Root rots continue to plague pea and lentil crops in Western Canada. Understanding management strategies to reduce the risk of infection in future years starts with identifying the issues in existing fields and tracking them over time. After we know what we are dealing with in a field, it is all about planning to reduce the spore load. For future pea and lentil crops, it is about proper risk evaluation and reducing the risk of infection through field choice, field histories, rotations, and management decisions.

Setting a Baseline on Fields Already Infected with Aphanomyces

Fall soil sampling can be a great time to establish a baseline not only for soil fertility but also for pathogen load. In root rots, the level of Aphanomyces euteiches in the soil sets the risk for future pea and lentil crops. Knowing which areas of the field were most affected is important, as it usually starts increasing in severity in compacted areas (field entrances or headlands), low-lying areas, and side-hill seeps, and then, under the right conditions, spore loads build to infect the whole field. Soil samples are a good way to document Aphanomyces oospore levels at a single point in time. You can either take a representative sample from across the field if the whole field is infected or from high-risk areas. Make sure to mark the areas sampled so you can go back over time and resample to ensure your results are from similar spots and to see if levels are changing. Collect soil samples from a 4-8 inch depth after the thatch or residue layer is removed. Remember, the soil contains living organisms, so it is important to handle it properly, keeping it cool and out of direct sunlight, and deliver it to a lab as soon as possible. Refer to the Testing for Aphanomyces document for the list of soil testing labs and the specific tests offered. If you are looking to establish a baseline, you will want to go with a lab that can accurately quantify the number of oospores per gram of soil, rather than just reporting whether Aphanomyces is present or not.

Reducing Risk of Infection in Upcoming Pea and/or Lentil Fields

1. Soil Sampling Fields Prior to Planting Peas or Lentils

It is important to sample fields prior to planting peas and lentils to understand whether Aphanomyces is present and at what levels. DNA tests provide data on either the presence or the quantification of oospores based on soil extraction. A challenge with these tests is obtaining a representative sample of an entire field in 1 gram (g) of soil, then extracting the oospores from the soil and breaking open their thick cell walls to extract the DNA. Researchers are exploring ways to improve test procedures and increase the consistency and reproducibility of tests, especially under dry soil conditions, where false-negative rates can be high.

There is also a bait test in which peas are planted in the soil, and then the pea roots are analyzed for infection by Aphanomyces. The bait test is very sensitive because the peas growing in the pot attract zoospores released from germinating oospores. The bait test also uses a larger soil sample than the DNA extraction tests, which rely on very small (0.5–1 g) amounts of soil to represent the entire field. Bait tests, when combined with DNA tests, are the most accurate for determining the presence and infectivity of Aphanomyces.

For predicting risk going into the upcoming season, it is about the presence or absence of the pathogen. Most labs have a detection threshold, and anything above that level is considered high risk. Some labs report results as positive or negative, classify risk as low, medium, or high, or provide actual quantitative numbers. The accuracy of the test is only as good as the sampling procedures and can be severely impacted by variability in the field. Due to the small amount of soil analyzed (<1 gram), there is a chance that Aphanomyces oospores may not be present in that sample or at very low levels, even if Aphanomyces is present in other areas of the soil. This means there is a chance of false-negative results. If negative results are obtained, they do not guarantee the absence of Aphanomyces; they could mean that, in the small sample size, the DNA level was below a detectable level or that there were issues with DNA extraction, as mentioned previously. There is a lot of variability in a field and within soil, so relying solely on soil sample results is not recommended.

2. Review Past Field History

a) Number of times peas/lentils have been grown in a field

It is important to know how many times peas or lentils have been grown on a field, as the higher the frequency, the greater the risk for the build-up of soil-borne pathogens. It has been suggested that growing susceptible crops just five times can increase soil pathogen levels under the right conditions. In Saskatchewan and Western Canada, peas and lentils have been grown for approximately 20–30 years on most farms. With a four-year rotation, this means peas or lentils have been on these fields 5–7 times. With shorter rotations, such as lentils in a two-year (lentil-durum) sequence, these fields would have had lentils 10 times in the past 20 years. This means that many acres in Saskatchewan that have long been pea and/or lentil fields are at the greatest risk. Going forward, avoid long-time pea or lentil fields by choosing fields that are relatively new to growing pulses or have been out of pulses to reduce risk.

b) Environment when peas/lentils were previously grown

It is not only the number of times susceptible crops were grown, but also how many of those years were conducive to Aphanomyces infection. Aphanomyces prefers water, and disease incidence is always worse in wetter years, particularly when May/June have higher-than-normal precipitation.

