Nematodes - groundnut
Contributors to this page: ICRISAT, Patancheru, India (RP Thakur, AG Girish, VP Rao).
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Potato tuber nematode, Potato rot nematode
Scientific name
Ditylenchus destructor Thorne.
Importance
Medium
Significance
It has been a problem in all the groundnut-producing areas of South Africa (Jones and De Waele 1988). It is suspected that the population in South Africa may be a separate ecotype or pathotype and may be confined to groundnuts.
Symptoms
The most common symptoms include stunting and chlorosis of affected plants. Hulls of groundnuts show black discoloration which appears first along the longitudinal veins. The kernels are shrunken. The infected testae are brown to black and the embryo shows a brown discoloration (Jones and De Waele 1988).
Host
Potatoes (Solanum tuberosum) are the main host of D. destructor, but the nematode can also occasionally be found on bulbous Trifolium spp. (clover), Daucus carota (carrots), Arachis hypogaea (groundnut) and Allium sativum (garlic). Overall, some 70 crops and weeds and a similar number of fungus species have been recorded as hosts (Thorne 1961).
Geographical distribution
Ditylenchus destructor is worldwide in distribution.
Biology and transmission
Adults of D. destructor are minute worm-like animals, 0.8-1.4 mm in length and 23-47 μm in diameter. Considerable morphometric variation occurs in adults according to their host and/or age. Males and females are similar in general appearance. In females, the ost-vulval sac extends about three-quarters of the distance to the anus, and the tail has a narrow rounded terminus. Males have ventrally curved, anteriorly expanded spicules. There are four juvenile stages (the first preceding hatching of the egg), superficially similar to adults, but differing in size and in lacking developed reproductive organs. Unlike the closely related species D. dipsaci, D. destructor is unable to withstand excessive desiccation, and for this reason it is usually more important in cool and moist soils. Without a resistant resting stage, the species overwinters in soil as adults or larvae and may even multiply by feeding on alternative weed hosts (e.g. Mentha arvensis, Sonchus arvensis) and on fungal mycelium. It may also possibly overwinter as eggs. These hatch in the spring and larvae are immediately able to parasitize hosts. Egg hatch at 28°C begins 2 days after egg laying, with an average interval of 4.4 days between egg laying and hatch, and development from egg to adult takes between 6 and 7 days (Hooper 1973).
Detection/indexing methods at ICRISAT
- Pre export field inspection and seed washing to detect the nematodes.
Treatment/control
- Seed dressing of groundnut prior to planting with thiram or benomyl wettable powder gave very good control (Fujimura et al. 1989).
Procedures followed in case of positive test at ICRISAT
- Incineration of the infested crop and rejection of the seed samples are used.
EPPO protocols
Ditylenchus destructor was considered to be an EPPO A2 quarantine pest (OEPP/EPPO 1978) but was deleted from the quarantine list in 1984 because of its minor importance and very wide distribution throughout the EPPO region, in particular in those areas where it would be likely to cause crop damage.
References and further reading
Fujimura T, Ichita T, Kimura T. 1989. Occurrence of potato-rot nematode, Ditylenchus destructor Thorne, in garlic and control. 1. Evaluation of treatments applied before planting and after harvest for control. Japanese Journal of Nematology18: 22-29.
Hooper DJ. 1973. Ditylenchus destructor. CIH Descriptions of Plant-parasitic Nematodes No. 21. CAB International, Wallingford, UK.
Jones BL, De Waele D. 1988. First report of Ditylenchus destructor in pods and seeds of peanut. Plant Disease 72: 453.
OEPP/EPPO. 1978. Data sheets on quarantine organisms No. 123, Ditylenchus destructor. Bulletin OEPP/EPPO Bulletin 8(2).
Thorne G. 1961. Principles of nematology, 533 pp. McGraw-Hill Book Co. Inc., New York, USA.
Scientific name
Aphelenchoides arachidis Bos.
Importance
High
Significance
Nematode infection does not suppress yield, but causes serious qualitative damage and predisposes seeds to fungal infection by Fusarium spp., Macrophomina phaseolina, Rhizoctonia solani and Sclerotium rolfsii. These infected seeds are under graded and unmarketable (Bridge and Hunt 1985; Minton and Baujard 1990; Stokes 1980).
Symptoms
Aphelenchoides arachidis occurs as an endoparasite on the tissues of the pods, testas, roots and hypocotyls. Testas infected with A. arachidis are thicker and more uneven than normal testas. Heavily infected seeds had translucent testas shortly after removal of the fully mature pods. These seeds are also a lighter brown and have darker vascular strands within testas (Stokes 1980).
Host
Arachis hypogaea (peanut) is the principal host; other agronomic crops and weeds can be infected without showing any symptoms or damage. These hosts include Zea mays (maize), Oryza sativa (rice), Saccharum offcinarum (sugarcane) and unidentified grasses (CABI 2001).
Geographical distribution
This nematode has been reported only from Nigeria.
Biology and transmission
Females of Aphelenchoides arachidis are characterized by a lateral field marked by two incisures, a stylet of 11-12 µm long with distinct knobs, a postvulval uterine sac extending for half the vulva-anus distance, and a sub-cylindroid tail with a bluntly rounded terminus provided by a mucro. The nematode develops and reproduces in the seed coat (testa), and also in the pod, root and hypocotyl tissues causing discoloration, necrosis and brown stripes within the testas. The nematode-infected seeds appear shrunken and dark. This pest can survive in low numbers in pods and seeds (Bridge and Hunt 1985). The nematode is disseminated by infected peanut hulls and seeds. Because of its limited distribution (Nigeria), A. arachidis has not caused major economic losses, but this pest can become a major economic pest if introduced in large peanut-producing areas (Minton and Baujard 1990).
Detection/indexing methods at ICRISAT
- Seed washing test is used to detect the nematode.
Treatment/control
- Hot water treatment at 60°C for 5 min. eliminates the nematode (Stokes 1980).
Procedures followed in case of positive test at ICRISAT
- Incineration of the plants and rejection of the seed samples in case of positive test.
References and further reading
Bridge J, Hunt DJ. 1985. Aphelenchoides arachidis. CIH description of plant parasitic nematodes Set 8, No. 116, pp. 3. St. Albans, UK: Commonwealth Institute of Helminthology.
CAB International. 2001. Aphelenchoides arachidis In: Crop protection compendium, global module, 3rdedition. Wallingford, UK: CAB International.
Minton N A, Baujard P. 1990. Nematode parasites of peanuts. Pp. 285-320 in (M. Luc, R. A. Sikora, and J. Bridge eds) Plant parasitic nematodes in tropical and subtropical agriculture. Wallingford, UK: CAB International.
Stokes DE. 1980. The effects of the testa nematode, Aphelenchoides arachidis, on peanuts. Nematology Circular No. 68, Florida Department of Agriculture and Consumers Services, Division of Plant Industry, Gainesville, FL, USA.
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