Rev 02/04/2021
Xiphinema americanum Cobb, 1913
Has same general configuration as female. Actually, males of X. americanum are seldom found.
Note diorchic testes, with one branch anterior and one branch posterior, around mid body. Connected to cloaca through seminal vesicle and vas deferens.
Vulva region and tail region.
Female is diovarial and amphidelphic.
Vulva at 46-54% of body length.
Xiphinema americanum is actually considered to be a group of some 39 species, which include X. brevicolle and X. pachtaicum.
Molecular techniques are further underscoring the variability within the group and adding to the body of knowledge that will be required for an eventual taxonomic revision (Vrain, 1993).
The group also includes X. californicum which is probably associated with grape in California (Lamberti and Ciancio, 1993).
Xiphinema americanum group. Adult stages of this nematode range in length from 1.3 to 2.2 mm. However, the X. americanum sensu stricto ranges from 1.6 to 1.8 mm in length with an odontostyle and odontophore length slightly greater then 100 µm .
The morphometrics of these species tend to overlap (Robbins and Brown, 1991).
Reported median body size for this species (Length mm; width micrometers; weight micrograms) - Click:
It is currently suggested that the true X. americanum is native to, and predominant in, the eastern U.S. whereas the western U.S. populations are actually X. californicum, X. pacificum and other species (Lamberti and Bleve-Zacheo, 1979). Although true X. americancum occurs in California it may not be as common as X. californicum.
Members of the X. americanum group are common throughout vineyard areas of California, with the notable exception of the drier and warmer Coachella Valley (Siddiqui et al., 1973). They are most numerous in vineyards with abundant other vegetation, usually those in areas with >45 cm annual rainfall.
Migratory root ectoparasite; all stages feed at root tips.
Deep penetration of root tip by stylet; causes hypertrophy of cells, wall thickening, etc.
Causes necrosis of feeder roots on strawberries.
Species in this group of nematodes are difficult to study biologically because they are not readily cultured in greenhouse or laboratory conditions. They have a wide host range that includes common weeds and grasses, strawberries, and soybeans, forest trees (spruce, pine, etc.), perennial orchards, grape. 90% of California pear orchards (French, 1964).
The breadth of the host range is probably also related to the genetic diversity within the species complex.
Ecophysiological Parameters:
Four larval stages were thought to occur in the soil as there is no molt in the egg. However, recent studies indicate that some members of the X. americanum group (including X. americanum sensu stricto and X. californicum) pass through only three juvenile stages before becoming adults (Halbrendt and Brown, 1992).
Molting does not occur in the egg and the first stage hatches.
The length of the life cycle has not been experimentally determined; speculated to be 1 year (Malek, 1969). However, two population peaks are sometimes observed per year, although not in our (Ferris and McKenry, 1974) studies in California. Current conventional wisdom is that the longevity of X. americanum individuals is 3 to 5 years, with only one generation produced per year (Halbrendt and Brown, 1993).
Research experience and conventional wisdom has been that X. americanum is difficult to culture in pots under greenhouse conditions (Cohn and Mordechai, 1969, Lownsbery and Mitchell, 1965), which may have deterred researchers from attempting to work with the nematode under controlled conditions. The earlier work suggested that maintenance of relatively constant moisture conditions was important (Lownsbery and Mitchell, 1964) and more recent studies suggest that regulated temperature conditions are favorable for the nematode and that sorghum and grasses are useful greenhouse hosts (Diop et al., 2001). The temperature and moisture conclusions are quite interesting because, under field conditions in California vineyards, X. americanum is most prevalent in the upper 30-45 cm of soil where aeration conditions are favorable but soil moisture and temperature conditions fluctuate the most (Ferris and McKenry, 1974).
Endosymbiont Bacteria
Palomeres-Rius et al (2016) screened 124 plant-feeding nematode populations for the presence of bacterial endosymbionts; that is, bacteria living within the body of the nematode. Potential bacterial endosymbionts were only detected in species of Xiphinema, particularly in species of the X. americanum (sensu lato) group.
One group of 16S rRNA sequences was related to ‘Candidatus Xiphinematobacter’ and a second to the bacterial family Burkholderiaceae, which includes fungal and plant endosymbionts.
