Field Pea – Seed or Soil-Borne Diseases

Root Rot Diseases – Seed Rot, Root Rot, Seedling Blights; Rhizoctonia Root Rot, Fusarium Root Rot, and Aphanomyces Root Rot
CAUSED BY	Pathogens (various fungal and fungus-like organisms) associated with root rot often appear as a complex, where more than one pathogen is present, making identification of the primary causal agent difficult. These diseases are caused by several fungi including Pythium, Rhizoctonia, Botrytis, and Fusarium species. These soil-borne fungi are seed or soil-borne and can infect the plant at any stage between germination and maturity, and any part of the root system up to a short distance above the soil surface. These fungi are common in the soil, and infection is more likely if the soil around the seed is excessively wet. Warm and moist conditions generally favour these diseases, but cold and wet is also detrimental because the cool temperatures slow plant development and add additional stress. Other factors, including abiotic conditions such as flooding and soil oxygen depletion can result in root cell death.
SYMPTOMS	Poor emergence, stunting, yellowing of leaf tissues, a less well developed root system, decay, and brown discolouration of roots. 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.
PREVENTION AND CONTROL	As prevention measure, seed treatments offer protection to the developing seedling, especially under cool, wet conditions when emergence may be delayed. A four to five-year rotation out of pea or other susceptible hosts (alfalfa and clovers). Use seed with high vigour, practice good soil fertility, liming of acid soils and ripping to reduce soil compaction. Avoid herbicides such as: MCPA, MCPB or a mix of MCPA and MCPB – these herbicides cause plant stress and make root rot more severe. For recommendations in managing root rot, see the link below. 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 current options.
IMPACT	Yield loss, stunting of plants.
RESOURCES	APG Tailgate Talk: Aphanomyces and Other Root Rots 2021 2019 Root Rot in Peas and Lentils in Western Canada 2019 Aphanomyces Root Rot in Pulse Crops 2018 Aphanomyces Root Rot Soil Testing

Aphanomyces Root Rot Rotation impact on reduction of Aphanomyces Root Rot with six year rotation on left versus four year rotation on right.
CAUSED BY	Caused by Aphanomyces euteiches f.sp. pisi. Aphanomyces euteiches is classified as an oomycete, or water mould and is not a true fungus. The resting spores, called oospores, are thick-walled and allow the pathogen to survive in the soil during harsh winter conditions. Can infect at any time in the growing season and spores persist for many years in the soil, making it the most difficult cause of root rot to manage (and therefore the most serious among the root rot pathogens). A single infected plant can spread to the roots of healthy neighbouring plants up to 18 cm or 7 inches away. Risk factors for Root Rot include stress factors that delay germination as well as slow emergence and growth of plants, these include: wet conditions, cool temperatures early in the season, shortened rotations, heavy textured soils, soil compaction, nutrient deficiency, herbicide residues, low seed vigour.
SYMPTOMS	Roots of plants infected with Aphanomyces root rot become soft and have a water-soaked, honey-brown or caramel-coloured appearance. Infection begins on the lateral roots, then spread to the main root, and eventually into the epicotyl, stopping where the green stem tissue begins. At this time, infected plants start to turn pale in colour and are especially noticeable when beside healthy plants. As the season progresses and conditions remain favourable, the main root typically becomes colonized by other pathogens (Fusarium is the most common) and turns dark brown to back. Also leads to yellowing from the bottom up and stunting of the plant as it is unable to extract nutrients and moisture from the soil.
PREVENTION AND CONTROL	A four to five-year crop rotation will minimize damage from this pathogen. Currently no reliable prevention or cure.
IMPACT	Aphanomyces Root Rot causes severe damage to the roots, causing infected plants to wilt and die prematurely. Survives in soil for up to 10 years.
RESOURCES	2017 Root Rot in Peas and Lentils in Western Canada Aphanomyces Root Rot in Peas and Lentils in Western Canada pdf (941 KB) Testing for Aphanomyces Root Rot pdf (493 KB)

