Invasive Species Compendium

Detailed coverage of invasive species threatening livelihoods and the environment worldwide

Datasheet

Phytophthora medicaginis
(Phytophthora root rot of lucerne)

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Datasheet

Phytophthora medicaginis (Phytophthora root rot of lucerne)

Summary

  • Last modified
  • 14 July 2018
  • Datasheet Type(s)
  • Invasive Species
  • Pest
  • Preferred Scientific Name
  • Phytophthora medicaginis
  • Preferred Common Name
  • Phytophthora root rot of lucerne
  • Taxonomic Tree
  • Domain: Eukaryota
  •   Kingdom: Chromista
  •     Phylum: Oomycota
  •       Class: Oomycetes
  •         Order: Peronosporales
  • Summary of Invasiveness
  • P. medicaginis is a pathogen that can be either soil or water borne; its invasiveness is limited to periods when the soil is very wet or flooded such as during flood irrigation, heavy rain periods and natural flood events. The success of invasion by...

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Pictures

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PictureTitleCaptionCopyright
Lucerne roots infected with P. medicaginis. Note dark brown lesions girdling the taproot.
TitleSymptoms on lucerne roots
CaptionLucerne roots infected with P. medicaginis. Note dark brown lesions girdling the taproot.
CopyrightJohn A.G. Irwin
Lucerne roots infected with P. medicaginis. Note dark brown lesions girdling the taproot.
Symptoms on lucerne rootsLucerne roots infected with P. medicaginis. Note dark brown lesions girdling the taproot.John A.G. Irwin

Identity

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Preferred Scientific Name

  • Phytophthora medicaginis E.M. Hansen & D.P. Maxwell, 1991

Preferred Common Name

  • Phytophthora root rot of lucerne

Other Scientific Names

  • Phytophthora megasperma Drechsler
  • Phytophthora megasperma f.sp. medicaginis T.L. Kuan & Erwin
  • Phytophthora sojae f.sp. medicaginis Faris

International Common Names

  • English: Phytophthora root rot of alfalfa; Phytophthora root rot of chickpea

Local Common Names

  • India: foot blight of gram

Summary of Invasiveness

Top of page P. medicaginis is a pathogen that can be either soil or water borne; its invasiveness is limited to periods when the soil is very wet or flooded such as during flood irrigation, heavy rain periods and natural flood events. The success of invasion by P. medicaginis is dependant on the presence of a susceptible host such as lucerne or chickpea. Flooding facilitates the transportation of zoospores and makes the host more receptive to infection (Kuan and Erwin, 1980b). P. medicaginis may also be spread as oospores in contaminated soil and infected plants but the very wet soil conditions are required to stimulate their germination, development of zoospores and infection of the host (Erwin and Ribeiro, 1996). The roots of infected plants develop a dark-brown rot and soon lose their efficiency in nutrient and water transport resulting in additional symptoms of yellowing of leaves, wilting, stunting and finally death. Large areas of plants in low lying locations or poorly drained soil are affected and often die (Lowe et al., 1987; Dale and Irwin, 1990) causing considerable economic loss.

Taxonomic Tree

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  • Domain: Eukaryota
  •     Kingdom: Chromista
  •         Phylum: Oomycota
  •             Class: Oomycetes
  •                 Order: Peronosporales
  •                     Family: Peronosporaceae
  •                         Genus: Phytophthora
  •                             Species: Phytophthora medicaginis

Notes on Taxonomy and Nomenclature

Top of page Phytophthora medicaginis was formally recognized as a distinct species by Hansen and Maxwell (1991), and is characterized most readily by its aggresive pathogenicity to lucerne and chickpea. In earlier reports it was included in the P. megasperma 'complex', sometimes with P. sojae as P. megasperma var. sojae, and sometimes as P. megasperma f.sp. medicaginis. However, it is morphologically, culturally and behaviourally distinct from the other species of the megasperma complex and molecular data shows it to be only distantly related to them. It is sometimes difficult to interpret the older literature because of varying species concepts. For example, some authors included all Phytophthora isolates recovered from lucerne within P. megasperma f.sp. medicaginis, without regard for oospore size or aggressiveness. Large-spored, mildly pathogenic 'megasperma-type' isolates are P. megasperma not P. medicaginis.

