Scutellonema bradys (yam nematode)
- Taxonomic Tree
- Notes on Taxonomy and Nomenclature
- Distribution Table
- Risk of Introduction
- Hosts/Species Affected
- Host Plants and Other Plants Affected
- Growth Stages
- List of Symptoms/Signs
- Biology and Ecology
- Seedborne Aspects
- Pathway Vectors
- Plant Trade
- Detection and Inspection
- Prevention and Control
- Distribution Maps
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PicturesTop of page
IdentityTop of page
Preferred Scientific Name
- Scutellonema bradys (Steiner & Le Hew, 1933) Andrássy, 1958
Preferred Common Name
- yam nematode
Other Scientific Names
- Anguillulina bradys (Steiner & Le Hew, 1933) T. Goodey, 1975
- Hoplolaimus bradys Steiner & Le Hew, 1933
- Rotylenchus blaberus Steiner, 1937
- Rotylenchus bradys (Steiner & Le Hew, 1933) Filipjev, 1936
- Scutellonema blaberum (Steiner, 1937) Andrássy, 1958
- Scutellonema dioscorea Lordello, 1959
International Common Names
- English: yam dry rot nematode
- French: nématode de l'igname
- SCUNBR (Scutellonema bradys)
Taxonomic TreeTop of page
- Domain: Eukaryota
- Kingdom: Metazoa
- Phylum: Nematoda
- Family: Hoplolaimidae
- Genus: Scutellonema
- Species: Scutellonema bradys
Notes on Taxonomy and NomenclatureTop of page S. bradys was first described in 1933 from infected yam tubers from Jamaica (Steiner and Lehew, 1933). These authors initially placed the species in the Genus Hoplolaimus.
DescriptionTop of page Measurements
15 Female (syntypes): L=0.88-1.11 mm; a=27-32; b=6.7-8.7; b'=5.2-6.0; c=32-56; V = 54-59; spear = 26-30 µm.
10 Male (T. Goodey's specimens from Nigerian yam): L=0.85-1.0 mm; a=26-36; b=6.6-9.0; b'=5.2-6.6; c=27-32; spear=25-28 µm; spicules=29-33 µm; gubernaculum=14-17 µm.
Female (based on syntypes): Body straight to slightly arcuate when relaxed; annules about 1.6 µm wide near middle; lateral fields about one-fifth body-width, with 4 incisures, areolated at phasmids and anteriorly, sometimes irregularly areolated on mid-body and tail. Lip region knob-like, offset by a constriction, with a labial disc and 6 to 8 (usually 7) annules lacking longitudinal striations. Cephalic sclerotization strong. Spear well developed with large oval to rounded basal knobs bearing flattened, indented or irregular anterior surfaces; anterior tapering portion a little less than half spear length. Hemizonid usually distinct, 2-3 annules long, 0-3 annules anterior to excretory pore and close to oesophago-intestinal junction. Hemizonion 1 annule long, about 8 annules behind the excretory pore. Oesophageal glands elongate, overlapping intestine dorsally and dorso-laterally; nucleus of dorsal gland anterior to those of subventrals. Ovaries paired, with oocytes in 1 or 2 rows. Spermathecae rounded, sometimes oval, usually packed with sperms. Vulva a transverse slit with conspicuous cuticular thickenings towards ends (? = 'vaginal glands' of Sher, 1964). Epiptygma inconspicuous. Intestine partially overlapping rectum. Tail variable with obtusely rounded striated terminus and 13 - 20 annules. Phasmids about 4 µm diameter, with pore-like aperture, at or up to 6 annules anterior to anal level (Siddiqi, 1972).
Male: Abundant. Similar to female except for sexual dimorphism. Testis outstretched; spermagonia in 3-4 rows; sperms about 4 µm in diameter. Bursa large, crenate, enclosing tail. Spicules slightly cephalated and ventrally arcuate, with large distal flanges. Capitulum (= telamon) prominent, about 10 µm long. Phasmids usually just postanal. Cuticular, non-protoplasmic terminal portion of tail 11-16 µm long (Siddiqi, 1972).
DistributionTop of page S. bradys is a nematode of West Africa which has spread to other yam growing areas of the world in South America and the Caribbean.
