Phytophthora medicaginis (Phytophthora root rot of lucerne)
- Summary of Invasiveness
- Taxonomic Tree
- Notes on Taxonomy and Nomenclature
- Distribution Table
- History of Introduction and Spread
- Risk of Introduction
- Hosts/Species Affected
- Host Plants and Other Plants Affected
- Growth Stages
- List of Symptoms/Signs
- Biology and Ecology
- Notes on Natural Enemies
- Means of Movement and Dispersal
- Seedborne Aspects
- Pathway Vectors
- Plant Trade
- Wood Packaging
- Impact Summary
- Environmental Impact
- Impact: Biodiversity
- Social Impact
- Detection and Inspection
- Similarities to Other Species/Conditions
- Prevention and Control
- Distribution Maps
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PicturesTop of page
IdentityTop of page
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 InvasivenessTop of page
Taxonomic TreeTop of page
- Domain: Eukaryota
- Kingdom: Chromista
- Phylum: Oomycota
- Class: Oomycetes
- Order: Peronosporales
- Family: Peronosporaceae
- Genus: Phytophthora
- Species: Phytophthora medicaginis
Notes on Taxonomy and NomenclatureTop of page
DescriptionTop of page
DistributionTop of page
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 TableTop of page
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.Last updated: 10 Mar 2020
|Continent/Country/Region||Distribution||Last Reported||Origin||First Reported||Invasive||Reference||Notes|
|Italy||Present||Present based on regional distribution.|
|Canada||Present||Present based on regional distribution.|
|United States||Present||Present based on regional distribution.|
|-New South Wales||Present, Localized|
History of Introduction and SpreadTop of page
Risk of IntroductionTop of page
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 AffectedTop of page
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 AffectedTop of page
Growth StagesTop of page
SymptomsTop of page
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/SignsTop of page
|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 EcologyTop of page
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).
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).
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).
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 EnemiesTop of page
Means of Movement and DispersalTop of page
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).
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 AspectsTop of page
Plant TradeTop of page
|Plant parts liable to carry the pest in trade/transport||Pest stages||Borne internally||Borne externally||Visibility of pest or symptoms|
|Growing medium accompanying plants||fungi/hyphae; fungi/sporangia; fungi/spores||Yes||Pest or symptoms not visible to the naked eye but usually visible under light microscope|
|Roots||fungi/hyphae; fungi/sporangia; fungi/spores||Yes||Yes||Pest or symptoms not visible to the naked eye but usually visible under light microscope|
|Seedlings/Micropropagated plants||fungi/hyphae; fungi/sporangia; fungi/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|
|Fruits (inc. pods)|
|Stems (above ground)/Shoots/Trunks/Branches|
|True seeds (inc. grain)|
Wood PackagingTop of page
|Wood Packaging not known to carry the pest in trade/transport|
|Loose wood packing material|
|Processed or treated wood|
|Solid wood packing material with bark|
|Solid wood packing material without bark|
Impact SummaryTop of page
|Fisheries / aquaculture||None|
ImpactTop of page
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 ImpactTop of page
Impact: BiodiversityTop of page
Social ImpactTop of page
DiagnosisTop of page
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 InspectionTop of page
Similarities to Other Species/ConditionsTop of page
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 ControlTop of page
Due to the variable regulations around (de)registration of pesticides, your national list of registered pesticides or relevant authority should be consulted to determine which products are legally allowed for use in your country when considering chemical control. Pesticides should always be used in a lawful manner, consistent with the product's label.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.
ReferencesTop of page
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