Aphanomyces euteiches f.sp. pisi
Encyclopedia
Aphanomyces euteiches f.sp.
pisi is a water mould, or oomycete, plant pathogen responsible for the disease Aphanomyces pea root rot
. The species Aphanomyces euteiches can infect a variety of legumes. Symptoms of the disease can differ among hosts but generally include reduced root volume and function, leading to stunting and chlorotic foliage. Aphanomyces root rot is an important agricultural disease in the United States, Europe, Australia, New Zealand, and Japan. Management includes using resistant crop varieties and having good soil drainage, as well as testing soil for the pathogen to avoid infected fields.
or perennials
in the legume family, including: pea (Pisum sativum), alfalfa (Medicago sativa)
, snap bean and red kidney bean (Phaseolus vulgaris), faba bean (Vicia faba), red clover (Trifolium pratense)
, and white clover (Trifolium repens)
. In North America, genetically distinct populations of A. euteiches demonstrate host specificity, but such specificity has not been observed in Europe.
Because A. euteiches is a root-infecting pathogen, primary symptoms
occur on roots and stem tissue below the soil line. Infected root often appear gray and water-soaked, eventually becoming soft and honey-brown or blackish-brown in appearance. Infection causes a reduction in root volume and function, including reduced nodulation
, leading to decreased water and nutrient up-take, which lead to above-ground secondary symptoms. Symptoms in the above-ground plant tissue can include chlorosis of the cotyledons
and necrosis
of the epicotyls
and/or hypocotyls
, stunting
, and wilting of foliage. Some symptoms can differ among hosts. Infected plants and patterns of infection in the field often correspond to areas in the field with poor soil drainage, which can be the result of soil compaction, soil texture
(high clay content), or excessive wetness due to irrigation or rainfall. Symptoms are generally similar among infected legumes, however timing and pattern of disease can differ among hosts and between annuals and perennials. In both peas and beans, lesions
tend to progress up the plant tissue, starting with the epicotyls and moving to the hypocotyls, eventually extending above the soil. Lesions on pea epicotyls turn black, eventually creating a pinched region above the cotyledons as the result of pinched tissue. Lesions on beans, on the other hand, have a characteristic water-soaked appearance, are grayish-green in color, and are firm to the touch. In alfalfa symptoms include chlorotic cotyledons which may eventually become necrotic.
Although the symptoms caused by A. euteiches can be difficult to distinguish from symptoms caused by other root-infecting plant pathogens (such as Pythium
, Rhizoctonia
, and Fusarium
), there are some distinct differences. Aphanomyces root rot rarely results in seed rot and/or pre-emergent damping-off. The characteristic lesions caused by the different pathogens also differ. Fusarium infection results in black or reddish vascular tissue, and Rhizoctonia infection results in sunken, cankerous lesions.
A. euteiches exhibits no macroscopic signs, but oogonia and oospores
can be seen in root tissue with a compound microscope.
Although A. euteiches can potentially infect hosts at any point in the growing season, infection usually begins during seedling emergence. The primary source of inoculum
is oospores, which can be found either in the soil or in infected plant debris from previously infected host plants. Oospores germinate as a response to chemical signals detected from the roots of new hosts plants. Germination can either be direct or indirect. Either way, infecting begins at the cell in the root tips of the host plant. In direct germination, the oospore produces hyphae which directly penetrate host cells at the plant’s root tips. In indirect germination, the oospore produces sporangia
which release zoospores
. The zoospores then encyst on the host plant cells, and germinate. After direct or indirect germination, coenocytic
hyphae of A. euteiches colonize host tissue through inter- and intra-cellular growth. For sexual reproduction, hyphae develop male and female gametangia called, respectively, antheridia
and oogonia
. The antheridium fertilizes an oogonium, which then develops into a single oospore, which is 20-35 micrometers in diameter.
When growing under optimal conditions, an infection of A. euteiches can result in symptoms within 10 days, and oospores can be formed between 7 and 14 days. Oospores become dormant after being formed, and can survive for more than 10 years.
stage, and when temperatures are between 22-28 degrees Celsius. Because zoospores require water for mobility, standing water in the soil increases host infection by making it easier for zoospores to move to host cells. After infection, though, the development of symptoms is favored by warm and dry soil conditions.
