The Rise of Aphanomyces - a concern for peas and lentils

Management strategies and plans for controlling the pathogen.

Allison Friesen M.Sc.

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In 2014 environmental conditions during seeding were less than favourable throughout most of Western Canada, with excessive rainfall and a prolonged cool spring leading to extreme seedling stress and significant stand loss in pea and lentil fields. Wet, cool conditions have been an ongoing trend in certain areas of Western Canada over the past 5 years and as a result the soil in these areas has become severely waterlogged. The impact of these saturated soils has been most evident in the pulse acres because of their higher sensitivity to wet soils. This has resulted in delayed seedling growth and increased disease pressure, especially root rots, leading to reduced yield and, in extreme instances, crop loss.

The infection levels of pathogens like Fusarium, Rhizoctonia, and Pythium have dramatically increased across the prairies as a result of these conditions. These diseases are manageable by the use of fungicide seed treatments like Insure® Cereal in wheat for instance. However, a growing concern is the recent increase in the incidence and prevalence of the pathogen Aphanomyces that could have a dramatic impact on pulse acres throughout most of Saskatchewan and parts of Alberta.

Aphanomyces euteiches is a pathogen that is found worldwide and is a large concern particularly in the pulse growing regions, due to the dramatic impact it has on seedlings and the lack of management options available to combat the effects. The first case of the pathogen in Canada was reported in the early 1930’s in Ontario however it was only positively identified in Saskatchewan soils in 2012. Doctor Sabine Banniza, a plant pathologist in the Department of Plant Sciences at the University of Saskatchewan, believes this pathogen has been present for much longer than 3 years, but its impact was not substantial so it went undetected until recently. To support this hypothesis, it has been found that the pathogen is present in soils across Saskatchewan, which is indicative of a pathogen that it has been prevalent in the soil for a significant amount of time.

Consequences for pea and lentil crops

Aphanomyces is a concern for Western Canada as the pathogen has been shown to have a detrimental effect on entire growing regions where field pea and lentil are common rotational crops. Examples for the impact of Aphanomyces come from France and the Great Lakes Regions of the U.S.A. where inoculum levels have become so high that pea production and the associated processing facilities around them have been relocated to areas that are drier and less conducive to the pathogen.

The dramatic effects forcing pea production and processing to move to new areas are due to the incredibly resilient characteristics of the pathogen. Unlike most other root rot fungi, Aphanomyces is a fungus-like pathogen that belongs to the Oomycete family. Oomycetes have the ability to form protective resting spores that can live in the soil for up to 10 years and are insensitive to most chemical treatments. This pathogen also produces motile spores, called zoospores, which have the ability to move short distances through the soil water matrix with whip-like tails that allow them to “swim” towards susceptible plants. Unfortunately, there are no known chemical or agronomic practices (aside from a lengthy rotation) that are effective at managing the disease. Even metalaxyl, a specific Oomycete fungicide that controls Pythium spp. from the same family as Aphanomyces, has no activity on the pathogen.

Aphanomyces is a true root rot, unlike other pathogens that cause seed rot, seedling blight, etc. It is specifically attracted to the chemical exudates released by the roots emerging from the seed. Once a seedling has been infected, few above-ground effects are observed early on. The most obvious visual effect is at and below the soil line where the shoot of the seedling will have a girdled appearance and root masses are decreased in size, have lower nodulation and display a distinct caramel-brown colouring (Figure 1). Above-ground symptoms are usually visible only a week or two after infection and consist of yellowing of the lower leaves, stunting and in severe cases seedling death.

Figure 1: Above and below ground symptoms of Aphanomyces Root Rot. A. Stunting and yellowing of leaves, B. Caramel colouring of the roots, C. Girdling of the stem.
Source B: U of S Cherly Cho, Dr. Sabine Banniza Lab

Management strategies for risk reduction

The growing prevalence of Aphanomyces in Western Canada is due to the increasing buildup of inoculum in our soils. This buildup is due partially to the wet conditions we have been experiencing over the last 5 years, which have made soils wetter and sometimes waterlogged and compacted. Combined, these conditions are favourable for spore reproduction and survival. However, the longevity of the spores in the soil and tight crop rotations of field pea and lentil are a large contributing factor to inoculum buildup as well. With crop rotations for pea and lentil being as short as 2 to 3 years, little reductions of oospore numbers in the soil will occur. Crop rotations of up to 6 to 8 years are recommended on infested soils to decrease the amount of inoculum in a field. For most growers, this seems impossible; however, with no current chemical controls available there are few other control options.

Even though increased time between susceptible pulse cops (pea and lentil) is the number one recommendation, some other management strategies are recommended by Saskatchewan Pulse Growers:

  • Field choice - Use fields with lighter soils that have better drainage and avoid or deal with compacted soils.
  • Plant health - Healthy seedlings are able to withstand infection and/or recover better.
    • Use starter nitrogen (10-15 lbs/ac), if soil is below 15 lbs available N per acre.
    • Use a proper inoculant to promote nodulation.
    • Use phosphorous, if soil is deficient or when seeding into cool soils.
    • Test seed quality to ensure best seed lots possible.
    • Use seed treatments for protection against other fungal pathogens.
      • Minimize physical seed damage.
      • Reduce auger and wind speeds.
    • Do not use seed from the bottom of the bin.
    • Roll crop at optimal conditions to reduce plant stress. Do not roll if soil is wet. In certain instances omitting a rolling operation where applicable is recommended.
    • Follow herbicide labels to reduce crop injury and avoid additional stress.
  • Select cultivars/crops with good partial resistance to Aphanomyces as an alternative to pea or lentil (e.g. chickpea, soybean and faba Bean are genetically more tolerant to Aphanomyces)

Plans and recommendations

Currently there are no cultivars of pea or lentil that have any resistance to Aphanomyces. However, Dr. Banniza is currently working with labs in both the U.S.A and France on breeding lines with resistant traits. In the meantime, Dr. Banniza suggests that growers look to alternative crops that have natural tolerance to Aphanomyces. In lighter, drier soil zones, chickpeas are viable options with cultivars such as CDC Frontier having partial resistance. For the wetter, black soil zones soybeans and faba beans are also great options.

Overall, the information we have about the Aphanomyces pathogen is still somewhat limited in Western Canada. Institutions like the University of Saskatchewan and Agriculture and Agri-Food Canada continue to research the infection process and survival of the pathogen, along with methods to quantify it in soil samples and breed for tolerant lines in pea and lentil. The Saskatchewan Pulse Growers are also working on viable solutions and creating recommendations for control, while BASF continues to actively screen for effective chemistries. In the meantime, growers can help manage the Aphanomyces pathogen by being more aware of seedling health, field selection and crop rotation.

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