Rev 02/04/2021
Xiphinema index Thorne and Allen, 1950
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Length of adults of this species range from 2.9 to 3.3 mm.
Males are extremely rare.
The tail of X. index adults has a distinct, finger-like, protuberance. Under the dissection microscope, the protruberance provides a convenient method for separating X. index from co-occurring Xiphinema species.
Reported median body size for this species (Length mm; width micrometers; weight micrograms) - Click:
Throughout the world, X. index is associated with its most important host, the grapevine.
In California this nematode appears to have been moved around with rootstocks. It commonly appears in plant material that has been collected in other regions and then planted for observation adjacent to a winery.
In the 1970s, McKenry estimated that 5% of California grape acreage was partially infested. In a 1993 study of 15 randomly selected replanted vineyards in the southern San Joaquin Valley, three were infested with X. index by the fourth year after replanting. In California, X. index is almost always found in association with either phylloxera or Meloidogyne spp. or X. americanum sensu lato. Generally found in Northern California, but sporadically as far south as Kern County.
In Australia, distribution of the nematode has been greatly slowed by the phylloxera quarantine.
Originally described from fig in soil samples taken by E.F. Serr near Planada, California (11 miles east of Merced) and sent to Gerald Thorne in Salt Lake City for diagnosis. From those samples, Thorne and Allen (1950) described both Xiphinema index and Paratylenchus hamatus.
B-rated pest in California Nematode Pest Rating System.
Migratory root ectoparasite; all stages feed at root tips.
Deep penetration of root tip by stylet; causes hypertrophy of cells, wall thickening, etc.
The host range of X. index is apparently narrow and includes grape, fig, apple, rose, pistachio and a few other, mainly perennial, species.
Ecophysiological Parameters:
The length of the life cycle of X. index is reported variously as 27 days in California (Radewald and Raski, 1962) to 7-9 months in Israel (Cohn and Mordechai, 1969).
Eggs are produced at the reate of one per 21 to 90 degree days (Brown and Coiro, 1985).
Reproduction apparently by parthenogenesis.
Recent microplot studies in the San Joaquin Valley indicate that on particularly suitable host selections, such as V. rupestris Scheele, X. index population levels peak between the 5th and 12th month after inoculation. Peak population levels are achieved over longer periods of time on poorer hosts such as Vitis champinii Planch..
In field plot evaluations by McKenry on sandy loam soil inoculated with X. index at planting, seventeen of eighteen grape cultivars did not show significant damage two years after inoculation. The single exception was V. vinifera L. cv. Rubired. Direct damage by X. index, in the absence of virus, is most commonly observed on very porous gravely silts or other coarse-textured soils.
Xiphinema index transmits grapevine fanleaf virus; first record of virus transmission by a nematode was observed by Hewitt, Raski and Goheen, 1958 - opened whole new field of study. Actually, Allen (Ph.D. thesis) had earlier tried to implicate nematodes without success in transmission of lettuce big vein - vector is soil fungus, Olpidium.
Vector of Grapevine Fanleaf Virus
Symptoms:
The virus is intimately associated with esophageal lining, acquired in 5-15 minutes of feeding, and persists for up to 9 months when nematode is not feeding. Virus is lost at molt and does not pass through egg stage.
Grapevine fanleaf virus causes reduced vigor, lack of fruit set, reduced yield.
Remove virus infected vines and initiate 5-year rotation for roots to die. Treat with 1,3-Dichloropropene (1,3-D) at high rates and deep levels (250 gpa). Nemacur, etc. postplant will reduce nematode level, but not eliminate it, and virus will still be present DBCP had a similar effect..
Rootstocks have been used in viticulture to protect against soil pests for 150 years (Reisch et al., 2012). Hybrids of Vitis vinifera and Muscadinia rotundifolia were developed by Olmo at UC Davis in late 1940s: O39-16 and O43-43. They were patented by Lider and Goheen in 1980s as conferring grapevine fanleaf resistance based on a rather limited series of field trials. In a rootstock trial at Rutherford, the scion Cabernet Sauvignon was high yielding and vigorous on O39-16 and O43-43 in relation to other rootstocks in the trial. However, scions on these rootstocks have tested ELISA positive for fanleaf at this and other locations since the late 1980s, even though leaf symptoms are not yet obvious. Further, O43-43 has been determined to be susceptible to phylloxera and is no longer recommended. Note: There may be considerable genetic variability among X. index populations. Rootstock trials in California, Israel, Australia and South Africa show conflicting results. In part this may be due to difficulties in accurate identification of the rootstock species or accessions. Dr. Andrew Walker and students (Viticulture and Enology, UC Davis) conducted (1994) a survey of genetic diversity in X. index populations.