. Resistant cultivars have been identified in alfalfa, bean, pea and red clover. Breeding for resistance has been successful in alfalfa and beans, however in beans it has been difficult to establish resistant cultivars that also meet consumer needs. The efficacy of chemical suppression is dependent on environmental factors such as temperature and soil moisture. Before widespread fungicide use was available, control consisted mainly of avoiding fields with high disease risk based on a procedure developed in 1958. Management practices should also include good soil drainage and soil testing to avoid infested fields. Crop rotation
can be implemented to slow the rate of build-up of A. euteiches, however because oospores can survive for up to 20 years in the soil, rotation is not an effective means of eradicating the inoculum. Therefore, once high levels of A. euteiches have been identified in a field, growers should abstain from planting susceptible hosts in that field.
) factories to relocate, a considerable threat for local economies. In fact, prevalence of Aphanomyces root rot has ultimately shifted pea production in the United States from being predominantly in the Midwest and Eastern parts of the country to drier states such as Idaho, Washington, and Oregon where A. euteiches is still common but conditions are less favorable..
In addition to fresh peas, alfalfa is another crop where Aphanomyces root rot (ARR) causes significant economic damage. In peas, A. euteiches is the only pathogen that causes Aphonomyces root rot, but in alfalfa a complex of pathogens are often involved in the disease, including Fusarium and Pythium spp.. Thus although A. euteiches had been known as a pathogen of pea since the 1920's, it was not until the 1980’s that scientists from the University of Wisconsin confirmed it as a significant pathogen of alfalfa.
In alfalfa, A. eutiches is especially damaging in conjunction with Phytophthora medicaginis
, which causes Phytophthora root rot (PRR) in alfalfa. After initial studies in Wisconsin, A. euteiches was identified as an economically significant alfalfa pathogen in other states as well. In Wisconsin, Iowa and Kentucky it often exceeds P. medicaginis in prevalence in fields where alfalfa is grown. Due to these diseases, conventional advice was to avoid growing alfalfa in any wet soils. However, with the development of ARR- and PRR-resistant varieties
, wet soil conditions have become less of a concern for alfalfa production, at least as far as disease pressure is concerned. Today, modern alfalfa varieties are required to have both PRR and ARR resistance.
An alfalfa, there is evidence that another interaction can occur between A. euteiches and P. medicaginis, another important alfalfa pathogen which causes Phytophthera root rot. In this case, colonization by A. euteiches may make it more difficult for P. medicaginis to take hold.
of root rot in peas. At the time, the disease had plagued Wisconsin
and the American Midwest, where monoculture
was commonplace in pea production for processing, for decades. The pathogen has since been recorded in Europe, Australia, New Zealand, Japan, and throughout the United States, suggesting that the disease may have already been widespread at the time of its discovery.
Forma specialis
Forma specialis is a taxonomic grouping allowed by the International Code of Botanical Nomenclature, that is applied to a parasite which is adapted to a specific host ; this classification may be applied by authors who do not feel that a subspecies or variety name is...
pisi is a water mould, or oomycete, plant pathogen responsible for the disease Aphanomyces pea root rot
Root rot
Root rot is a condition found in both indoor and outdoor plants, although more common in indoor plants with poor drainage. As the name states, the roots of the plant rot. Usually, this is a result of overwatering. In houseplants, it is a very common problem, and is slightly less common in outdoor...
. The species Aphanomyces euteiches can infect a variety of legumes. Symptoms of the disease can differ among hosts but generally include reduced root volume and function, leading to stunting and chlorotic foliage. Aphanomyces root rot is an important agricultural disease in the United States, Europe, Australia, New Zealand, and Japan. Management includes using resistant crop varieties and having good soil drainage, as well as testing soil for the pathogen to avoid infected fields.
Hosts and symptoms
Hosts of Aphanomyces euteiches can be annualsAnnual plant
An annual plant is a plant that usually germinates, flowers, and dies in a year or season. True annuals will only live longer than a year if they are prevented from setting seed...
or perennials
Perennial plant
A perennial plant or simply perennial is a plant that lives for more than two years. The term is often used to differentiate a plant from shorter lived annuals and biennials. The term is sometimes misused by commercial gardeners or horticulturalists to describe only herbaceous perennials...
in the legume family, including: pea (Pisum sativum), alfalfa (Medicago sativa)
Alfalfa
Alfalfa is a flowering plant in the pea family Fabaceae cultivated as an important forage crop in the US, Canada, Argentina, France, Australia, the Middle East, South Africa, and many other countries. It is known as lucerne in the UK, France, Australia, South Africa and New Zealand, and known as...
, snap bean and red kidney bean (Phaseolus vulgaris), faba bean (Vicia faba), red clover (Trifolium pratense)
Red clover
Trifolium pratense is a species of clover, native to Europe, Western Asia and northwest Africa, but planted and naturalised in many other regions....
, and white clover (Trifolium repens)
White clover
Trifolium repens, the white clover , is a species of clover native to Europe, North Africa, and West Asia...