Description

Top of page The morphological descriptions of this and other species of Phytophthora are based on in vitro observations and are often insufficient for identification. P. megasperma var. sojae (Waterhouse, 1963) describes P. medicaginis as well as several other distinct species of the P. megasperma complex (Hansen et al., 1986). Homothallism, relatively small oogonia and ovoid, non-papillate sporangia are key features. The formal species description of Hansen and Maxwell (1991) includes: Homothallic features, with paragynous or amphigynous antheridia and oogonia 37-39 µm diameter, occasionally 32-43 µm. Oospores are 28-34 µm diameter, with oospore walls 2-4 µm thick. Sporangia are ovoid to ellipsoid or pyriform, without papillae, and 50-60 µm long by 24-30 µm wide. Growth is slow at 4-5 mm per day on agar media at 25-26°C and little or no growth occurs at 5°C or 35°C. The colony is dense, without pattern, and with an appressed margin.

Distribution

Top of page P. medicaginis has been introduced to most areas of the world where lucerne is grown. Its distribution is usually restricted to localised areas with poor drainage. These include areas that are topographically flat or have depressions or areas with heavy textured soils with high clay content (Wilkinson and Millar, 1982; Barta and Schmitthenner, 1986; Myatt et al., 1993). In regions where P. medicaginis is present and damaging, it is usually not found in every field (Aguirre et al., 1983; Basu, 1983; Dale and Irwin, 1990; Munkvold and Carlton, 1995). Despite its reported occurrence in most lucerne production areas throughout the world, P. medicaginis has not been reported from Europe. Many reports carry some degree of uncertainty, because of continuing changes in the species concept. CMI (1987), for example, does not distinguish the several species of the 'P. megasperma complex'. The accompanying list of countries is therefore based on an interpretation of the older literature. Where a P. megasperma-type fungus is associated with a serious root rot of lucerne, it is a fair assumption that P. medicaginis is involved. Reports of P. medicaginis in lucerne or chickpea since its formal description (Hansen and Maxwell, 1991) are generally reliable. The pathogen may well be present but unreported in other countries with a history of lucerne cultivation.

The situation in Europe deserves special attention. If in fact the fungus is absent or uncommon in large parts of Europe, then special efforts should be made to prevent its introduction. It is important to note that the fungus identified as P. megasperma from chickpea in Spain (Trapero-Casas et al., 1992) is in fact a distinct taxon (Liew and Irwin, 1994; Irwin et al., 1996).

Distribution Table

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The distribution in this summary table is based on all the information available. When several references are cited, they may give conflicting information on the status. Further details may be available for individual references in the Distribution Table Details section which can be selected by going to Generate Report.

Continent/Country/RegionDistributionLast ReportedOriginFirst ReportedInvasiveReferenceNotes

Asia

JapanPresentMatsumoto and Araki, 1978; Matsumoto and Sato, 1985; Hansen et al., 1986
PakistanPresentSaleem et al., 1993
TurkeyPresentKarakaya et al., 1995