Until the 1970s, the goal of finding a rootstock with resistance to X. index was thought to be unattainable. Hybrids of V. vinifera and Muscadinia rotundifolia Small that had been developed by Olmo in the late 1940s appeared in mid-1980s field screens to survive GFLV (Lider et al., 1988). They were reported as resistant and tolerant of X. index. One of these rootstocks 'VR-O39-16' is a very poor host for X. index and several other ectoparasites, although it is a good host for most endoparasitic nematodes (McKenry and Kretsch, 1994). At several locations in California, scions on this rootstock have become infected by the virus, but at a much slower rate than those on susceptible rootstocks. It appears that 'VR-O39-16' slows the rate of virus infection and reduces the damage, and reduces population levels of X. index in infested vineyards. 'Freedom' rootstock is also a poor host for X. index and is resistant to most common endoparasites. Scions on 'Freedom' will, however, succumb to GFLV in the field more rapidly than those on 'VR-O39-16'. The use of 'Freedom' rootstock or 'VR-O39-16' rootstocks in replanting a virus-infested vineyard site must be coupled with pre-plant treatments which will kill old virus-infected roots. The V. riparia Michx. x. V. rupestris rootstock, 'Schwarzmann', also possesses a high level of resistance to X. index (McKenry and Kretsch, 1994).
Rootstocks with combined resistance to X. index and to virulent pathotypes of Meloidogyne spp. (UCD GRN1, UCD GRN2, UCD GRN3, UCD GRN4, and UCD GRN5) were released to nurseries in California in 2009 and were available commercially in 2011. Prior to release, selection of the new rootstocks and testing of the durability of their broad resistance occurred over a period of 15 years (Ferris et al., 2012)
Host Plant Resistance, Non-hosts and Crop Rotation alternatives:
The natural antagonists of X. index, have not been studied. however, population levels seldom rise to high levels in many field sites. That may be indicative of the activity of biological antagonist as well as unfavorable physical conditions.
Management Guidelines - Pre-plant
AVOIDANCE
The wide host ranges of ectoparasitic nematodes make pest avoidance difficult to achieve. One exception is X. index and GFLV. The use of virus-free and nematode-free rootings provide the best method of controlling the complex and certification programs have reduced its dissemination through vegetative propagation (Esmenjaud et al., 1993). The wide host range and current abundance of X. americanum in U.S. soils is an indication that quarantine of the nematode is not possible, but the certified virus-free nursery stock program continues to have merit. On-farm nurseries should be maintained nematode-free and located away from old vineyard sites.
CULTURAL CONTROLS
By their nature, ectoparasitic nematodes can survive in soil or persist on weed hosts for long periods of time. Fallowing will only reduce populations, not remove them. The persistence of old grape roots in replant sites is a problem. New grape roots tend to follow the channels in the soil left by old roots. GFLV can be moved from these reservoirs to healthy planting stock through exploratory feeding probes by X. index. It may be difficult to develop 'immune' rootstocks on which the nematode cannot reproduce and never even attempts to feed. Replanting of a virus-infected vineyard with a rootstock that is resistant to X. index within 6-7 years after removing a virus infected vineyard will not be successful in the long term unless the nematode does not attempt to feed. The site will probably not be safe until 1-2 years after the last old root is dead. Such time intervals may not be economically feasible. Resistance to GFLV will be an important solution to the problem (Esmenjaud, 1986).
Old roots are most easily killed before the old vine is removed by using systemic herbicides. Once the vine is removed the only reliable method of killing roots in the top 2 m of soil is by soil fumigation. Soil ripping to that depth on 30 or 60 cm centers may increase the speed of root death, but this tactic has not been field tested. Heat is a useful root killing agent but we are unable to deliver the required heat using current methods. Soil flooding for months will not kill old grape roots or nematodes. Selected rotation crops will reduce population levels of certain nematode species over time. This will have little value for endoparasites but should be tested with ectoparasites.