. In North America, genetically distinct populations of A. euteiches demonstrate host specificity, but such specificity has not been observed in Europe.
Because A. euteiches is a root-infecting pathogen, primary symptoms
Symptom
A symptom is a departure from normal function or feeling which is noticed by a patient, indicating the presence of disease or abnormality...
occur on roots and stem tissue below the soil line. Infected root often appear gray and water-soaked, eventually becoming soft and honey-brown or blackish-brown in appearance. Infection causes a reduction in root volume and function, including reduced nodulation
Root nodule
Root nodules occur on the roots of plants that associate with symbiotic nitrogen-fixing bacteria. Under nitrogen-limiting conditions, capable plants form a symbiotic relationship with a host-specific strain of bacteria known as rhizobia...
, leading to decreased water and nutrient up-take, which lead to above-ground secondary symptoms. Symptoms in the above-ground plant tissue can include chlorosis of the cotyledons
Cotyledon
A cotyledon , is a significant part of the embryo within the seed of a plant. Upon germination, the cotyledon may become the embryonic first leaves of a seedling. The number of cotyledons present is one characteristic used by botanists to classify the flowering plants...
and necrosis
Necrosis
Necrosis is the premature death of cells in living tissue. Necrosis is caused by factors external to the cell or tissue, such as infection, toxins, or trauma. This is in contrast to apoptosis, which is a naturally occurring cause of cellular death...
of the epicotyls
Epicotyl
In plant physiology, the epicotyl is the embryonic shoot above the cotyledons. In most plants the epicotyl will eventually develop into the leaves of the plant. In dicots, the hypocotyl is what appears to be the base stem under the spent withered cotyledons, and the shoot just above that is the...
and/or hypocotyls
Hypocotyl
The hypocotyl is the stem of a germinating seedling, found below the cotyledons and above the radicle .-Dicots:...
, stunting
Stunt (botany)
In botany and agriculture, stunting describes a plant disease that results in dwarfing and loss of vigor. It may be caused by infectious or noninfectious means.-Infectious:A stunt caused by infectious means usually is too late to cure.* Nematodes...
, and wilting of foliage. Some symptoms can differ among hosts. Infected plants and patterns of infection in the field often correspond to areas in the field with poor soil drainage, which can be the result of soil compaction, soil texture
Soil texture
Soil texture is a qualitative classification tool used in both the field and laboratory to determine classes for agricultural soils based on their physical texture. The classes are distinguished in the field by the 'textural feel' which can be further clarified by separating the relative...
(high clay content), or excessive wetness due to irrigation or rainfall. Symptoms are generally similar among infected legumes, however timing and pattern of disease can differ among hosts and between annuals and perennials. In both peas and beans, lesions
Lesion
A lesion is any abnormality in the tissue of an organism , usually caused by disease or trauma. Lesion is derived from the Latin word laesio which means injury.- Types :...
tend to progress up the plant tissue, starting with the epicotyls and moving to the hypocotyls, eventually extending above the soil. Lesions on pea epicotyls turn black, eventually creating a pinched region above the cotyledons as the result of pinched tissue. Lesions on beans, on the other hand, have a characteristic water-soaked appearance, are grayish-green in color, and are firm to the touch. In alfalfa symptoms include chlorotic cotyledons which may eventually become necrotic.
Although the symptoms caused by A. euteiches can be difficult to distinguish from symptoms caused by other root-infecting plant pathogens (such as Pythium
Pythium
Pythium is a genus of parasitic oomycete. Most species are plant parasites, but Pythium insidiosum is an important pathogen of animals...
, Rhizoctonia
Rhizoctonia
Rhizoctonia is a genus of anamorphic fungi in the order Cantharellales. Species do not produce spores, but are composed of hyphae and sclerotia and are asexual states of fungi in the genus Thanatephorus. Rhizoctonia species are saprotrophic, but are also facultative plant pathogens, causing...
, and Fusarium
Fusarium
Fusarium is a large genus of filamentous fungi widely distributed in soil and in association with plants. Most species are harmless saprobes, and are relatively abundant members of the soil microbial community. Some species produce mycotoxins in cereal crops that can affect human and animal health...
), there are some distinct differences. Aphanomyces root rot rarely results in seed rot and/or pre-emergent damping-off. The characteristic lesions caused by the different pathogens also differ. Fusarium infection results in black or reddish vascular tissue, and Rhizoctonia infection results in sunken, cankerous lesions.
A. euteiches exhibits no macroscopic signs, but oogonia and oospores
Oospore
An oospore is a thick-walled sexual spore that develops from a fertilized oosphere in some algae and fungi. Also the result of plasmogamy/karyogamy in oomycetes, which in turn leads to the development of hyphae, then mycelium....
can be seen in root tissue with a compound microscope.