Africa

South AfricaPresentHansen et al., 1986; Thompson, 1987; Thompson, 1988

North America

CanadaPresentPresent based on regional distribution.
-AlbertaPresentDamirgi et al., 1979
-OntarioRestricted distributionBasu, 1983
-QuebecPresentRichard and Martin, 1991
MexicoRestricted distributionRomero, 1978; Aguirre et al., 1983
USAPresentPresent based on regional distribution.
-ArizonaPresentHine et al., 1972
-CaliforniaPresentErwin, 1965
-IllinoisPresentFrosheiser, 1969
-IowaRestricted distributionFrosheiser, 1969; Munkvold and Carlton, 1995
-KentuckyRestricted distributionVincelli et al., 1994
-MinnesotaPresentFrosheiser, 1967; Frosheiser, 1969; Haan et al., 1996; Schwingle et al., 2007
-MinnesotaPresentFrosheiser, 1967; Frosheiser, 1969; Haan et al., 1996; Schwingle et al., 2007
-MississippiPresentFrosheiser, 1967; Hamm and Hansen, 1981
-NebraskaPresentFrosheiser, 1969
-New YorkPresentWilkinson and Millar, 1981; Hansen et al., 1986
-OhioPresentErwin, 1965; Frosheiser, 1967
-OklahomaPresentPorter et al., 1986
-OregonPresentHansen et al., 1986
-VirginiaPresentBush et al., 2006
-WashingtonPresentElgin et al., 1972; Shaw, 1973; Hansen et al., 1986
-WisconsinPresentFrosheiser, 1969; Pratt and Mitchell, 1973; Hamm and Hansen, 1981; Hansen et al., 1986
-WyomingPresentGray et al., 1983; Gray and Wofford, 1987

South America

ArgentinaPresentHijano, 1991

Europe

CroatiaPresentCvjetkovic, 2008
GreecePresentKouyeas and Chitzanidis, 1978
HungaryPresentSzabó and Lakatos, 2008
IrelandPresentShafizadeh and Kavanagh, 2005
ItalyPresentPresent based on regional distribution.
-SicilyPresentCacciola et al., 2007
NorwayPresentTalgø, 2009
Russian FederationPresentNugaeva, 2000

Oceania

AustraliaPresentErwin, 1965
-New South WalesRestricted distributionIrwin et al., 1995; Liew and Irwin, 1997; Southwell and Crocker, 2005
-QueenslandRestricted distributionPurss, 1959; Irwin, 1974; Vock et al., 1980; Dale and Irwin, 1990

History of Introduction and Spread

Top of page The centre of origin of P. medicaginis is unknown. However, because this pathogen has a narrow host range of chickpea and lucerne, it has been suggested that it has co-evolved with these hosts in Transcaucasia (Irwin et al., 1995). Single introductions from this centre may have resulted in the Australian and USA populations of P. medicaginis. These introductions were most likely of an accidental nature involving the transport of contaminated soil or infected plant material.

Risk of Introduction

Top of page The spread of P. medicaginis is by survival structures (oospores and chlamydospores) in infected plant tissue and in soil and by zoospores in water. Oospores and chlamydospores may be spread short distances within fields during tillage, or further to different fields in soil adhering to vehicles, tillage implements or animals' feet. Zoospores in their encysted state may spread even greater distances during flood events (Erwin and Ribeiro, 1996). In regions where P. medicaginis is already present and damaging, it is usually not found in every field (Aguirre et al., 1983; Basu, 1983; Dale and Irwin, 1990; Munkvold and Carlton, 1995).

Soil contaminated with oospores of P. medicaginis adhering to tillage implements or to the footwear of travellers or the feet of live export animals are the most likely means of accidental introduction of this pathogen. These carriers may travel vast distances with modern transport, particularly air transport, and oospores can survive for 3.5 years in the soil (Pratt and Mitchell, 1975; Stack and Millar, 1985b).

Floodwaters carrying zoospores also are a likely means of accidental introduction to new areas (Erwin and Ribeiro, 1996). However, the spread by this means is restricted to areas below the source of infection in the local geomorphic drainage system.

Alfalfa seedlings (sprouts) infected with P. medicaginis also may pose a potential threat in the introduction of this pathogen to new regions. Alfalfa sprouts used in the food industry may be transported great distances by air, rail or road transport and P. medicaginis infecting such plants would readily survive these journeys. Although it may be very unlikely for alfalfa sprouts to be infected with P. medicaginis because this pathogen is not seedborne and water used for their germination should not be contaminated, the possibility remains. Alfalfa or lucerne seedlings are very susceptible to infection by P. medicaginis and are used to bait the fungus in the isolation of this pathogen (Erwin and Ribeiro, 1996).

It is unlikely that P. medicaginis would be deliberately introduced into a region because it has a narrow host range of mainly agricultural plants and would not be a suitable candidate for the biological control of weeds, other pathogens or animal pests.

