Xiphinema americanum, the American dagger nematode, is a species of plant pathogenic nematodes. It is one of many species that belongs to the genus Xiphinema. It was first described by N. A. Cobb in 1913, who found it on both sides of the United States on the roots of grass, corn, and citrus trees.Cobb, N. A. (1913). "New nematode genera found inhabiting freshwater and non-brackish soils.". J. Wash. Acad. Sci. 3: 432–435.4 Not only is Xiphinema americanum known to vector plant viruses, but also X. americanum has been referred to as "the most destructive plant parasitic nematode in America", and one of the four major nematode pests in the Southeastern United States.Christie, J. R. (1952). "Some new nematodes of critical importance to Florida growers.". Proc. Soil Sci. Soc. Fla. 30: 39Thorne, G. (1961). Principles of Nematology. McGraw-Hill. New York. 553pWang, S., Gergerich, R., Wickizer, S., Kim, K. (2002). "Localization of transmissible and nontransmissible viruses in the vector nematode Xiphinema Americanum". Phytopathology 92: 646-653
Morphology and anatomy The length of the adult Xiphinema americanum ranges from 1.3 to 3.0 millimeters. The dagger nematode is characterized by a 100 μm odontostyle which is used for deep penetration of root tips with its spear-like stylet. The odontostyle is connected to the lining of the cheilostome by a folded membrane called the "guiding ring". The guiding ring is attached to a flanged odontophore.
Females: The body is usually in an "open C" conformation. The shape of the body tapers towards the extremities. The two rings of the odontophore are located 3 μm apart. The Xiphinema americanum esophagus is dorylaimoid with an enlarged posterior portion that occupies roughly 1/3 of its total length. The esophagus contains a muscular bulb which is 80 μm long and 20 μm wide. The valve between the esophagus and the intestine is amorphous.Lamberti, F.; Golden, M. (1984). "Redescription of Xiphinema americanum Cobb, 1913 with comments on its morphometric variation". Journal of Nematology 16(2): 204–206. The vulva is 46-54% of the total body length, and is located equatorially with a transverse slit shape, with the vagina having a diameter of 1/3 of the body diameter. The ovaries normally occur in pairs, and are amphidelphic and relexed. The prerectum of X. americanum measures 120–140 μm long, with a rectum that is roughly the same length as the body diameter at the anus. The tail contains 2-3 pairs of caudal pores, is conoid, and curves dorsally with a subacute terminus.
Males: The males have a similar overall configuration as the females, but are slightly smaller in length. Males of X. americanum, however, are rarely found in nature. The male has diorchic testes that are connected to the cloaca, with one anterior branch and one posterior branch. It is common to find more coil in the posterior region. The males also have paired spicules but lack a gubernaculum and bursa.
Life cycle Identifying Xiphinema americanum as a separate species has been a difficult task because of overlapping morphological aspects; however, differences in the life cycles of X. americanum may differentiate it from other species. Findings may also suggest that two subgroups of X. americanum should be made due to the finding of either 3 or 4 juvenile stages.Halbrendt, J. M.; Brown, D. J. F. (1992). "Morphometric evidence for three juvenile stages in some species of Xiphinema americanum sensu lato". Journal of Nematology 24: 305–309. The eggs of X. americanum are laid directly into the soil in water films, and are not associated with an egg mass. No molt occurs within the egg, which means that the first stage juvenile is the stage that enters the soil. Before becoming sexually mature adults, the X. americanum nematodes undergo three to four juvenile stages with a molt occurring between each.
Measurements of the functional and replacement odontostyles allows for the determination of the current stage in development.Halbrendt, J. M.; Brown, D. J. F. (1993). "Aspects of biology and development of Xiphinema americanum and related species". Journal of Nematology 25(3): 355–360. Compounding the issue of determining the life cycle of X. americanum is their difficulty with being grown in culture or greenhouse conditions. It has been suggested that this is due to X. americanum's sensitivity to moisture tension, temperature fluctuation, physical handling, or oxygen deprivation.Lownsberry, B. F.; Maggenti, A. R. (1963). "Some effects of soil temperature and soil moisture on population levels of Xiphinema americanum". Phytopathology 53: 667– 668.Griffin, G. D.; Barker, K. R. (1966). "Effects of soil temperature and moisture on survival and activity of Xiphinema americanum". Proceedings of the Helminthological Society of Washington 33: 126-130.Van Gundy, S. D.; Stolzy, L. H.; Szuszkiewicz, J. E.; Rackham, R. L. (1962). "Influence of oxygen supply on survival of plant-parasitic nematodes in soil". Phytopathology 52: 628–632.
Field evidence taken over a 2-year observation period indicates that X. americanum are most likely k-selected; they most likely have a long life span and a low reproduction rate.Jaffee, B. A.; Harrison, M. B.; Shaffer, R. L.; Strang, M. B. (1987). "Seasonal population fluctuations of Xiphinema americanum and Xiphinema rivesi in New York and Pennsylvania orchards". Journal of Nematology 19: 369–378. Unpublished results have shown greenhouse observations of X. americanum to develop from egg to adult in 7 months. Other results have suggested that X. americanum can live as long as 3–5 years.
