Root Rot in Pea and Lentil in Western Canada

Root rot of pea and lentil is a disease that affects the below ground portion of the developing plant, leading to poor performing pulse crops. The organisms that cause the disease are soil borne and can infect the plant at any stage. Unfortunately once root rot has set in, there is nothing that can be done. Understanding the disease, identifying the risks for root rot infection, and thorough planning for prevention are the only options.

Root Rot Symptoms

• Poor emergence, stunting, yellowing of leaf tissue, a reduced root system, decay, and brown discolouration of roots.
• Nodules are often reduced, pale in colour, or have not developed.
• Typically occur in patches and may expand if conditions are favourable for the pathogens over several growing seasons. Symptoms are often associated with areas of flooding or waterlogging.
• Difficult to identify root rot pathogen(s) once plants are heavily damaged or dead, due to the presence of other organisms that feed on decaying tissue.
• Pathogens associated with root rot often appear in the form of a complex, where more than one pathogen is present, making identification of the primary causal agent difficult.

Root Rot Pathogens

Various fungal and fungus-like organisms make up the complex that cause/contribute to root rot. Other conditions can also contribute to root rot, including abiotic factors such as flooding and soil oxygen depletion. Abiotic factors can result in root cell death and symptoms that are similar to root rot, as well as facilitate infection by root rot pathogens.

Fusarium Root Rot

Fusarium species isolated from pulses in Saskatchewan include F. avenaceum, F. solani, F. redolens, F. oxysporum, F. graminearum, F. equiseti, F. culmorum, and F. poae. These are non-specialized pathogens that can also infect cereals, causing root rot and head blight. A distinguishing feature of fusarium root rot is a red discolouration of the vascular tissue below the soil line.

Aphanomyces and Pythium Root Rots

Pythium spp. and Aphanomyces euteiches are organisms that belong to a group of fungal-like root pathogens commonly referred to as “water moulds”. As the name indicates, they are particularly adapted to wet, waterlogged soils. Pythium spp. can be controlled with certain seed treatments, however there is no effective seed treatment available against A. euteiches. Because of the lack of seed treatment and the longevity of its spores in the soil, A. euteiches is the most difficult and therefore most serious pathogen among the root rot pathogens.

A distinguishing feature of aphanomyces root rot is the development of caramel coloured roots (below). Later, roots darken and the cortex is sloughed off.

Other Forms of Root Rot

Rhizoctonia solani is also a root rot fungus that may be present in soil. Botrytis and Sclerotinia are other pathogens that may be present on seed and able to cause seedling diseases as well.

Risk Factors for Root Rot

Stress factors that delay germination and slow emergence and growth of plants contribute to an increased risk of root rot infection.

• Wet Conditions: Wet feet stresses plants and reduces rhizobial activity. Root rot fungi need water to germinate and infect roots
• Cool Temperatures Early in the Season: Slow plant growth and slow nitrogen availability from organic matter
• Shortened Rotations: Increase level of pathogens in soil
• Heavy Textured Soils: More prone to waterlogging and compaction
• Soil Compaction: Root growth impeded and less aeration
• Nutrient Deficiency: Slows seedling growth and weakens plant

Key Facts

• Plant roots and nitrogen fixing bacteria need oxygen. When the soil is saturated, roots function poorly, and Rhizobia activity is slow, resulting in yellow growth.
• Cool conditions slow seedling metabolism and root growth. This also slows mineralization of nitrogen from organic matter.
• Under cloudy skies, plants turn pale green and yellow due to reduced photosynthetic activity.
• Seed with low vigour and stressed plants are more susceptible to seedling diseases.
• Seed treatments are ineffective past the seedling stage and foliar fungicides will not work on root diseases.
• Root rots are most severe under waterlogged conditions. However, crops can be diseased even under ideal moisture conditions, and crops can also suffer due to wet feet regardless of pathogen pressure.

Heavy Disease Pressure

When a pathogen is able to build up in the soil due to conditions conducive for its development in consecutive seasons (such as waterlogging and tight rotations), it may continue to cause issues even when conditions return to what would be considered normal or ideal for crop production.

More than One Susceptible Crop

Depending on the pathogen, root rot can infect various crops in the rotation, or survive as a saprophyte (feeding on dead plant material) until the next susceptible crop is grown, and/or conditions are favourable for disease.

Root Rot Diagnosis

The purpose of diagnosing root rot is not to implement an immediate fix as there are no effective treatment options. However, proper diagnosis will aid in future crop management decisions and may reveal trends among varieties, crop rotations, management practices that affect the soil, or other inputs and stresses. This information also supports researchers in breeding efforts.

Send Samples to a Lab

Diagnostic laboratories may be able to examine freshly infected roots for spores, plate samples for fungal identification, or confirm disease using DNA testing. The following labs offer analysis for root rot disease:

BDS Laboratories – Qu’Appelle, SK
306-699-2679 | www.bdslabs.com

Discovery Seed Labs Ltd. – Saskatoon, SK
306-249-4484 | www.seedtesting.com

20/20 Seed Labs Inc – Nisku, AB; Winnipeg, MB
1-877-420-2099 | www.2020seedlabs.ca

Individual labs may differ in testing methods and sample requirements. Please check with the lab prior to sending samples.