P. medicaginis is not known from Europe, and is potentially damaging to lucerne and chickpea if introduced there. In both international and local situations, sanitary precautions, aimed at limiting transport of infected plants and infested soil, will greatly reduce the risk of introduction or local spread.

Hosts/Species Affected

Top of page P. medicaginis has a narrow host range in nature and is primarily a pathogen of the family Fabaceae. It is only known as an agricultural pathogen. Lucerne (Medicago sativa) is the primary host (Erwin and Ribeiro, 1996). Other species of Medicago, notably the annual species, are also susceptible under field test conditions (Haan et al., 1996). Chickpea (Cicer arietinum) is susceptible and damaged in the field in USA, Australia (Irwin and Dales, 1982) and India (Suryanarayana and Pathak, 1968). In Spain, root rot of Cicer is caused by other Phytophthora species distinctly different from P. medicaginis in genetic, morphological and cultural parameters (Liew and Irwin, 1994); P. medicaginis has not been reported (Erwin and Ribeiro, 1996). Sainfoin (Onobrychis viciifolia) is susceptible in the seedling stage (Gray and Wofford, 1987). Hedysarum spp. have been shown to be susceptible in glasshouse tests with H. carnosum being as equally susceptible as chickpea (Southwell and Crocker, 2005).

P. medicaginis infects and causes significant disease in a variety of unrelated hosts under conditions of artificial inoculation, especially when soil flooding is prolonged or repeated (Hamm and Hansen, 1981; Wilcox and Mircetich, 1987). However, it has never been isolated from these other plant species in the field, and host preference remains an important diagnostic characteristic of this pathogen.

Host Plants and Other Plants Affected

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Plant nameFamilyContext
Actinidia deliciosa (kiwifruit)ActinidiaceaeMain
Cicer arietinum (chickpea)FabaceaeMain
Daucus carota (carrot)ApiaceaeMain
Medicago (medic)FabaceaeOther
Medicago sativa (lucerne)FabaceaeMain
Onobrychis viciifolia (sainfoin)FabaceaeOther

Growth Stages

Top of page Pre-emergence, Seedling stage, Vegetative growing stage

Symptoms

Top of page Germinating seed and seedlings of lucerne are infected and killed rapidly, with the disease appearing as damping off. Older plants are also infected, with characteristic reddish-brown or black root lesions developing into root rot disabling some or all of the root system. Mature plants may be killed, especially following repeated infection in poorly drained areas, but more often exhibit chlorosis and reduced growth (Erwin, 1954; Schmitthenner, 1964). Infected plants are prone to winter injury, including frost heaving.

In chickpea, Phytophthora root rot is characterized by chlorosis and desiccation of foliage, decay of lateral roots, small brown lesions that develop into extensive dark-brown lesions on the taproot, and the development of girdling lesions with reddish-brown margins extending to the collar (Vock et al., 1980).

List of Symptoms/Signs

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SignLife StagesType
Leaves / abnormal colours
Leaves / wilting
Leaves / yellowed or dead
Roots / cortex with lesions
Roots / necrotic streaks or lesions
Roots / reduced root system
Roots / stubby roots
Whole plant / damping off
Whole plant / dwarfing
Whole plant / early senescence
Whole plant / plant dead; dieback

Biology and Ecology

Top of page Genetics

P. medicaginis has a chromosome number of 12-15 (Hansen and Maxwell, 1991). Molecular markers have been used to assess the genetic diversity of P. medicaginis populations using isolates collected from Australia and the USA and these were found to be very uniform (Liew and Erwin, 1994; Irwin et al., 1995). P. medicaginis does not hybridize with other Phytophthora spp. such as P. sojae (Layton and Kuhn, 1988; Nygaard et al.,1989) or P. megasperma (Hansen, 1987).

Physiology and Phenology

The long-term survival strategy of P. medicaginis is through the production of thick-walled oospores. These may survive for up to 3.5 years in the soil (Pratt and Mitchell, 1975; Stack and Millar, 1985b). The fungus may also survive as chlamydospores in infected plant tissue (Basu, 1980).