The study suggests a highly specific symbiotic relationship and a high degree of phylogenetic congruence and long-term evolutionary association between hosts and endosymbionts.
The bacterial endosymbionts are transmitted vertically to progeny. They are observed to colonize the ovary wall of females of Xiphinema species, they apparently invade developin oocytes and are seen in uninucleate eggs in the uterus. They become associated with intestinal epithelium during nemattode development and then with the overy in adult females (Palomares-Rius, 2016; Vandekerckhove et al., 2002; Brown et al., 2015).
Endosymbionts may have effects that range from parasitism (harmful to the host), through commensalism (neutral effects) to mutualism (beneficial to host and enosymbiont). The effects of the realtionships between the endosymbionts and Xiphinema spp. is unknown at this time)s. In the latter case, both host and endosymbiont would benefit from the realtionship.Although the nat
There are other examples of bacterial endosymbionts in nematodes. In many cases their roles in the biology of the nematode are unknown (Palomares-Rius et al, 2016), for example:
Associations between the insect-pathogenic bacteria Photorhabdus and Xenorhabdus is necessary for completion of the life cycle of Heterorhabditis and Stienernema (Ruby. 2008).
Detection of Wolbachia in the burrowing nematode Radopholus similis and R. arabocoffeae (Haegeman, et al. 2009).
Verrucomicrobia has been detected in theXiphinema americanum group (Vandekerckhove et al., 2002).
Bacteroidetes has been detected in populations of Heterodera glycines and Globodera rostochiensis (Noel & Atibalentja 2006).
Nematode causes root stunting and tip galling.
It causes necrosis on strawberry roots. It is also common in declining vineyards at high population densities. Marked growth response in grapevines were obtained after applications of DBCP.
From field observations, certain populations identified as X. californicum can build to 4-5 times the population level of a typical X. americanum sensu stricto. Population levels of X. americanum sensu lato in excess of 500 actual individuals/250 cm3 in non-weedy sites should be suspected as a probable cause for vine damage (McKenry, 1992).
Xiphinema americanum is a NEPO virus vector - polyhedral particles.
Xiphinema americanum sensu lato vectors:
Populations within the X. americanum group are capable of transmitting various strains of tomato ringspot virus (TRSV):
However, some populations keyed to the same species may not be able to transmit the virus (Brown et al., 1993).
Virus particles can be acquired within 24 hours of exposure to root. They occur in the esophageal lumen near stylet extension and in the glandular region of the esophagus.
In California, TRSV strains have not posed serious production problems in grape vineyards like those due to grapevine fanleaf virus. This could be a direct result of quarantines on virus infected grape cultivars. Vine damage attributable to the nematode alone is not great. At high inoculum levels, there was significant damage to newly planted 'French Colombard' in one experiment but not in others. Damage may occur when other factors also reduce root systems or force a shallow root system (Ferris and McKenry, 1975).
Some conventional grape rootstocks tested by McKenry in California have shown some resistance to X. americanum sensu stricto include 'VR-O39-16', 'Freedom', 'Dog Ridge', and 'Schwarzmann' (McKenry and Kretsch, 1994).
In contrast, V. vinifera and '1613C' are good hosts. In vineyards, 'VR-O39-16' and 'Freedom' do build-up populations of X. americanum sensu lato, however.
Host Plant Resistance, Non-hosts and Crop Rotation alternatives:
CIH Descriptions of Plant-parasitic Nematodes, Set 2, No. 29 (1973).
Cohn E., and Mordechai, M. 1969. Investigations on the life cycles and host preference of some species of Xiphinema and Longidorus under controlled conditions. Nematologica 15:295-302.
Diop, M.T., Diémé, J-H., Mountport, D. and Baujard, P. 2001. Laboratory culture of two Xiphinema americanum-group species (Nematoda: Longidoridae) from Senegal. Nematology 3:411-415.
Ferris, H. and M. V. McKenry. 1974. Seasonal fluctuations in the spatial distribution of nematode populations in a California vineyard. Journal of Nematology 6:203-210.
Lownsbery, B.F. and Mitchell, J.T. 1965. Some effects of chemical amendments and cultural conditions on population levels of Xiphinema americanum. Pl. Dis. Reptr 49, 994-998.