A record of S. bradys on yam in Korea (CABI/EPPO, 2000; EPPO, 2006) published in previous editions of the Compendium has been removed as it was based on a misidentification. Although S. bradys was reported in yam fields of Geyongbuk province, Korea in 1998 (Park et al., 1998), it was not supported with morphometric data or a description of the species. S. bradys was not found in 160 soil samples taken during a survey of major yam-growing regions of Geyongbuk province in 2006 (Park and Khan, 2007).
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: 23 Apr 2020
|Continent/Country/Region||Distribution||Last Reported||Origin||First Reported||Invasive||Reference||Notes|
|Benin||Present||Baimey et al. (2009); CABI and EPPO (2011); EPPO (2020)|
|Burkina Faso||Present||CABI and EPPO (2011)|
|Cameroon||Present||Bridge et al. (1995); CABI and EPPO (2011); EPPO (2020)|
|Côte d'Ivoire||Present||Baudin (1956); CABI and EPPO (2011); EPPO (2020)|
|Gambia||Present||Merny and Fortuner (1973); CABI and EPPO (2011); EPPO (2020)|
|Ghana||Present||ADDOH (1971); CABI and EPPO (2011); EPPO (2020)|
|Guinea||Present||CABI and EPPO (2011); EPPO (2020); CABI (Undated)|
|Kenya||Present||Coyne et al. (2016); EPPO (2020)|
|Mali||Present||CABI and EPPO (2011)|
|Nigeria||Present||GOODEY (1935); UNNY and JERATH (1965); Bridge (1972); CABI and EPPO (2011); EPPO (2020)|
|Senegal||Present||CABI and EPPO (2011); EPPO (2020)|
|Sudan||Present||CABI and EPPO (2011)|
|Tanzania||Present||Coyne et al. (2016); EPPO (2020)|
|Togo||Present||CABI and EPPO (2011); EPPO (2020); CABI (Undated)|
|India||Present||NADAKAL and THOMAS (1967); CABI and EPPO (2011); EPPO (2020)|
|-Kerala||Present||NADAKAL and THOMAS (1967); CABI and EPPO (2011); EPPO (2020)|
|Pakistan||Present||CABI and EPPO (2011); EPPO (2020)|
|South Korea||Absent, Invalid presence record(s)||CABI and EPPO (2011); EPPO (2020); CABI (Undated)|
|Barbados||Present||CABI and EPPO (2011); EPPO (2020)|
|Costa Rica||Present||Humphreys-Pereira et al. (2014)|
|Cuba||Present||DECKER et al. (1967); CABI and EPPO (2011); EPPO (2020)|
|Dominica||Present||Belliard and Kermarrec (1978); CABI and EPPO (2011); EPPO (2020)|
|Dominican Republic||Present||CABI and EPPO (2011); EPPO (2020)|
|Guadeloupe||Present||Kermarrec et al. (1987); CABI and EPPO (2011); EPPO (2020)|
|Guatemala||Present||CABI and EPPO (2011); EPPO (2020); CABI (Undated)|
|Haiti||Present||Jatala and Bridge (1990); CABI and EPPO (2011); EPPO (2020)|
|Jamaica||Present||CABI and EPPO (2011); EPPO (2020); CABI (Undated)|
|Martinique||Present||Kermarrec et al. (1987); CABI and EPPO (2011); EPPO (2020)|
|Puerto Rico||Present||Steiner and Buhrer (1934); AYALA and AGOSTA (1971); CABI and EPPO (2011); EPPO (2020)|
|Trinidad and Tobago||Present||CABI and EPPO (2011); EPPO (2020)|
|United States||Present, Localized||CABI and EPPO (2011); EPPO (2020); CABI (Undated)|
|-Arkansas||Present||CABI and EPPO (2011); EPPO (2020)|
|-Florida||Present||CABI and EPPO (2011); EPPO (2020); CABI (Undated)|
|Brazil||Present, Localized||LORDELLO (1959); CABI and EPPO (2011); EPPO (2020)|
|-Alagoas||Present||CABI and EPPO (2011); EPPO (2020)|
|-Bahia||Present||Coimbra et al. (2006); CABI and EPPO (2011)|
|-Paraiba||Present||CABI and EPPO (2011); EPPO (2020)|
|-Pernambuco||Present||LORDELLO (1959); CABI and EPPO (2011); EPPO (2020)|
|-Sao Paulo||Present||Lordello et al. (2005); CABI and EPPO (2011)|
|Venezuela||Present||CABI and EPPO (2011); EPPO (2020)|
Risk of IntroductionTop of page S. bradys poses a considerable phytosanitary risk because of its survival and ease of dissemination within yam tubers. Yams are propagated from whole tubers or pieces of tuber and are thus the principal means of dissemination of S. bradys in the yam growing areas of the world. Comparatively low populations of the nematodes in tubers do not produce external symptoms of damage (Bridge, 1973) and therefore the risk of dissemination by this means is greater. Infested seed tubers rather than soil are probably the main source of nematode inoculum in yam fields. The spread of the nematode in infested tubers has been highlighted between islands in the Caribbean (Kermarrec et al., 1981, 1987).