Fusarium Root Rot
CAUSED BY	Caused by Fusarium solani (Mart.) Sacc. f.sp. pisi (Jones) Snyd. and Hans. Fusarium root rot is most prevalent under poor crop rotations, high soil temperatures (22˚ to 30˚C), moist soils, acidic soils (pH 5.1 – 6.2) and low fertility. Soil compaction by farm machinery also increases both incidence and severity.
SYMPTOMS	Reddish brown streaks initially develop on the primary and secondary roots. These eventually join together to form a dark reddish-brown colour on the primary root up to the soil line. Greying, yellowing, death of lower foliage and stunting can occur if infection is severe. Foliar symptoms often appear following warm temperatures and heavy rainfall.
PREVENTION AND CONTROL	As prevention measure, seed treatments offer protection to the developing seedling, especially under cool, wet conditions when emergence may be delayed. For recommendations in managing root rot, see that attached resource below. 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 current options.
IMPACT	Yield losses of 26% to 57% have been reported in both dryland and irrigated areas.
RESOURCES	2017 Root Rot in Peas and Lentils in Western Canada Aphanomyces Root Rot in Peas and Lentils in Western Canada Testing for Aphanomyces Root Rot

Pythium Root Rot
CAUSED BY	Caused by a number of different Pythium species. In Alberta, P. irregulare and P. ultimum are the most common Infection very dependent on seed quality – cracked seeds leak more nutrients and are more likely to attract Pythium and be infected than intact seed. More prevalent in cool, wet, poorly drained soils.
SYMPTOMS	Seeds are rotted – when removed from the soil, they emerge with a layer of soil around them (this is full of whitish, threadlike fungal hyphae). Emerging plant and roots, if produced, may be soft and watery; cotyledons may or may not be rotted. Germinating seeds are only susceptible for 48 to 72 hours – once the root emerges, the seed is no longer susceptible to infection (new developing tissue, however, remains susceptible). Infection can occur at the tip of feeder roots where young tissue can be destroyed – this can lead to root pruning and/or reduction in length. Depending on the severity of the infection, seedlings may become stunted and chlorotic and collapse as the root base decays and turns tan to light brown.
PREVENTION AND CONTROL	As prevention measure, seed treatments offer protection to the developing seedling, especially under cool, wet conditions when emergence may be delayed. For recommendations in managing root rot diseases, see the resource section below. 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 current options.
IMPACT	Infected plants tend to lack vigour and often yield poorly.
RESOURCES	2017 Root Rot in Peas and Lentils in Western Canada Aphanomyces Root Rot in Peas and Lentils in Western Canada Testing for Aphanomyces Root Rot

Rhizoctonia Root Rot
CAUSED BY	Caused by Rhizoctonia solani Kuehn. Seedlings generally less susceptible as they get older. High soil temperatures (24˚C to 30˚C) are known to cause higher rates of infection.
SYMPTOMS	Infection occurs (and symptoms appear) close to the soil surface. Symptoms on seedlings first appear as water-soaked lesions that eventually turn reddish brown. Plant’s growing point may die as it emerges from the ground – this can lead to multiple shoots emerging and then dying. On older plants, symptoms appear as reddish-brown sunken lesions on the lower stem, which can cause girdling and often leads to stunted plants.
PREVENTION AND CONTROL	As prevention measure, seed treatments offer protection to the developing seedling, especially under cool, wet conditions when emergence may be delayed. For recommendations in managing root rot diseases, see the resources section below. 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 current options.
IMPACT	Stunted plants.
RESOURCES	2017 Root Rot in Peas and Lentils in Western Canada Aphanomyces Root Rot in Peas and Lentils in Western Canada Testing for Aphanomyces Root Rot

Other seed or soil-borne diseases that are less common include Anthracnose, Bacterial Blight, Brown Spot, Cladosporium Blight, Pink Seed, and Septoria Blotch.

Anthracnose
CAUSED BY	Caused by Colletotrichum pisi Pat. Seed or soil-borne. Since spores are spread by splashing rain, Anthracnose thrives under warm, moist weather, frequent showers and dense canopies. Spores can survive on stubble for up to two years.
SYMPTOMS	Appears as grey to cream-coloured spots on leaves and tan to brown lesions on stems. The entire lower stem may become covered in lesions, giving it a brown, rough appearance. Pod lesions are circular and sunken with reddish-brown margins and reddish centres. Close examination of pod lesions often reveals fruiting bodies (acervuli) with orange-pink spores. Leaf and stipule lesions are oval, with brown margins and greyish centres, and stem lesions are elongated – they appear a copperish colour when moist and greyish when dry. This stage is generally followed by die back of leaves and extensive loss of leaves – entire plants may die back and stems take on a blackish colour. Severely diseased plants have an overall reddish-brown appearance.
PREVENTION AND CONTROL	Use disease-free seed. Use four to five-year crop rotations out of pea and other susceptible crops such as lentil and faba bean.