Reproductive Biology

The life cycle of P. medicaginis is similar to that of other homothallic species of Phytophthora (Erwin, 1965). The mycelium is self-fertile and oospores are formed, in culture and in infected roots in soil, following the fusion of oogonia and antheridia. Oospores are thick-walled spores that can survive adverse conditions for long periods. Typically, they germinate to produce a sporangiophore and sporangia in water-saturated soil (Salvatore et al., 1973). Germination of oospores is stimulated by exudates from host roots and occurs at pH 3.5-10.5 with an optimum of pH 6 (El-Hamalawi and Erwin, 1986). Zoospores are released into the soil solution and follow chemotactic gradients to host roots, where they encyst and then germinate, forming vegetative hyphae. Hyphae penetrate and colonize host roots, killing host tissue as they advance. Chlamydospores may be formed on the vegetative hyphae. They are typically thin-walled, in contrast to oospores, but also serve as survival structures (Basu, 1980). Oogonia and antheridia form on the hyphae in roots and fuse, especially as soil dries or host tissue dies.

Primary inoculum is usually oospores or chlamydospores in infected root fragments in the soil. Germination and release of zoospores can occur in water saturated soil at temperatures from 5 to 30°C but growth is best at about 25°C (Erwin and Ribeiro, 1996). Zoospores swim for only a few millimetres through the soil matrix, but can be carried for much longer distances in flowing water such as in flood irrigation and during periods of natural flooding following heavy rain events. They remain motile for several hours and may encyst on any surface, especially if agitated. However, they concentrate on host roots, frequently where lateral roots emerge from the taproot (Marks and Mitchell, 1971; Chi and Sabo, 1978), on nodules of Rhizobium (Gray and Hine, 1976) and on stomata near the soil line on stems of chickpea (Dale and Irwin, 1991b).

Environmental Requirements

P. medicaginis occurs in temperate to sub-tropical environments where lucerne or chickpea are grown. Low lying and poorly drained areas and soils of heavy texture high in clay content are common habitats of P. medicaginis because they are prone to water saturation following heavy rain, flooding or flood irrigation (Lehman et al., 1968; Pulli and Tesar, 1975; Wilkinson and Millar, 1982; Alva et al., 1985, 1986; Barta and Schmitthenner, 1986; Myatt et al., 1993).

Flooding from either irrigation or natural rainfall events may act as a trigger event causing P. medicaginis to become invasive because its zoospores can be transported readily from one site to another (Erwin and Ribeiro, 1996) and flooding increases the attractiveness and susceptibility of lucerne seedlings to this pathogen (Kuan and Erwin, 1980b).

Associations

Aphanomyces euteiches can be associated with P. medicaginis in lucerne fields, with both pathogens causing damage to the roots of these plants. In a study of lucerne fields in 45 of 99 counties in Iowa, USA, Munkvold and Carlton (1995) detected P. medicaginis and A. euteiches in soil and root samples in 26 and 31 counties, respectively. Of the soil samples infested with P. medicaginis, 81% were also infested with A. euteiches. In some cases where A. euteiches is also present in lucerne fields in North America it may be the more serious pathogen (Abbo and Irwin, 1990; Holub and Grau, 1990a).

Notes on Natural Enemies

Top of page Little is known about natural enemies of P. medicaginis, although many elements of the soil microflora and fauna undoubtedly limit the fungus under some conditions (Sneh et al., 1977). Under experimental conditions, antagonistic bacteria inhibit the fungus and reduce disease (Handelsman et al., 1990; Myatt et al., 1993). A number of Streptomyces isolates were found by Xiao et al. (2002) to reduce disease in lucerne caused by P. medicaginis both in vitro and in naturally infested field soil.

Means of Movement and Dispersal

Top of page Natural dispersal

The natural dispersal of P. medicaginis is by zoospores in wet soil conditions. Zoospores swim for only a few millimetres through the soil matrix, but can be carried for much longer distances in flowing water such as in flood irrigation and during periods of natural flooding following heavy rain events (Erwin and Ribeiro, 1996).

