Scouting for Aphanomyces root rot and other root rots is crucial in disease management. You can detect the disease early by digging up plants and inspecting the root systems. Initially, the above-ground portions of infected plants will appear green. However, as the root system decays, the stems and leaves will gradually turn light green and then yellow.
If Aphanomyces is suspected, soil and root samples can be submitted to a commercial lab for DNA testing. Confirming the presence of Aphanomyces in your soil or plants will aid disease management and future crop planning.
Fusarium root rot often occurs in conjunction with Aphanomyces root rot and can mask Aphanomyces root rot symptoms. Therefore, when requesting an Aphanomyces root rot DNA test, it is crucial to include Fusarium spp. testing as well, ensuring comprehensive disease diagnosis.
To measure root rot severity, researchers use a 0–5 Aphanomyces root rot rating scale in combination with a 1–7 Fusarium root rot scale when both pathogens are present. Since most infections in the field are typically combined, the Fusarium 1–7 scale is often used, highlighting the difficulty of identifying the honey-brown discolouration from Aphanomyces when Fusarium is also present.
Testing & Results
Scout for suspect plants showing signs of root rot, such as wilting, yellowing, and discoloured roots. Growers and agronomists can submit fresh root tissue, whole plants, and soil for root rot pathogen testing.
Plant and root tissue samples taken in season are the best method for confirming a root rot or Aphanomyces infection. Outside of the growing season, post-harvest or pre-seeding soil DNA analysis using a soil bait test is recommended.
Sample Collection
Assess five areas of the field in a W or zigzag pattern. Visually inspect the crop for patchy emergence, chlorosis, necrosis, wilting, or other symptoms. Pay particular attention to low spots, where water flows or accumulates, and compacted areas like field entrances or headlands. Be sure to sample these areas when attempting to confirm or diagnose a root rot complex infection.
- Plant samples: Dig around the plant to obtain the whole plant and root system.
- Soil samples: Remove the residue layer, then take a soil sample from the central rooting depth of four to eight inches or less if the A-horizon is shallow. A minimum of two cups of soil is required for testing.
To test the plant, place plant samples in a labelled plastic bag with a paper towel, with the roots wrapped in foil, or in a labelled brown paper bag. Soil samples can be placed in labelled, sealable plastic bags. Keep all plant and soil samples cool (4oC) by placing them in a cooler with ice packs or storing them in a fridge until delivered to the lab for testing. Do not collect or ship dead plants or wash seedlings before submitting. Samples should be shipped or dropped off the day they are collected. Shipped samples should be in a container or cardboard box to avoid crushing. Avoid sending samples immediately before the weekend.
Types of Tests
- A qPCR (DNA) test is fast, affordable, and usually accurate for detecting the presence of oospores in the soil. Some labs can provide the number of oospores per gram of soil to be used as a baseline for Aphanomyces root rot management. DNA tests are usually accurate, although, in dry conditions, they may return a false negative when the pathogen is present. A follow-up bait test, although taking longer, can confirm the DNA test.
- Soil Bait Testing is another technique that involves planting seeds of susceptible crops whose roots act as bait to attract Aphanomyces. The roots are then analyzed for the presence of oospores. Baiting is a slower process but can be more sensitive if low levels of Aphanomyces are present.
List of Soil Certified Testing Labs
The North American Proficiency Testing Program (NAPT) can accredit labs that analyze soil and plant tissue for nutrient analysis.
- List of accredited labs across North America | North American Proficiency Testing
- List of labs across Canada | Alberta Pulse Growers
- Testing for Aphanomyces and Other Root Rot Pathogens | SPG
- Scouting and Sampling for Root Rot in Pulses | SPG
Results
Infected plants are concentrated sources of disease and represent a significant increase in the inoculum present in a field.
The results from a lab test in the fall can mean several things, depending on the reporting lab. The first is positive or negative. This means that the Aphanomyces pathogen is present in the sample but does not indicate the number of spores per gram of soil or the level of infection in the root tissue.
Some labs take the positive/negative test one step further in their reporting. A positive result means that the pathogen is present at levels capable of causing disease. A negative result cannot guarantee that the pathogen is not present in the field but is just not present in the soil sample.
Finally, some labs provide a quantitative result reporting on the number of oospores per gram of soil, usually at an additional cost. This number is helpful since research by Agriculture and Agri-Food Canada has determined the risk of A. euteiches causing disease is 100 oospores per gram of soil in the Dark Brown soils and 750 oospores per gram in the Brown and Black soils. However, the presence of Fusarium spp., particularly in the Brown soils, increases disease severity. Because Fusarium spp. is so widespread in the Prairies, the risk threshold for A. euteiches has been adjusted.
The number of A. euteiches oospores at which the pathogen may start to impact the crop has been set at 100 oospores per gram of soil for all soil types.
Suppose A. euteiches are confirmed in a field. In that case, growers should follow recommended best management practices, including removing peas or lentils from their current rotation, considering incorporating non-susceptible or more tolerant pulse crops such as faba beans, chickpeas, or soybeans, and maintaining a minimum of six to eight years between susceptible hosts such as peas, lentils, dry beans, and alfalfa.
Root Rot Diagnosis
An accurate diagnosis of root rot aids future crop management decisions. Results may reveal trends among varieties and management practices affecting soil or other inputs and stresses.
The following are essential to gather for root rot diagnosis:
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Field History |
Crop rotation, last year in pulses, and the number of pea or lentil cycles. |
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Herbicide History |
Herbicides have been used throughout the year and history. |
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Environment |
Moisture situation leading up to the problem, including previous year(s). |
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Soil Information |
Texture, organic matter, pH, signs of compaction, flooding or water runs. |
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Seeding Information |
Variety, seeding date, depth, seed treatments, inoculant, fertilizer amount and placement as applicable. |
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Field Information and Maps |
Legal land location, map of good and bad areas, notes on topography and patterns in the field where symptoms are present and not present—mark waterways, side hill seeps, heavier soil, etc. |
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Patterns in Field |
Note any patterns that may be visible. Patterns may relate to equipment such as misses, overlap areas, swath and chaff rows from harvest, and compacted regions. Note seeding and sprayer direction. Patterns may also relate to other factors, such as field edges. |
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Photos and Samples |
Good photos are critical. Aerial photos are great for identifying patterns. Collect plant and soil samples from both good and bad areas for analysis. |
