Root rot pathogens, with their devastating impact on plant performance, pose a significant threat to pea and lentil production. They can substantially reduce yields by up to 70% in peas. These seed or soil-borne diseases infect the plant between germination and maturity, striking at any part of the root system and crown of the plant.
Root rot complex, with its brown discoloration of plant roots, hampers the development of a robust root system. This, in turn, restricts plant growth as the roots and crown cannot supply the required nutrients to the aboveground biomass. The result? Yellowed leaves, root decay, and premature death. Root rot infections may not be immediately apparent as the damage typically occurs in the roots. Moreover, root rot complex can also impact seed protein content in peas and lentils. The pathogens restrict root development, limiting the plant’s ability to form nodules with nitrogen-fixing rhizobia.
Root rot complex pathogens include Aphanomyces euteiches, Fusarium spp., Pythium spp., and Rhizoctonia solani. They are considered a complex as more than one pathogen is typically present at any given time. Aphanomyces euteiches and Pythium spp., for instance, belong to a group of fungal-like root pathogens and are considered water moulds or oomycetes that thrive in saturated soils.
Aphanomyces euteiches is arguably the most damaging root rot pathogen due to the extreme durability of the resting spores, also known as oospores, as they can survive in the soil for ten-plus years, the absence of pea or lentil varietal genetic resistance, and the lack of effective control measures. Fusarium spp. and the other pathogens in the root rot complex typically infect peas and lentils at the same time as Aphanomyces, increasing the severity of the disease. Fusarium is the most common root rot disease in pulses, causing early-season seedling wilt, commonly known as fusarium wilt. Fusarium spp. can infect cereals and other rotational crops and persist throughout a crop rotation. However, its impacts are less severe as pea and lentil varieties have partial resistance and control options like seed treatments. Pythium spp. causes seed rot, while Rhizoctonia solani causes wirestem. However, these pathogens are less problematic as specific seed treatments can control both.
Research efforts to solve root rot complex have been underway since 2012 and have increased our understanding of the root rot pathogens, their life cycle, virulence, diagnosis, and the interaction between the plant, pathogen, and the environment. More work is needed to understand what factors could potentially impact the survival of the resting oospores, novel management strategies and control options, and the breeding of resistant or partially resistant varieties.
Learn about the pathogens that cause root rot complex in peas and lentils, agronomic best management practices, and current research findings.
Factors that Increase the Risk of Root Rot
Read more about field choice.
Environment & Pathogen
Wet Conditions
High soil moisture is a critical factor in the development of root rot and can be caused by excessive rainfall, poor soil drainage or flooding, soil compaction, and heavy textured soils (35–45% clay content).
Plant roots and nitrogen-fixing bacteria need oxygen. When the soil is saturated, roots function poorly, and rhizobia activity is reduced, resulting in stunted growth.
Root rot tends to be more severe under saturated conditions. Research has suggested that root rot can occur at a 45% soil saturation level or higher, and the severity of the infection increases as soil moisture increases. However, some root rot pathogens, like Rhizoctonia and some Fusarium spp., can occur even under ideal or drier moisture conditions.
Soil moisture is critical for Aphanomyces, allowing the zoospores to swim to the roots, where the initial infection occurs.
Years of above-average moisture tend to increase outbreaks and spread of the disease. The pathogen’s zoospores need moisture to move through the soil, but they can lie dormant as oospores, waiting for ideal conditions for years.
A few days after infection, the pathogen enters its sexual stage. It produces oospores, the primary source of inoculum for future infections in susceptible crops. Aphanomyces can survive resting for 10–15 years, even when no host is present.
Oospore production can occur as little as 10–14 days after the first root infection.
Even though waterlogged and saturated soils are the ideal environments for Aphanomyces, dry conditions or even drought do not significantly affect oospore decay. Dr. Sabine Banniza, Pulse Pathologist at the University of Saskatchewan, has stated, “The spores produced by this fungus are very resilient, so a year of drought will not make a big dent in the number of viable spores in the soil of an infested field. They will sit there and wait out the drought, and when there is some moisture in the soil, and it does not take much, and another pea or lentil crop is planted, they will revive and infect.”
Heavy Textured Soils
Heavy textured soils are more prone to waterlogging and compaction. Heavy clay soils (35–40% clay content) favour the development of Aphanomyces root rot, although it can occur in any soil type.