Hosts/Species AffectedTop of page All the main yams, Dioscorea spp., grown for food are susceptible hosts of S. bradys. These are the species D. alata, D. cayenensis, D. dumentorum, D. esculenta and D. rotundata (Baudin, 1956; Caveness, 1967; Smit, 1967; Bridge, 1982). In addition other yams known to be infected by S. bradys are D. bulbifera, D. trifida and D. transversa (Decker et al., 1967; Ayala and Acosta, 1971; Belliard and Kermarrec, 1978; Kermarrec et al., 1987).
Two wild Dioscorea spp. growing in forests in Nigeria and Cameroon have been shown to be natural hosts (Bridge, 1982; Bridge et. al., 1995).
A wide range of other crops and some weeds have been shown to support low root populations of S. bradys including yam bean (Pachyrrhizus erosus), greengram (Phaseolus aureus), pigeon pea (Cajanus cajan), kenaf (Hibiscus cannabinus), okra (Hibiscus esculentus), tomato (Lycopersicon esculentum), sorghum (Sorghum vulgare), loofah (Luffa cylindrica), roselle (Hibiscus sabdariffa) and Synedrella nodiflora. These alternative hosts permit the yam nematode to survive in soil in the absence of yams, but only cowpea (Vigna unguiculata), melon (Cucurbita melo) and sesame (Sesamum indicum) in addition to yams have been found to actually increase populations of the nematode (Luc and de Guiran, 1960; Adesiyan 1976; Bridge, 1982; Jatala and Bridge, 1990).
Host Plants and Other Plants AffectedTop of page
|Abelmoschus esculentus (okra)||Malvaceae||Other|
|Cajanus cajan (pigeon pea)||Fabaceae||Other|
|Catharanthus roseus (Madagascar periwinkle)||Apocynaceae||Wild host|
|Celosia argentea (celosia)||Amaranthaceae||Other|
|Cocos nucifera (coconut)||Arecaceae||Other|
|Corchorus olitorius (jute)||Tiliaceae||Other|
|Dioscorea (yam)||Dioscoreaceae||Wild host|
|Dioscorea alata (white yam)||Dioscoreaceae||Main|
|Dioscorea bulbifera (air potato)||Dioscoreaceae||Main|
|Dioscorea cayenensis (Guinea yam)||Dioscoreaceae||Other|
|Dioscorea esculenta (Asiatic yam)||Dioscoreaceae||Main|
|Dioscorea trifida (cushcush yam)||Dioscoreaceae||Other|
|Hibiscus cannabinus (kenaf)||Malvaceae||Other|
|Hibiscus sabdariffa (Roselle)||Malvaceae||Other|
|Luffa aegyptiaca (loofah)||Cucurbitaceae||Other|
|Manihot esculenta (cassava)||Euphorbiaceae||Other|
|Pachyrhizus erosus (yam bean)||Fabaceae||Other|
|Pueraria phaseoloides (tropical kudzu)||Fabaceae||Other|
|Sesamum indicum (sesame)||Pedaliaceae||Other|
|Solanum lycopersicum (tomato)||Solanaceae||Other|
|Sorghum bicolor (sorghum)||Poaceae||Other|
|Synedrella nodiflora (synedrella)||Asteraceae||Other|
|Urena lobata (caesar weed)||Malvaceae||Other|
|Vigna radiata (mung bean)||Fabaceae||Other|
|Vigna unguiculata (cowpea)||Fabaceae||Main|
Growth StagesTop of page Post-harvest, Vegetative growing stage
SymptomsTop of page S. bradys causes a characteristic disease of yam (Dioscorea spp.) tubers known as 'dry rot disease'. Dry rot of yams occurs in the outer 1 to 2 cm of tubers directly associated with S. bradys. The initial stage of dry rot consists of cream and light-yellow lesions below the outer skin of the tuber. There are no external symptoms at this stage. As the disease progresses it spreads into the tuber, normally to a maximum depth of 2 cm but sometimes deeper. In these later stages of dry rot, infected tissues first become light brown and then turn dark brown to black. External cracks appear in the skin of the tubers and parts can flake off exposing patches of dark brown, dry rot tissues. The most severe symptoms of dry rot are seen in mature tubers especially during storage when it is often associated with general decay of tubers. No foliar symptoms have been observed on yams growing in soil infested with S. bradys (after Jatala and Bridge, 1990).