Agricultural practices

Soil contaminated with oospores of P. medicaginis may be relocated a metre or more to nearby uncontaminated areas by soil tillage practices. Contaminated soil adhering to tillage implements or the hooves of livestock may be transported much greater distances.

Seedborne Aspects

Top of page There is no evidence that P. medicaginis is seedborne.

Pathway Vectors

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VectorNotesLong DistanceLocalReferences
Soil, sand and gravel Yes

Plant Trade

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Plant parts liable to carry the pest in trade/transportPest stagesBorne internallyBorne externallyVisibility of pest or symptoms
Growing medium accompanying plants hyphae; sporangia; spores Yes Pest or symptoms not visible to the naked eye but usually visible under light microscope
Roots hyphae; sporangia; spores Yes Yes Pest or symptoms not visible to the naked eye but usually visible under light microscope
Seedlings/Micropropagated plants hyphae; sporangia; spores Yes Yes Pest or symptoms not visible to the naked eye but usually visible under light microscope
Plant parts not known to carry the pest in trade/transport
Bark
Bulbs/Tubers/Corms/Rhizomes
Flowers/Inflorescences/Cones/Calyx
Fruits (inc. pods)
Leaves
Stems (above ground)/Shoots/Trunks/Branches
True seeds (inc. grain)
Wood

Wood Packaging

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Wood Packaging not known to carry the pest in trade/transport
Loose wood packing material
Non-wood
Processed or treated wood
Solid wood packing material with bark
Solid wood packing material without bark

Impact Summary

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CategoryImpact
Animal/plant collections None
Animal/plant products None
Biodiversity (generally) None
Crop production Negative
Environment (generally) None
Fisheries / aquaculture None
Forestry production None
Human health None
Livestock production Negative
Native fauna None
Native flora None
Rare/protected species None
Tourism None
Trade/international relations None
Transport/travel None

Impact

Top of page There is apparently no specific information available on the economic impact of P. medicaginis on a regional scale. It is widespread in many lucerne production areas and reduces stand density, winter hardiness (Tu, 1980; Basu and Butler, 1991; Wiersma et al., 1997) and forage yield of individual plants and entire fields (Lowe et al., 1987). A 75% stand reduction in the first year after sowing a susceptible cultivar in naturally infected fields (Gray et al., 1983) is perhaps representative of severe damage. In a different study, individual infected and healthy plants were monitored for 2 years; forage yield of root-rotted plants was reduced by 68% (Gray et al., 1988). In Wisconsin, USA, lucerne cultivars with good field resistance to Phytophthora root rot yielded 40% more than susceptible cultivars and combined resistance to Phytophthora and Aphanomyces increased yield still more (Wiersma et al., 1995). Estimating damage from comparisons of susceptible and resistant cultivars probably underestimates total loss, because even 'resistant' cultivars may be infected and damaged, especially under very wet conditions (Erwin and Ribeiro, 1996).

In eastern Australia, Phytophthora root rot caused by P. medicaginis is regarded as one of the major constraints to lucerne production on poorly drained soil types (Irwin, 1974; Rogers et al., 1978).

P. medicaginis is a major disease of chickpea in Australia (Vock et al., 1980; Irwin and Dales, 1982; Brinsmead et al., 1985; Knights et al., 2003). Large areas of chickpea in a field can be killed causing growers to abandon part or all of affected crops (Dale and Irwin, 1990b). Yield losses can exceed 50% for individual crops and reach 20% on a regional basis in years with above average rainfall (Knights et al., 2003).

Environmental Impact

Top of page The impact of P. medicaginis on the natural environment has not been reported. However, it is likely to be minimal because of its narrow host range of agricultural plants within the Fabaceae. Lucerne and chickpea are the primary hosts (Erwin and Ribeiro, 1996; Irwin and Dales, 1982). Annual species of Medicago are also susceptible under field test conditions (Haan et al., 1996). Annual medics and many other legumes are common in the natural environment. However, the effect of P. medicaginis on many of these legumes and the interaction of these effects with other plants are unknown.

