot in our fields yet, others have, even when following a recommended crop rotation. Prompted by increasing reports of the root rot, researchers in Alberta, Saskatchewan and Manitoba are taking a hard look at this disease and possible ways to overcome it.

Root rot symptoms

Above ground root rot symptoms can include patches or large areas of yellowed and stunted plants with decaying leaves. Below ground roots will be thin, poorly nodulated and discoloured, often with a reddish streak inside the main tap root. While seed treatment may provide some control for early season root rots that effect seedling emergence, late season cases of root rot show up after the efficacy period of treatments.

Two years ago my neighbour, Greg, had a pea field that performed poorly. Unaware of root rot, he suspected chemical residue was the culprit, but a review of the chemicals used on that field did not reveal any obvious cause of the damage. Last year, he sowed the adjacent field to peas. It was almost completely affected and he only harvested about 21 bushels per acre.

“Up until the six-node stage the crop was looking really good,” he explained. “It was looking okay until it flowered and then it went south from there.” Greg and his brother have been growing peas about one in every four years for the last 20 years. They consider peas to be one of their most profitable crops, but after two years of severe losses Greg said they planned to quit growing peas. However, last fall he realized he had a bin of leftover treated seed, so he’s going to take his chances in 2014 on a piece of ground that hasn’t had peas on it for six years.

Greg’s field was one of 145 that was surveyed in 2013 in a research project funded by the Alberta Pulse Growers and the Alberta Crop Industry Development Fund. Dr. Syama Chatterton, crop pathologist at AAFC in Lethbridge, along with Dr. Michael Harding and Robyne Bowness of Alberta Agriculture, conducted an Alberta-wide survey for the presence of root rot in pea fields. Ninety-eight per cent of the fields visited showed symptoms of root rot, and yet clearly, 98 per cent of pea growers in Alberta did not experience the loss that my neighbour did.

Overall, the disease severity in the province was three on a scale of severity from one to seven (seven being completely decayed). The ubiquity of the disease combined with the range of severity suggests that while the root rot-causing pathogens are native to Prairie soils, they are found at different levels and may be exacerbated by different environmental conditions. This year, for example, the Vegreville area saw high disease severity, but as Dr. Chatterton said, that can change from year to year.

“When you get higher than normal precipitation at the beginning of the growing season that tends to be when the root rots are more severe.”

In each field the surveyors randomly selected 10 sites where they examined a one metre section of the row, counting the plants that showed above ground signs of root rot. After that five to 10 roots were dug up at each sampling site, then taken back to the lab where they were washed and rated on the seven-point scale. Any roots with a rating of four to five were kept and the pathogens on each root were isolated and identified to help the researchers discover which strains caused the root rot.

I asked Dr. Chatterton if the soil could be fumigated to kill the pathogens, but she said this would not only be too expensive, it would also kill the beneficial microbes. One goal of the field pea survey (which will continue for three more years) is to develop a field indexing system where a producer would be able to take a soil sample from a previously affected field to a lab which would test it for the root rot pathogens and let him know if it is safe to plant peas in that field again.

From the Alberta Farmer Express website: Plant diseases to look for in 2014

A complex pathogen

In an email, Dr. Debra McLaren, crop production pathologist at Agriculture and Agri-Food Canada (AAFC) in Brandon, wrote that, “the development of root rot resistant cultivars is generally recommended as the best method for the control of these root diseases.” There is no easy work-around for plant breeders and pathologists, however, as root rot is caused by various strains of three major pathogens: Fusarium, Pythium and Rhizoctonia. Further complicating matters is the indication that the pathogen composition in the soil may change depending on the environment. As Dr. Chatterton pointed out, research in other crops shows that some pathogens are higher in dry years, while others are more prevalent in wet years.

In 2013 AAFC scientists, with support from Manitoba Pulse Growers, ran field trials with about 60 pea varieties grown in that province to assess their reaction to Fusarium solani, Fusarium avenaceum and Rhizoctonia solani. Included in the trials were Carman peas, a variety previously identified as having moderate resistance to these pathogens, and as two varieties known to be susceptible. These trials will continue until 2017.

On our farm

We are still selling a large volume of peas to our seed customers, although we are concerned that demand may drop if growers find root rot affecting their yields. We are definitely keeping our eyes open for signs of the disease on our farm.

Since root rot pathogens are soil-borne, I think twice about doing custom work with my own equipment or having custom operators on my land. One extension specialist said not to worry, since root rot pathogens are likely already in my soil. Another said to follow the same equipment hygiene practices as I would to avoid transferring clubroot. One suggestion was to go into fields with known root rot problems last, but I wonder how practical this is in crunch times.

I had hoped that introducing fababeans to our crop rotation would relieve some of the rotation pressure, but unfortunately, they are likely susceptible to most of the same root rot pathogens as peas.

“Fababean root system is a little more robust so it could outgrow the pathogen whereas the pea root and more shallow not as robust,” said Dr. Chatterton. So while fabas might not take the yield hit from late season root rot, they might not give the soil any more of a break from the pathogen build up than a crop of peas would.

Dr. Chatterton describes the root rot pathogens as, “weak and opportunistic” so she said the best control against the disease is “to maintain cultural practices that will give your peas a good steady growth rate,” avoid soil compaction, choose well-drained fields, avoid over-fertilizing and any mechanical damage to roots and stems.

I hesitated to write this column. I don’t want to spread undue alarm about pea diseases because there are still many benefits to this crop and they have been good to us and many other farmers for the 20-plus years we have been growing them. However, we need to talk about the reality of what we see in our field so the source of the problem can be correctly identified and treated. The history of agriculture shows that we have learned to overcome other, once-devastating crop diseases. I am optimistic that the research of our scientists into cultural and chemical controls, as well as the development of resistant cultivars, will lead to results that diligent farmers can implement. †

Sarah Weigum grows pedigreed seed and writes in Three Hills, Alta. Follow her on Twitter: @sweigum.

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Sarah Weigum grows pedigreed seed and writes at Three Hills, Alta.

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