Certain Sorghum bicolor L. x Sorghum sudanense Stapf hybrids grown in summer have negative effects on ectoparasites but the number of years required to eliminate a nematode problem is unknown. Addition of manure, composts and ammonia does not kill old roots, but there are various reports of the benefits of ammonia as a nematode control agent (Mojtahedi and Lownsbery, 1976).
CHEMICAL CONTROLS
Soil fumigation with methyl bromide or 1,3-dichloropropene is effective for killing old roots 1.5 and 2 m deep in soil. Such treatments can also give 99.9% reduction of all nematode species in the top 1.5 to 2 m of soil when properly applied. Two to six years after such treatments, the nematodes do return unless resistant rootstocks are replanted. The use of high rates of 1,3-dichloropropene was halted in the state of California in spring 1990. Methyl bromide use is to be phased out by the year 2000, and adequate replacement fumigants must be environmentally safe. Environmental problems with nematicides have also occurred elsewhere (Rupp, 1990).
The future of all general biocides as a fumigation replacement is unclear. An approach we have taken is the integration of "softer" root killing agents or tools coupled with "softer" soil treatments which reduce soil-dwelling population levels. The practice of removing a vineyard and replanting within one year is not a current option in vineyard management where nematode problems occur. There will be much discovery in this area in the decades ahead. One treatment strategy worthy of study involves use of root killing and soil cleansing treatments followed by one or more years of non-host crops.
Management Guidelines for Established Vineyards
Movement of contaminated grape harvesting equipment and tractors from a site infected with X. index should be avoided.
BIOLOGICAL CONTROL
Refer to the previous chapter on this subject however, the addition of biological agents to soil has been inconsistent and usually ineffective. It may be possible, however, with the advent of drip irrigation systems, to apply microbe-produced toxins directly to the soil in irrigation water. Zoosporic fungal parasites have been isolated from X. rivesi and X. americanum. Such fungi require free water for their movement through soil. Their occurrence suggests research to investigate water management protocols that will enhance fungal parasitism of dagger nematodes.
By reducing vine stress through more frequent irrigation the damage caused by nematodes can be reduced.
The use of grassy cover crops in vineyards infested with X. index should be studied. However, legume cover crops in vineyards should be monitored to assess population levels of Mesocriconema xenoplax, which may increase. Populations of X. americanum will also increase on most cover crop selections. The difficulty in choice of cover crops is that the host range of most ectoparasites is quite broad.
Nematode species with long bodies tend to be in shallow and non-disturbed sites of a vineyard, so placement of any treatment is important. Tillage and soil disturbance can reduce population levels short periods of time, but root surface area is also reduced.
Drip-irrigation-applied fertilizers that release ammonia may reduce population levels of ectoparasitic nematodes when applied repeatedly. Fifteen kg/ha of nitrogen in urea salt when re-applied three to five times at 30 to 45 day intervals can reduce population levels of most ectoparasites by half. More field testing of these strategies is necessary. Since grapevines do not have a high nitrogen requirement and some vineyards already have excess nitrogen, this technique should be tested and implemented with caution.
Organophosphate and carbamate nematicides are lethal to ectoparasitic nematodes when used as single treatments at high rates via drip irrigation. Treatments with currently-available commercial nematicides only reduce populations about 50% for 6 to 8 months after treatment. Multiple treatments with low rates of phenamiphos (1 kg/ha) are ineffective against ectoparasites. When multiple treatments are used against endoparasites for several years, population levels of ectoparasites such as X. americanum can be observed to increase above the non-treated population levels.
In the case of ectoparasites, the value of systemic nematicides will be minimal unless the toxicant is available during feeding or leaks out into the rhizosphere (Edwards, 1991).
Future Research
Except for X. index, ectoparasitic nematodes have not received the same research attention as endoparasites. Long-term experiments on the damage potential and management of ectoparasitic nematodes in vineyards are needed.
The apparent increase in incidence of X. index associated with reduced use of conventional soil fumigants is important and should be monitored. Other problems with ectoparasitic nematodes may similarly emerge. New pre-plant control strategies will be needed in the near future. Resistance and tolerance of rootstocks will continue to be important areas of study.
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