Impact: Biodiversity

Top of page There are no reported impacts of P. medicaginis on biodiversity. P. medicaginis does not hybridize with other Phytophthora spp. such as P. sojae (Layton and Kuhn, 1988; Nygaard et al.,1989) and P. megasperma (Hansen, 1987).

Social Impact

Top of page There are no documented studies of the social impact of P. medicaginis on human activities which would indicate a low impact. However, root rot of lucerne and chickpea caused by this pathogen can result in high economic losses on individual farms and, logically, this reduces the livelihood of these primary producers.

Diagnosis

Top of page Isolation of P. medicaginis is best accomplished from active lesions on the roots of symptomatic but not dead plants, and by baiting from the soil. Roots should be carefully washed and pieces 2-4 mm long cut from the margins of lesions. Pieces of larger roots can be surface sterilized and then placed on selective agar medium containing pimaricin and antibacterial antibiotics (Erwin and Ribeiro, 1996). In some isolation procedures washing the roots for 2 h under running tap water has been used instead of surface sterilizing between the initial washing and plating onto the selective medium (Southwell and Crocker, 2005). Baiting is accomplished by floating newly germinated lucerne seedlings on flooded soil samples, then transferring them to selective media as discoloration develops (Erwin and Ribeiro, 1996).

Distinguishing P. medicaginis from other species of Phytophthora on morphological features alone is difficult, even for those familiar with the genus. The pattern of growth on agar media is distinctive for those familiar with this fungus. All the characteristics of the sporangia, oogonia, antheridia and oospores as originally described (Hansen and Maxwell, 1991) need to be considered to distinguish this fungus. Strong pathogenicity to lucerne and chickpea is an important species character. In critical cases, identification should be confirmed with molecular techniques. Vandemark and Barker (2003) have developed a real-time fluorescent PCR method that can not only detect P. medicaginis in lucerne but quantify it. This will be particularly useful in breeding programmes aiming to improve plant resistance to this pathogen.

Detection and Inspection

Top of page Phytophthora root rot is indicated by the pattern of disease in low-lying areas of affected fields and the characteristic dark lesions on tap roots of symptomatic plants. Commercially available ELISA test kits can be used to confirm the presence of Phytophthora, but not the species involved. Liew et al. (1998) have developed a PCR-based technique for the specific detection of P. medicaginis and used it successfully in detecting P. medicaginis in lucerne stems infected under greenhouse conditions and field-infected lucerne roots.

Similarities to Other Species/Conditions

Top of page Other Oomycetes can cause similar symptoms on lucerne and chickpea seedlings in the field, but severe damage to mature plants is usually caused by P. medicaginis. Aphanomyces euteiches is often present with P. medicaginis and may be damaging in seedling stands of lucerne in parts of North America. In some cases it may be the more serious pathogen (Abbo and Irwin, 1990; Holub and Grau, 1990a). It causes damping off and decay of the fine roots, but is usually not associated with lesions on tap roots. Seedlings attacked by Aphanomyces often remain upright, in contrast to the collapse of seedlings infected by Pythium or P. medicaginis (Stuteville and Erwin, 1990). Pythium species are generally confined to the fine roots, killing young seedlings, but causing little or no damage on mature plants.

Other species of Phytophthora may be present on lucerne or chickpea, but most lack the host specificity of P. medicaginis (Schmitthenner, 1964) and are not associated with severe disease. Isolation and identification of the pathogen is necessary to confirm. The situation is confused by recent changes in species concept and nomenclature (Hansen and Maxwell, 1991). P. megasperma, with larger oospores and a broad host range, is reported from alfalfa growing in Ontario, Canada (Barr, 1980; Faris et al., 1983; 1986; 1989), and Oregon, USA (Hansen and Hamm, 1983; Hansen and Maxwell, 1991). Species of Phytophthora from lucerne in California, USA, that grow at temperatures greater than 30°C are not P. medicaginis (Ribeiro et al., 1978; Forster and Coffey, 1993). In Spain, a fungus damaging chickpea has been confused with P. megasperma, but is in fact a new species, not P. medicaginis. (Liew and Irwin, 1994). Phytophthora megasperma isolates with small oospores like P. medicaginis, but lacking host specificity, have been recovered from lucerne field soils in Japan (Matsumoto and Sato, 1985; Hansen et al., 1986).