For example, in 2020, many lentil fields in West-central Saskatchewan were devastated by root rots. Many of these fields were last planted to peas or lentils in 2016, four years previously. In that area, 2016 was a wet year, which meant a higher risk of infection and likely large increases in the Aphanomyces oospore load in those fields. In the years since, even in drier-than-normal growing conditions, one or two timely rains have resulted in heavy root-rot infestations, which continue to be linked to fields that had peas or lentils during the wet 2016 growing season. Looking at the environment, the last time or the last few times peas and/or lentils were grown is important to determine whether the field could be at higher risk in future years.

In general, Saskatchewan experienced above-average rainfall early in the season in 2010, 2012, 2014, and 2016 (see Agriculture and Agri-Food Canada for historical climate conditions). Avoiding fields with peas or lentils in those years will help reduce the risk of severe Aphanomyces infection. For example, a tight two-year lentil rotation in 2020 in Southwest Saskatchewan would have had lentils in 2010, 2012, 2014, 2016, and 2018, of which four out of five years were above average for early-season precipitation. In this example, the two-year lentil rotation is at high risk of root rot due to inoculum load buildup from intensified rotations in previous seasons with above-average rainfall. Growers who practice a three-year rotation on lentils may be better situated to grow lentils in any given year, as there is a greater chance that the last time their fields had an aphanomyces host crop was not under high moisture conditions, and the frequency of the host crop in rotation is significantly less. However, it is highly recommended that growers considering growing lentils or peas in any given year examine their past rotation as a whole and consider testing their fields for aphanomyces oospore loads utilizing soil bait tests in the fall to establish the risk potential on a field-by-field basis. Figure 1 provides a visual example of a field of peas grown on a half-section that was combined from two quarter sections into a half-section field in 2014. On the left side, peas were grown in a 4-year rotation, with peas planted in 2006, 2010, and again when this image was taken in 2014. On the right, peas were grown in an 8-year rotation; peas in 2006, canola in 2010, and peas in 2014. One additional year of pulses during a high moisture year (2010) led to a heavy oospore load and subsequent disease pressure on the left side of the field.

3. Rotation

For whole fields that are infected, it is recommended to move away from peas or lentils for 8-10 years to allow spore loads to decrease below threshold levels. It is not so much about the exact years between susceptible crops, but about the severity in the past, the environment during the years the crops were grown, and what has been on the field since.

Aphanomyces has very long-lived resting spores that remain viable for 10+ years. That means the degradation of the oospores occurs very slowly over time. In fact, 10 years may not be enough under very high infection levels, and many areas of the world have abandoned some fields to pea production as a result.

Reducing the spore load in the soil for Aphanomyces has been challenging. Research looking at incorporating green manures from oats, corn, rapeseed, and mustard has shown some positive impact on the severity of disease under greenhouse conditions, and a research project by Dr. Syama Chatterton with Agriculture and Agri- Food Canada, Lethbridge, has just recently started to evaluate green manure and grain crop impacts on spore load in Western Canada. Dr. Chatterton and her team are investigating oats, rye, mustard, forage mixes, and non-host legumes, but field trial results are not yet available.

4. Choosing the Right Field: Texture and Drainage

Field choice can influence the risk of Aphanomyces, as it is all about water management. Fields with heavier textured soils (high clay content) and poor drainage are most susceptible to waterlogging or holding onto water and are therefore riskier. Aphanomyces needs free water to move to the roots and infect the crop. Choosing fields that are drier, lighter in soil texture, and have good drainage can help reduce the risk of Aphanomyces. Soil compaction can also affect the soil’s ability to drain, and therefore, managing compaction or avoiding fields with compaction is a good strategy to reduce risk.

5. Choosing the Right Field: Managing Fusarium

The root rot complex consists of more than just Aphanomyces, and we know the most severe fields are usually infected with more than one pathogen. In fact, it is usually Aphanomyces and Fusarium together that are the most devastating. Therefore, it is not only about managing Aphanomyces but also Fusarium. Fusarium is a common pathogen of all crops grown in Western Canada and can cause root rots and Fusarium Head Blight (FHB). One suggestion for managing root rots in peas and lentils is to avoid fields that were heavily infected with FHB and where a lot of infected residue remains. The heavily infected residue could increase Fusarium loads in the soil and increase the risk of Fusarium root rots. In the presence of Aphanomyces, these fields could then be severely impacted by root rot. Therefore, reduce risks by choosing fields that are free from or have low levels of Fusarium-infected residue.