Reproduction by fertilization from a male is rare if not nonexistent due to the lack of male X. americanum individuals, and therefore females reproduce parthenogenetically. All of the stages of X. americanum occur in the soil, with no particular stage as an important survival stage. In places with low winter temperatures, however, the egg is the primary survival structure.Griesbach, J. A.; Maggenti, A .R. (1990). "The morphometrics of Xiphinema americanum sensu lato in California". Revue de Nématologie 13: 93–103.
Host range X. americanum is a virtually non-specific plant nematode, causing it to have over one hundred different plant hosts. The most common plant hosts infected by X. americanum are common weeds and grasses, strawberries, soybeans, forest trees (spruce, pine, etc.), perennial orchards, and grapes.Ferris, H. "Nemabase". Host Range of a Genus and Species of Plant-feeding Nematodes. Retrieved 2011-10-21. Although large dagger nematode populations are rare in corn, Xiphinema americanum is the most commonly found dagger nematode species found in the United States, and significant damage can occur when large populations are present early in the growing season. This broad host range is due to the genetic diversity within the X. americanum species. For a complete list of the host range of X. americanum visit the Nemaplex Host Database.
Distribution Xiphinema americanum is found widely throughout most of the world and is found on all of the continents, except for Antarctica. The region with the highest population of X. americanum is thought to be the Eastern United States. The states with the highest population of the American Dagger Nematode are Arkansas, California, Pennsylvania, Rhode Island and Virginia.Robbins, R.T. (1993) "Distribution of Xiphinema americanum and Related Species in North America". Journal of Nematology 25(3):344–348. Other countries where X. americanum is found include Australia, Belize, Brazil, Chile, Guatemala, India, Japan, Korea Democratic People's Republic, Korea Republic, Mexico, New Zealand, Pakistan, Panama, South Africa, Sri Lanka, Uruguay, and areas of the Caribbean as well.Tarjan, A.C. (1969) "Variation within the Xiphinema americanum group (Nematoda: Longidoridae)". Nematologica 15, 241–252.
This species of nematode is also found to be sensitive to soil pH, and they are found most frequently in soils with a pH of 6.0 or higher.Norton, D.C.; Hoffmann, J.K. (1974) "Distribution of Selected Plant Parasitic Nematodes Relative to Vegetation and Edaphic Factors". Journal of Nematology April 6(2): 81–86.
Feeding habits Xiphinema americanum is a plant parasite that lives entirely in the soil and is attracted to young, growing roots due to source–sink dynamics. These nematodes are migratory ectoparasites and all of the life stages of the American Dagger Nematode feed at the root tips of plants. Since it is a migratory ectoparasite, they remain outside the root or other feeding areas and feed on epidermal cells or on cells deeper in the root. This ectoparasitism allows the nematode to move freely to different hosts throughout its life cycle. This nematode is considered to be an obligate parasite. It can survive within plant debris, however it needs living plant tissue to feed.
Once the nematode arrives at a root tip, it feeds by puncturing several successive layers of the plant's cells with its odontostyle; while penetrating, the nematode secretes enzymes that result in cell hypertrophy and thickening. The nematode is then able to begin extracting the cell's cytoplasm. The feeding period of X. americanum can last anywhere from several hours to several days, with the average being around 36 hours at each feeding site along the plant's roots. While feeding, it is common for the nematodes to remain still with their bodies either outstretched or curled, and following the feeding period they move slowly along the length of the root with their stylet remaining protruded and in search of a new feeding site. Unlike some species of nematodes, the observation of food passing into the gut of X. americanum is not seen. Although the nematodes are non-specific in their Host Range, they generally feed on plants that are in poorer condition for a shorter amount of time.Cohn, E. (1970). "Observations on the feeding and symptomology of Xiphinema longidorus on selected host roots". Journal of Nematology 2:167–173.
Symptoms and economic importance The symptoms that plants exhibit in response to the pathogenicity of Xiphinema americanum are similar to those of other migratory ectoparasitic nematodes of roots. It is common to see poor growth and or stunting of the plant, yellowing or wilting of the foliage, and reduced root systems which can include root necrosis, lack of feeder or secondary roots, and occasional tufts of stubby rootlets.Plant nematodes, their bionomics, and control. University of Florida Press. (January 1, 1959). Agric. Exp. Stn., University of Florida, Gainesville. 256p
Young, shortleaved yellow pine trees with moderate swelling of roots with clusters of short, stubby branches were the first demonstration of X. americanum pathogenicity in 1955.Xiphinema americanum. C.I.H. Descriptions of plant parasitic nematodes, Set 2, No. 29. Commonwealth Institute of Helminthology, Commonwealth Agricultural Bureaux. Herts, England.White, L. V. (1955). Xiphinema americanum Cobb: its relationship to certain perennial crops. M.S. thesis, Univ. Ark., Fayetteville, Ark. X. americanum can also cause severe effects on foliage, sometimes causing chlorosis and complete defoliation as seen on Guatemalan coffee trees.Lordello, L.G.E. (1972). Nematode pests of coffee, pp 409–434. In Economic nematology. J. M. Webster (ed.), Academic Press, New York. 563p The dagger nematode causes the devitalization of root tips and overall root death when they feed at the root tips and root sides of strawberry plants. Reddish-brown lesions that turn black and necrotic with time result at the sites of feeding, and result in reduced root systems and stunted tops.Perry, V. G. (1958). Parasitism of two species of dagger nematode (Xiphinema americanum and X. chambersi) to strawberry. Phytopathology. 48: 420–423.