Prevention and Control

Top of page Control of Phytophthora root rot of lucerne is possible in most situations by combinations of improved water management, use of resistant cultivars and crop rotation. Assuring good drainage is the first step in almost all situations. This can be accomplished by selecting well-drained soils for establishment of lucerne crops or grading fields to eliminate low spots (Erwin and Ribeiro, 1996), by installing drainage tile or ditches to remove water, by subsoiling to break up impervious soil layers, and by avoiding excessive irrigation (Lehman et al., 1968).

Phytophthora root rot in chickpea in Australia has been partially controlled by using moderately resistant varieties, by seed applications of fungicides such as metalaxyl, by alternative crop rotations and by reduced irrigation regimes (Myatt et al., 1993).

Lucerne and chickpea cultivars vary widely in their tolerance of Phytophthora root rot, and selections with useful field resistance are available in most regions where root rot is a problem (Lehman et al., 1967; Irwin, 1974; Irwin and Maxwell, 1980; Frosheiser, 1980; Brinsmead et al., 1985; Lowe et al., 1987; Dale and Irwin, 1991a; Ansar et al., 1995). However, resistance is relative and losses may still be heavy in seedling stands and in chronically poorly drained soils (Erwin and Ribeiro, 1996). If Aphanomyces euteiches is also present, then it may be necessary to use cultivars with dual tolerance (Holub and Grau, 1990b). In the future, new resistant cultivars produced by genetic engineering may be available (Masoud et al., 1996). A genetic linkage map has been developed for lucerne grown in northern Australia (Musial et al., 2005). Quantitative trait loci (QTLs) involved in resistance to P. medicaginis were identified in a backcross population. This genetic linkage map provides an entry point for future molecular-based improvement of disease resistance in lucerne in Australia.

There has been a recent advance in chickpea breeding for resistance to Phytophthora root rot with the evaluation of wild Cicer species for resistance to P. medicaginis. Cicer echinospermum accessions have shown significantly greater resistance to P. medicaginis than chickpea genotypes, both in the glasshouse and in the field (Knights et al., 2003). C. echinospermum accessions can be readily crossed with chickpea to give fertile progeny and so provide a readily available source of resistance for this pathogen. However, while this source of resistance provides extended scope in breeding programmes, it is incomplete and chickpea with this resistance remain partially susceptible under extreme disease pressure. Thus, other disease control methods in conjunction with plant resistance should not be overlooked.

Crop rotations of 3 years or longer are necessary to reduce P. medicaginis populations in fields because of the long-lived oospores (Pratt and Mitchell, 1975; Stack and Millar, 1985b). Populations may quickly recover to damaging levels after a rotation unless drainage is improved and resistant cultivars are employed. Chemical control of Phytophthora root rot is possible, but is seldom economically feasible. The Oomycete-active fungicides will limit development of root rot in seedlings and in established plants, but will not eradicate the pathogen from infested soil. It may be feasible to provide critical short-term protection to seedling stands by fungicidal seed treatments before sowing (Rhodes and Myers, 1989). The usefulness of metalaxyl to control P. medicaginis may soon be reduced with the discovery of an isolate of the fungus insensitive to this fungicide (Stack and Millar, 1985a). Protection of seedlings can be obtained under experimental conditions by seed treatment with biological agents such as the bacterium Bacillus cereus (Silo-Suh et al., 1994). Myatt et al. (1993) found that Pseudomonas cepacia (7 strains) and Pseudomonas fluorescens (2 strains) were able to limit or delay chickpea seedling disease caused by P. medicaginis in a pasteurized soil. However, commercial application of this type of control has yet to be demonstrated.

An integrated disease management programme incorporating host resistance and including cultural, chemical or biological methods needs to be implemented to best control P. medicaginis.

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