Disease Cycle
Aphanomyces root rot is an example of a monocyclic disease, causing only one infection cycle per season. This is in contrast to to polycyclic diseases, which can infect new hosts and produce multiple disease cycles within a single season. A. euteiches is not usually a saprophyte in nature, but can be grown in culture in the lab.Although A. euteiches can potentially infect hosts at any point in the growing season, infection usually begins during seedling emergence. The primary source of inoculum
Inoculation
Inoculation is the placement of something that will grow or reproduce, and is most commonly used in respect of the introduction of a serum, vaccine, or antigenic substance into the body of a human or animal, especially to produce or boost immunity to a specific disease...
is oospores, which can be found either in the soil or in infected plant debris from previously infected host plants. Oospores germinate as a response to chemical signals detected from the roots of new hosts plants. Germination can either be direct or indirect. Either way, infecting begins at the cell in the root tips of the host plant. In direct germination, the oospore produces hyphae which directly penetrate host cells at the plant’s root tips. In indirect germination, the oospore produces sporangia
Sporangium
A sporangium is an enclosure in which spores are formed. It can be composed of a single cell or can be multicellular. All plants, fungi, and many other lineages form sporangia at some point in their life cycle...
which release zoospores
Zoospore
A zoospore is a motile asexual spore that uses a flagellum for locomotion. Also called a swarm spore, these spores are created by some algae, bacteria and fungi to propagate themselves.-Flagella:...
. The zoospores then encyst on the host plant cells, and germinate. After direct or indirect germination, coenocytic
Coenocyte
A coenocyte is a multinucleate cell. It can result from multiple nuclear divisions without accompanying cell divisions, or from cellular aggregation followed by dissolution of the cell membranes inside the mass. Coenocytes are found in fungi and some protists, such as algae and slime mold. Some...
hyphae of A. euteiches colonize host tissue through inter- and intra-cellular growth. For sexual reproduction, hyphae develop male and female gametangia called, respectively, antheridia
Antheridium
An antheridium or antherida is a haploid structure or organ producing and containing male gametes . It is present in the gametophyte phase of lower plants like mosses and ferns, and also in the primitive vascular psilotophytes...
and oogonia
Oogonium
"Oogonium" may refer to either a primordial oocyte in a female fetus or the female gametangium of certain thallophytes.-in the fetus:Oogonia are formed in large numbers by mitosis early in fetal development from primordial germ cells...
. The antheridium fertilizes an oogonium, which then develops into a single oospore, which is 20-35 micrometers in diameter.
When growing under optimal conditions, an infection of A. euteiches can result in symptoms within 10 days, and oospores can be formed between 7 and 14 days. Oospores become dormant after being formed, and can survive for more than 10 years.
Environment
Aphanomyces in a soil-borne disease, and the entire lifecycle is completed in the host root and surrounding soil. The pathogen A. euteiches does best in warm, wet soil conditions, but can survive at a range of moderate temperatures. Infection is most prevalent when host plants are in the seedlingSeedling
thumb|Monocot and dicot seedlingsA seedling is a young plant sporophyte developing out of a plant embryo from a seed. Seedling development starts with germination of the seed. A typical young seedling consists of three main parts: the radicle , the hypocotyl , and the cotyledons...
stage, and when temperatures are between 22-28 degrees Celsius. Because zoospores require water for mobility, standing water in the soil increases host infection by making it easier for zoospores to move to host cells. After infection, though, the development of symptoms is favored by warm and dry soil conditions.
Management
The most effective management technique available to farmers is the use of disease-resistant cultivarsCultivar
A cultivar'Cultivar has two meanings as explained under Formal definition. When used in reference to a taxon, the word does not apply to an individual plant but to all those plants sharing the unique characteristics that define the cultivar. is a plant or group of plants selected for desirable...
. Resistant cultivars have been identified in alfalfa, bean, pea and red clover. Breeding for resistance has been successful in alfalfa and beans, however in beans it has been difficult to establish resistant cultivars that also meet consumer needs. The efficacy of chemical suppression is dependent on environmental factors such as temperature and soil moisture. Before widespread fungicide use was available, control consisted mainly of avoiding fields with high disease risk based on a procedure developed in 1958. Management practices should also include good soil drainage and soil testing to avoid infested fields. Crop rotation
Crop rotation
Crop rotation is the practice of growing a series of dissimilar types of crops in the same area in sequential seasons.Crop rotation confers various benefits to the soil. A traditional element of crop rotation is the replenishment of nitrogen through the use of green manure in sequence with cereals...
can be implemented to slow the rate of build-up of A. euteiches, however because oospores can survive for up to 20 years in the soil, rotation is not an effective means of eradicating the inoculum. Therefore, once high levels of A. euteiches have been identified in a field, growers should abstain from planting susceptible hosts in that field.