Information to Gather for Diagnosis and Discussions

• Field History: Crop rotation, last year in pulses
• Herbicide History: Herbicides used throughout the year and past history
• Environment: Moisture situation leading up to the problem including previous year(s)
• Soil Information: Texture, organic matter, pH, signs of compaction, flooding, or water runs
• Seeding Information: Variety, seeding date, seeding depth, seed treatments, inoculant, and fertilizer amount and placement as applicable
• 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.
• 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 areas. Note seeding and sprayer direction. Patterns may also relate to other factors such as field edges (see picture below)
• Photos and Samples: Good photos are critical. Aerial photos are great for identifying patterns. Plant and soil samples from both good and bad areas for analysis

Prevention of Root Rot

Following best management practices can help get crops off to a good start, ensuring they are better able to handle stress and tolerate disease pressure. The two most critical factors are environment (moisture) and presence of pathogen (rotation). Field choice is key but other factors may play a role.

In the case of Aphanomyces, it is important not to grow a susceptible host for a minimum of six years, maybe longer. Aphanomyces can infect peas, lentils, alfalfa, dry beans, some varieties of red clover, some varieties of faba bean, and possibly some of the native weedy legume species. Faba bean and chickpea varieties with partial resistance can be used to maintain pulse crops in rotation. Soybeans are another option for a nitrogen (N) fixing crop that is more resistant to Aphanomyces.

Peas and lentils fix their own N but until the nodules form, the crop relies on soil N. Starter N is not usually recommended with peas and lentils, as extra N can delay nodulation and maturity. However, under conditions where soils are low in N (less than 15 lb/acre in the top 12 inches) at the start of the season, application of 10 to 20 lbs of N may be beneficial. As a rule of thumb, if soil tests indicate more than 20 lbs/acre of nitrate nitrogen, then no additional N is needed. If below 15 lb/acre, then consider starter N.

Options for Reducing Risk of Root Rots

Field Choice

• Lighter textured soils (sandier) with good drainage
• Out of peas/lentils for at least three years (four year rotation) and maybe up to six years if Aphanomyces positively identified
• Manage or avoid compacted fields or areas

Soil Testing and Fertility

• Apply nutrients as needed
• Starter nitrogen if soils

Seed Testing

• Plant good quality seed
• Apply seed treatments as warranted for seed borne disease or if planting early into cool soils (see next table)

Seeding Decisions

• Use appropriate inoculant and good application methods
• Choose more resistant crops – faba bean, chickpea, and soybean (only for Aphanomyces root rot)
• Minimize seed damage and watch airspeed of seeder
• Seed into warm moist soil – the quicker the emergence the more vigorous the seedlings

After Seeding

• Monitor crop for signs of stress
• Follow herbicide labels – increased injury can occur when plants are stressed

Phosphorous (P) is important for good root development and to support the nitrogen fixation process. Good P levels are important for early growth, especially under cool conditions associated with early seeding. Maximum safe rates of seed placed P are 20-25 lbs/acre for lentils and 15-20 lbs/acre for peas based on narrow opener (15 per cent seedbed utilization), and good moisture conditions. If higher P rates are required, banding is the best strategy.

Seed Treatments

Root rot pathogens can be controlled to a certain degree using seed treatments. However, fungicidal effects will only last two to three weeks against early season disease pressure.

Making informed decisions before root rot symptoms appear is the best option. Once the seed is in the ground it is important to monitor plant health by checking above and below ground portions of the plant throughout the season.

Root Rot Research

Provincial pulse organizations Alberta Pulse Growers, Manitoba Pulse Growers Association, and Saskatchewan Pulse Growers are currently funding ongoing research to determine the extent of root rot issues in the prairies, assess the impact of various factors on root rot development, develop management options, and develop new varieties with improved root rot resistance. Get in touch with your local pulse organization to learn more.

For More Information on Root Rot

Saskatchewan

Saskatchewan Pulse Growers
www.saskpulse.com | 306-668-5556

Sherrilyn Phelps, PAg.
Agronomy and Seed Program Manager, Saskatchewan Pulse Growers
sphelps@saskpulse.com | 306-480-9767

Faye Dokken-Bouchard, PAg.
Provincial Specialist, Plant Disease, Saskatchewan Ministry of Agriculture
faye.bouchard@gov.sk.ca | 306-787-4671

Sabine Banniza, PhD.
Professor, Plant Pathology, Crop Development Centre, University of Saskatchewan
sabine.banniza@usask.ca | 306-966-2619

Alberta

Alberta Pulse Growers
www.pulse.ab.ca | 780-986-9398

Jenn Walker
Research Officer, Alberta Pulse Growers
jwalker@pulse.ab.ca

Syama Chatterton, Ph.D.
Plant Pathologist, Lethbridge Research Centre, Agriculture and Agri-Food Canada
syama.chatterton@agr.gc.ca

Manitoba

Manitoba Pulse Growers Association
www.manitobapulse.ca | kristen@manitobapulse.ca | 204-745-6488

Holly Derksen, MSc
Field Crop Pathologist, Manitoba Agriculture, Food and Rural Development
holly.derksen@gov.mb.ca