Shortened Rotations
Shortening rotations increases the level of pathogens in the soil. Pathogen numbers increase when pea or lentil are planted in a field.
Under the right conditions, growing susceptible crops five times in the same field over 20 years can increase soil pathogen levels. Many farmers experienced this in 2016 and again in 2020 when rain in June, followed by a hot, dry summer, caused dramatic yield reductions in infected fields.
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 conditions are favourable for disease.
Presence of Inoculum
The amount of disease inoculum, or oospores, in the soil contributes to root rot and may continue to cause issues even when conditions return to ‘normal’ or ideal for crop production.
Heavy disease pressure occurs when a pathogen builds up in the soil due to conditions favourable for its development in consecutive seasons (such as waterlogging and tight rotations).
Aphanomyces begins to impact plant health at 100 oospores per gram of soil. High oospore levels can take more than ten years to decline to levels that no longer cause outbreaks of root rot.
Root rot infections can occur in drier weather in fields with high disease inoculum. One possible explanation is that direct oospore germination does not require as much moisture as zoospores, and their hyphae can infect roots that encounter oospores.
Another possibility is that zoospores are formed early when water is available, and root infection occurs.
The research found the top 20 cm (eight inches) contained the highest inoculum levels within the soil profile, although there was still enough inoculum to cause infection as deep as 40–60 cm (16–24 inches).
Warm Soil Temperatures
Warm soil temperatures contribute to root rot, with the optimal soil temperature of 16°C for root rot infections and 20–28°C for disease development.
Host
Stressed plants are more susceptible to seedling diseases. Stress factors that delay germination and slow emergence and growth contribute to an increased risk of root rot infection.
Cool Temperatures
Cool temperatures early in the season slow plant growth and seedling metabolism. They also slow the mineralization of nitrogen from organic matter, directly contributing to seedling access to available N and nodule development.
Soil Compaction
Soil compaction reduces root growth. It can also contribute to the severity of Aphanomyces root rot by reducing air-filled soil pores and contributing to the high-moisture conditions favoured by A. euteiches. Because soil is variable across a field, root rot can also vary. Patches of root rot can develop in low spots that remain more saturated or in compacted areas such as field entrances.
Nutrient Deficiency
Nutrient deficiency slows seedling growth. Well-balanced fertility will give your pulse crop the best chance of resisting or tolerating a root rot infection.
Infected roots are less able to access nutrients, so ensuring a good nutrient supply can help reduce stress and, therefore, possibly reduce the impact of root rots on yield. This includes adequate phosphorous and inoculating peas and lentils to ensure proper nitrogen is present as the crop develops.
Herbicide Residues
Keep records of history and follow herbicide labels to ensure that herbicide residues in the soil have fully degraded before planting.
The presence of herbicides can inhibit germination or exacerbate other factors, such as root disease.
Avoid herbicide damage to the pea or lentil crops, as this stress could make them more vulnerable to root rot.
Low Seed Vigour
Seed quality is essential, and sourcing seeds with good germination, vigour, and disease-free qualities can help reduce the introduction of pathogens into fields.
Fusarium can be seed-borne, but Aphanomyces has not been detected on seed. If Fusarium is present in the seed, it is recommended that you use a registered seed treatment to protect your emerging crop.
A commercial disease seed test will indicate the presence of seed-borne diseases.
Weed & Pest Control
Building crop resiliency and eliminating crop stress is the best method for preventing or withstanding a root rot complex infection in your peas or lentils. Good crop competition via seeding rates and ensuring a weed-free environment are key agronomic best management practices for providing early-season resiliency to root rot infections.
Aphanomyces can also infect other plants, including weed species that grow on the Prairies. These weeds can provide a bridge from year to year and maintain oospore levels.
Weed control is essential 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 greater risk of preventing spore loads in the soil from decreasing and being unnecessarily stressed from heavy weed pressure.
Insect damage can also result in stressed plants and reduced early season vigour. Pea leaf weevil can cause significant damage from larval feeding on rhizobia in pea nodules and foliar damage from adult feeding. Consider your pea leaf weevil pressure risk before seeding and use insecticide seed treatment to reduce damage. For more information on pea leaf weevil and root rot:
- Interactions Between Fusarium Root Rot and Pea Leaf Weevil in Field Peas | SPG
- Pea Leaf WeeviI | SPG