6. Crop Class and Variety Selection

Peas and lentils are equally susceptible to infection by A. euteiches, and a lot of work is underway to develop new varieties that confer some resistance, but it will be some time before they are available commercially. Fusarium is also important to consider. Recent work by Dr. Sabine Banniza at the University of Saskatchewan has shown that there are differences among varieties and classes of peas and lentils in resistance to Fusarium avenaceum under controlled conditions (Fusarium Root Rot in Pulse Crops). If variety choice can influence tolerance to Fusarium, it may help reduce the overall severity of root rots when combined with Aphanomyces infection. Crop types with tannins in the seed coat (e.g., maple peas) do provide some protection against Fusarium infection and other seed-borne pathogens. More recent work by Dr. Chatterton has shown that the tannins help reduce seed infection but do not provide season-long resistance.

The species of Fusarium that is present can also influence the severity of infection. Dr. Chatterton has found that peas are susceptible to the aggressive nature of both Fusarium solani f.sp. pisi and F. avenaceum. With lentils, there is more tolerance to F. solani than peas, and green lentils appear to be more tolerant than reds to F. avenaceum. Therefore, lentils and peas do differ to some degree in tolerance to Fusarium species, which may affect the severity of root rots when combined with Aphanomyces infection. Consider growing varieties of peas or lentils with tannin in the seed coat and/or those with greater tolerance to Fusarium species to help reduce the risk of early infection or the severity of root rot infections.

7. Seed Quality and Insect Pressure

Seed quality is important, and sourcing seed with good germination, vigour, and disease-free status can help reduce the introduction of pathogens into fields. Fusarium can be seed-borne, but to date, no Aphanomyces has been detected on seed. Managing the introduction of Fusarium is important for managing root rots. If Fusarium is present on the seed, a seed treatment is important for managing it.

Seed treatments are available for control of Fusarium and early-season suppression of Aphanomyces. These seed treatments will not protect high-risk fields or fields with known Aphanomyces infection. For lower-risk fields, seed treatments can help keep infection levels down and should be considered. Another consideration is nodule damage caused by pea leaf weevils, as it has been shown to increase the severity of Fusarium root rot. In areas with high pea leaf weevil pressure, an insecticidal seed treatment may reduce the severity of root rot infection.

8. Nutrients

Any stress imposed on a crop can increase its susceptibility to disease. Managing pulses requires a nutrient plan to address soil deficiencies and supply the necessary macronutrients for healthy, vigorous growth. Good phosphorus management is important for growth, nodulation, and yield. Infected roots are also less able to access nutrients, so ensuring a good supply is available can help reduce stress and, therefore, possibly reduce the impact of root rots on yield. Initial results from a project led by Jessica Enns with Western Applied Research Corporation have demonstrated that good, balanced fertility can help to protect yield when grown on Aphanomyces-infested land. They showed that using a blend consisting of actual nutrient levels of 20-50-20-10 pounds per acre (lb/ac) of nitrogen, phosphorus, potassium, and sulphur (N-P-K-S) had a 9–10 bushel per acre (bu/ac) yield advantage over straight MAP at 4.5-20-0 actual lb/ac in an Aphanomyces-infested field.

Preliminary results of work led by Drs. Michelle Hubbard and Luke Bainard found that adding nitrogen fertilizer to peas or lentils may reduce root rot symptoms in some situations. However, nitrogen fertilizer addition can also reduce nodulation and has been shown to have variable impacts on yield, increasing yield in some circumstances and decreasing it in others. This study also found no impact of arbuscular mycorrhizal fungi on root rot.

Nutrient management should be included in your crop input plans prior to the upcoming growing season to reduce the risk of yield loss should Aphanomyces root rot appear.

9. Weed Management and Weed Pressure

Aphanomyces infects peas and lentils and can infect other plants, including some of the weed species we grow on the prairies (ex. shepherd’s purse, chickweed, vetches). These weeds provide a bridge from year to year and can keep oospore levels up. Therefore, weed control is important in all years—even when peas or lentils are not grown. Fields that continue to have weed pressures that are hosts to Aphanomyces are at more risk of not having a decrease in spore loads in soil. Managing weeds is important in all years of a rotation. It is also important to avoid herbicide damage to pea or lentil crops, as this stress could make them more vulnerable to root rot.

Summary

There are many unknowns about managing Aphanomyces; some factors to consider to minimize infection risk and influence infection severity when planning next season’s pea or lentil crops (Table 2). Use this risk assessment to identify fields that will be lower risk and to determine which management decisions can be made prior to the start of seeding. High-risk fields should not be seeded with peas or lentils until a soil test is negative or at least 8–10 years have elapsed since the last pea or lentil crop. Lower-risk fields are not no-risk fields. Soil tests can yield high false-negative rates, and soil can vary in spore load, which can affect test results. Use the Intermediate and Minor risk factors to help guide your field choice and management decisions prior to planting.