List of Symptoms/SignsTop of page
|Roots / necrotic streaks or lesions|
|Vegetative organs / dry rot|
|Vegetative organs / internal rotting or discoloration|
|Vegetative organs / surface cracking|
Biology and EcologyTop of page S. bradys is a migratory endoparasite of roots and tubers and will also be present in soils around host plants.
In yams, S. bradys invades the young, developing tubers through the tissues of the tuber growing point, alongside emerging roots and shoots, through roots and also through cracks or damaged areas in the tuber skin (Bridge, 1972). Nematodes feed intracellularly in yam tuber tissues resulting in rupture of cell walls, loss of cell contents and the formation of cavities (Goodey, 1935; Bridge, 1973; Adesiyan et al., 1975a). They are mainly confined to the sub-dermal, peridermal and underlying parenchymatous tissues in the outer 1-2 cm of tuber. S. bradys continues to feed and reproduce in yams stored after harvesting. Populations can increase 9 to 14-fold in D. rotundata tubers over a 5 to 6 month storage period, and 5 to 8-fold in D. alata and D. cayenensis respectively over the same period (Bridge, 1973; Adesiyan, 1977). In tubers with partial dry rot, more nematodes are found in the oldest, apical portions, adjacent to the stems (Adesiyan, 1977).
Seedborne AspectsTop of page S. bradys is not disseminated in true seed.
Pathway VectorsTop 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|
|Bulbs/Tubers/Corms/Rhizomes||adults; eggs; juveniles||Yes||Pest or symptoms not visible to the naked eye but usually visible under light microscope|
|Growing medium accompanying plants||adults; eggs; juveniles||Yes||Pest or symptoms not visible to the naked eye but usually visible under light microscope|
|Roots||adults; eggs; juveniles||Yes||Yes||Pest or symptoms not visible to the naked eye but usually visible under light microscope|
|Seedlings/Micropropagated plants||adults; eggs; juveniles||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)|
ImpactTop of page All information on the economic importance of S. bradys is derived from research on yams (Dioscorea spp.). The primary importance of S. bradys on yams is in the direct damage it causes to the tubers resulting in dry rot disease (Jatala and Bridge, 1990). The nematodes produce a marked reduction in the quality, marketable value and edible portions of tubers, and these reductions are more severe in yams that have been stored. Weight differences between healthy and diseased tubers harvested from the field have been estimated to be 20 to 30% in the Côte d'Ivoire (Smit in Bridge, 1982) and 0 to 29% in Nigeria (Wood et al., 1980). Weight reduction due to moisture loss is more likely to occur in late harvested tubers left in dry soil (Bridge, 1982). Water loss from tubers continues during storage and is significantly greater in tubers infected with S. bradys compared with healthy tubers (Adesiyan et al., 1975b). Nearly 47% of all yam tubers on sale in Nigerian markets were infested with S. bradys (Bridge, 1973) and both dry rot and wet rot diseases of tubers have been observed in all Nigerian yam barns and markets sampled (Adesiyan and Odihirin, 1977).