Bacterial Blight
CAUSED BY	Caused by Pseudomonas syringae pv. pisi (Psp). Overwinters mainly as a seed-borne pathogen – seed can remain infected for up to three years. Rate of infection increases with soil moisture. Injury from hail, frost, wind, animals or machinery predisposes plants to infection. Secondary spread occurs by rain splash, wind, machinery, animals and irrigation.
SYMPTOMS	Shiny, water-soaked lesions initially appear near nodes and stipules – these spread to the stems, pod stems and tendrils and become darker in colour. The underside of leaves and stipules develop water-soaked spots that appear dark green to brown on the upper surface – with age, spots develop an angular shape with dark margins and a light coloured, papery look in the centre. Lesions appear translucent when held up to the light. Lesions on pods are also water-soaked at first, turning darker as they mature and becoming sunken – pods are often infected through the suture. Infected seeds may or may not show injury symptoms – watery, dark spots sometimes appear on the seed but usually no visible symptoms appear.
PREVENTION AND CONTROL	Since the pathogen is mostly seed-borne, use disease-free seed. Seed production should be limited to more arid areas and irrigation should be avoided.

Brown Spot
CAUSED BY	Caused by Pseudomonas syringae pv. Syringae. Can be seed or soil-borne – does not survive for long on the seed but survives well in the soil. 24˚C and higher temperatures and high humidity favour brown spot.
SYMPTOMS	Initially appear as small, water-soaked lesions on leaves, petioles and stems – these eventually turn a tan colour and appear burnt. Stem lesions are sunken and tend to elongate upwards – they often cause stems, petioles and growing points to become distorted. Infected leaves eventually dry up and fall off.
PREVENTION AND CONTROL	Use of disease-free seed. Four to five-year rotation out of susceptible crops (e.g. dry bean).

Cladosporium Blight
CAUSED BY	Caused by Cladosporium pisicolum – often referred to as ‘scab’. Seed or soil-borne Occurs under conditions of high humidity and temperatures of 16˚ to 21˚C.
SYMPTOMS	First appear as inward curling as leaves unfold. Leaf lesions can be irregular or circular in shape. Grey at first, eventually turning a tan to brown colour (margins on lesions are thin and dark brown). Shredding of leaves can occur as necrotic areas fall out or tear from wind. Brownish-black lesions develop on most other plant parts except for the pods (cankers often form as these lesions age and crack). Infected pods are often pockmarked and feel bumpy – dark, irregular pimples occur randomly on pods (seed infection can occur if pods are penetrated). Severely infected seeds develop black, circular spots with well-defined borders – a light infection may show up as light, scattered black spots on the seed.
PREVENTION AND CONTROL	Use disease-free seed. Use four to five-year crop rotations.

Pink Seed
CAUSED BY	Caused by Erwinia rhapontici, this disease was first found on pea in southern Alberta in 1988. Bacteria enter pea pods through wounds caused by insects or mechanical injury and infect the seed inside the pods (seed quality is affected more than yield). More common in irrigated than in dryland pea fields because high humidity and mechanical injury from irrigation allow bacteria to penetrate through wounds to infect the pea plant. The pathogen can overwinter on seed and residue of infected crops even under Canadian Prairie winter conditions.
SYMPTOMS	The symptoms can be very striking with infected seeds becoming shriveled and turn an intense pink to red color. The bright colouring can sometimes be confused with seed treatments. Unlike seed treatments, the pink colour does not wash off the seed. Tan to dark-brown lesions develop on pods only. Planting infected seeds can result in infection of the seedling and lower parts of the plants such as roots and basal stems. However, there is no evidence that the disease spreads further upward to the seeds produced by those infected plants (Hsieh and Haughn, 2010).
PREVENTION AND CONTROL	Use disease-free seed. Burial of infected residue to a depth of 6 cm (or 2.4 in) does not reduce survival (Huang et al 2003).
IMPACT	Infection can affect seed yield and seed quality.
RESOURCES	Pink Seed in Pulses

Septoria Blotch
CAUSED BY	Caused by Septoria pisi West Either seed or soil-borne, although seed transmission is not considered important. Temperatures of 21˚ to 27˚C (with high humidity) favour development of this pathogen.
SYMPTOMS	Blotchy lesions with no distinct margins – these develop mainly on the lower, older leaves, pods and stems of the pea plant. Early blotches are yellowish-green, eventually darkening and enlarging as they mature (irregular in size and shape). Entire leaves or stipules may be covered by blotches as they coalesce. Black specks (pycnidia) develop profusely over the surface of the affected areas.
PREVENTION AND CONTROL	A four to five-year crop rotation.

Special thanks to Saskatchewan Pulse Growers and Manitoba Pulse and Soybean Growers.