Importance
Pea (Pisum sativum) is the crop where Aphanomyces causes the greatest economic damage. Aphanomyces root rot can account for yearly losses of about 10% in the fields where it occurs, and may cause losses of entire fields in conditions that are favorable to the pathogen. In agricultural regions that produce large amounts of susceptible crops and have favorable weather conditions for A. euteiches, careful monitoring is of paramount importance. Such regions include the American Midwest, particularly the Great Lakes region. In areas where peas are grown for processing, widespread Aphanomyces infection can cause processing (canningCanning
Canning is a method of preserving food in which the food contents are processed and sealed in an airtight container. Canning provides a typical shelf life ranging from one to five years, although under specific circumstances a freeze-dried canned product, such as canned, dried lentils, can last as...
) factories to relocate, a considerable threat for local economies. In fact, prevalence of Aphanomyces root rot has ultimately shifted pea production in the United States from being predominantly in the Midwest and Eastern parts of the country to drier states such as Idaho, Washington, and Oregon where A. euteiches is still common but conditions are less favorable..
In addition to fresh peas, alfalfa is another crop where Aphanomyces root rot (ARR) causes significant economic damage. In peas, A. euteiches is the only pathogen that causes Aphonomyces root rot, but in alfalfa a complex of pathogens are often involved in the disease, including Fusarium and Pythium spp.. Thus although A. euteiches had been known as a pathogen of pea since the 1920's, it was not until the 1980’s that scientists from the University of Wisconsin confirmed it as a significant pathogen of alfalfa.
In alfalfa, A. eutiches is especially damaging in conjunction with Phytophthora medicaginis
Phytophthora medicaginis
Phytophthora medicaginis is a plant pathogen that causes root rot in alfalfa and chickpeas . It is present in nearly all parts of the world where alfafa is grown. Plants are most susceptible to the pathogen when seedlings become flooded but it can also cause disease in mature plants.- External...
, which causes Phytophthora root rot (PRR) in alfalfa. After initial studies in Wisconsin, A. euteiches was identified as an economically significant alfalfa pathogen in other states as well. In Wisconsin, Iowa and Kentucky it often exceeds P. medicaginis in prevalence in fields where alfalfa is grown. Due to these diseases, conventional advice was to avoid growing alfalfa in any wet soils. However, with the development of ARR- and PRR-resistant varieties
Disease resistance in fruit and vegetables
There are a number of lines of defence against pests and diseases in the organic garden, principal among these being the practice of good husbandry, creating healthy soil and ensuring high standards of garden hygiene...
, wet soil conditions have become less of a concern for alfalfa production, at least as far as disease pressure is concerned. Today, modern alfalfa varieties are required to have both PRR and ARR resistance.
Host-parasite interaction
A study indicates that A. euteiches abundance is less severe when the infected plant also has significant mycorrhizal development. Mycorrhizal development can be decreased by management practices, excessive use of nitrogen and phosphorus fertilizer, and repeated tillage.An alfalfa, there is evidence that another interaction can occur between A. euteiches and P. medicaginis, another important alfalfa pathogen which causes Phytophthera root rot. In this case, colonization by A. euteiches may make it more difficult for P. medicaginis to take hold.
Origin
A. euteiches was first described by Drechsler in 1925 as the causal pathogenPathogen
A pathogen gignomai "I give birth to") or infectious agent — colloquially, a germ — is a microbe or microorganism such as a virus, bacterium, prion, or fungus that causes disease in its animal or plant host...
of root rot in peas. At the time, the disease had plagued Wisconsin
Wisconsin
Wisconsin is a U.S. state located in the north-central United States and is part of the Midwest. It is bordered by Minnesota to the west, Iowa to the southwest, Illinois to the south, Lake Michigan to the east, Michigan to the northeast, and Lake Superior to the north. Wisconsin's capital is...
and the American Midwest, where monoculture
Monoculture
Monoculture is the agricultural practice of producing or growing one single crop over a wide area. It is also known as a way of farming practice of growing large stands of a single species. It is widely used in modern industrial agriculture and its implementation has allowed for large harvests from...
was commonplace in pea production for processing, for decades. The pathogen has since been recorded in Europe, Australia, New Zealand, Japan, and throughout the United States, suggesting that the disease may have already been widespread at the time of its discovery.