DiagnosisTop of page Nematodes will be found in soil and roots particularly at the end of the growing season with yams. Nematodes are extracted from soil and roots by standard nematode extraction procedures. However, in yams, most nematodes will be found in tuber tissues; sampling of these is the most appropriate means of assessing the populations and importance of S. bradys. Peelings of a known thickness (1 or 2 cm) are cut from the outer tuber tissue, chopped finely and teased apart or preferably macerated before placing on a support tissue or sieve in water as for roots. Between 30 and 50% of nematodes will emerge from tissues in the first 3 days but they will continue migrating from the tissues for over 20 days.
Detection and InspectionTop of page In yams, the presence of dry rot in tubers can be assessed by direct observation. Severely infested tubers will have surface cracking or flaking of the epidermis exposing the black dry rot tissues underneath. In tubers without obvious external symptoms of damage, it will be necessary to scrape away the surface layers, or section tubers to determine whether dry rot occurs. In other crops, symptoms of damage are not known. Confirmation that S. bradys is the nematode present and causing any damage can only be done after extraction and microscopic examination.
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.Introduction
Management of S. bradys can be achieved by one or more of the following measures: (1) controlling nematodes in field soil by cultural or chemical means (2) use of planting material that is naturally free of nematodes, or treatment of seed material (tubers and setts with yams) prior to planting to reduce or eliminate nematodes from propagative material, and (3) in the case of yams, treatment of tubers after harvesting to prevent storage losses (Jatala and Bridge, 1990).
In Field Soil
In Cuba, keeping fallow land free of all host plants is a suggested means of reducing damage by S. bradys to yams (Decker et al., 1967) but this is unlikely to be economic or practical in most situations. Yams are frequently intercropped, sometimes with as many as five other crops (Coursey, 1967). Control of weed hosts and the exclusion of other crop hosts of S. bradys from around yams will help to reduce nematode damage. Soil populations of S. bradys will be reduced if a non-host or poor host crops, such as groundnut, chilli pepper, tobacco, Indian spinach (Beta vulgaris var. benghalensis), cotton, maize or sorghum are grown prior to yams (Adesiyan, 1976).
Application of chemical nematicides has, at best, only produced moderate yield increases and control of S. bradys (Anon., 1964; Ayala and Acosta, 1971) and information on the economics of this means of control is lacking for large scale use.
Resistance to S. bradys in yams has yet to be confirmed and all the main food yams (D. alata, D. bulbifera, D. cayenensis, D. esculenta, D. rotundata) are susceptible to damage. However, resistance could prove to be the most practical and economic means of managing S. bradys if found in commercially acceptable cultivars.
Clean Planting Material
In yams, using nematode-free planting material is a practical and economic means of preventing damage by S. bradys and also their dissemination. Seed tubers showing symptoms of dry rot (cracking and flaking) should not be used for planting. The presence of dry rot in tubers without external symptoms can be determined by scraping away sections of tuber skin, or by the use of tuber pieces rather than whole tubers enabling the grower to examine for dry rot symptoms before planting.
Bulbils or aerial tubers of the yam D. bulbifera and some forms of D. alata, which are used for propagation will be completely free of nematodes. A number of yams, such as D. alata, D. rotundata and D. dumentorum, can be produced from vine cuttings (Coursey, 1967). Even true seed can be used for propagating D. rotundata (Sadik and Okereke, 1975). Although these methods of propagation are not a practical means of producing ware tubers, they can be used to produce nematode-free seed tubers (Jatala and Bridge, 1990). The use of 'microsetts' or 'minisetts' cut from mature tubers (International Institute of Tropical Agriculture, 1984) can provide clean planting material if the mother seed yams selected are free of nematodes.
Some traditional practices in Africa, such as the use of wood ash on yam tubers or mixing cow dung in yam mounds before planting are reported to decrease nematode numbers (Adesiyan and Adeniji, 1976).
Hot water treatment can reduce or eliminate S. bradys from yam tubers. The expense of heating equipment, and the difficulties of maintaining constant temperatures, are the main prohibitive factors against its large scale use. However, it is feasible for small scale operations and for establishing nematode-free planting material. Most studies have shown that a water temperature of 50-55°C for up to 40 min gives the best control of S. bradys without damaging tubers (Jatala and Bridge, 1990).
ReferencesTop of page
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