Rev 12/18/23
Length, female: 0.40-0.78 mm; length, male: 0.53-0.61 mm.
Female:
Annules retrorse (200 or fewer, usually 100-150) and visible at low magnification, with smooth or slightly rough posterior margins, especially towards tail. Anastomoses rare.
Head broad, first annule entire or emarginated laterally, sometimes deeply.
Submedian lobes well developed. Lip region conspicuous, elevated. Typically there are four distinct and well separated labial plates alternating with the submedian lobes, but this arrangement varies greatly, fusion between plates occurring or reduction in their size or number.
Vulva distinctly "open" with lips separated, anterior lip bearing two protruberances variable in form and visible only in ventral view. Vagina always sigmoid in lateral view.
Tail broadly rounded to more conoid; terminus generally a simple rounded or lobed button.
Male:
Juveniles: Posterior margins of annules crenated.
Reported median body size for this species (Length mm; width micrometers; weight micrograms) - Click:
Mesocriconema xenoplax was described as Criconemaodes xenoplax by Raski (1952) from soil around Vitis longii (rootstock 1613) to which Vitis vinefera cv Thompson Seedless was grafted, from a location near Fresno, California, USA.
Mesocriconema xenoplax has been reported from North and South America, Europe, Africa, India, Australia, and Japan.
This species is found in 38% of California prune orchards and in all of the four important prune-growing districts and throughout grape-growing regions.
This slow-moving nematode can build to its highest population levels in highly porous soils, including coarse sands and well-aggregated silt or clay soils.
Extraction of this nematode from soil is best accomplished with sugar flotation and centrifugation techniques (Jenkins, 1964). It is poorly extracted by methods that are useful for other Tylenchida. Consequently, until development of appropriate extraction techniques, ring nematode populations were often not detected.
Populations of Mesocriconema xenoplax can be expected in any sandy soil where a woody perennial has grown. However it has a wide host range that includes certain grasses and legumes. In contrast, other ring nematode species of similar size may feed only on grasses and not on the roots of woody perennials.
All Prunus species, including peach, almond, apricot, cherry, and plum; also lettuce, carnation, and pine.
Type host - Thompson Seedless grapevine on 1613 rootstock from Fresno County (Raski 1952).
Vitis vinifera cultivars, including 'Grenache' and 'Flame Seedless', are among its favored hosts. It is also hosted well by '3309C' (V. riparia x V. rupestris) (McKenry and Kretsch, 1994).
Ecophysiological Parameters:
The female produces 3-5 eggs/day. The first molt occurs inside the egg. Observations of egg maturity in 10-12 days at 20-22ï ¿ ½C (Seshadri, 1965) and hatch in 15 days at 20 C (Thomas, 1959) are in reasonable accordance with an estimate of 154 + 5 degree days (base 9 C) measured for a South Carolina population (Wescott and Burrows, 1991).
All stages feed and the life cycle is complete in 24 to 30 days.
This nematode has the ability to increase rapidly. It is adversely affected by low moisture and high soil temperature. Highest population levels are observed in upper soil in fall-winter with lower populations near the soil surface in summer. Most of these observations emerge from studies of the nematode on
Population increases at a greater rate on Nemaguard than on Lovell peach rootstock - (Livingston, California - Ferris et al., 2004).
Nematode is adversely affected by low moisture and high soil temperatures. Highest population observed in upper soil in fall and winter; lower population observed near surface of soil in summer.
Damage by this nematode has been most extensively studied on Prunus spp. and Juglans spp.. It causes pruning and necrosis of fine feeder roots, especially on young plants, but also feeds on older parts of the root.
It predisposes some Prunus spp. and Malus spp. to infection by Pseudomonas syringae pv syringae, resulting in tree mortality due bacterial canker (BC) and to winter frost damage. The combined effect of the nematode, bacterium and cold injury result in enhanced tree mortality in the southeastern U.S., a condition known as peach tree short-life (PTSL).
Feeding by the nematode results in:
The mechanisms through which M. xenoplax and other stresses predispose Prunus trees to BC and PTSL remain uncertain; however, evidence is accumulating. Speculated mechanisms include:
It is important to note that Pseudomonas syringae (the bacterium) is not in the soil and does not interact directly with the nematode. It apparently invades the above-ground parts of the tree at pruning wounds and leaf scars.
In grape, the nematodes feed on young roots, usually near the root tip, often in large numbers. The feeding results in local darkening and destruction of the root tissue, and leads to breakdown of the roots. The root systems of plants attacked by these nematodes are visibly smaller than those of nematode-free plants. Significant reductions in top growth can occure (Klingler, 1975).
Mae Nofsinger (former museum scientist, UC Davis) inspects the damage.
Symptoms of bacterial canker include death of shoots and limbs; vascular discoloration, gummosis, "sour sap," and reduced flower production. [Note Livingston experiment - no effects of high nematode population levels on photosynthesis, some vigor decrease, root decrease, - McKenry observations on pruning of feeder roots].
Plant death does not occur in grapevines, but reductions in vine growth are observed in sandy areas. On Concord grape (V. labrusca) in Washington, M. xenoplax stunts vines and causes stunting and necrosis of feeder roots (Santo and Bolander, 1977). Effects on 'Thompson Seedless' have been less notable (Raski and Radewald, 1958) but 'Grenache' in sandy areas appears damaged (McKenry, unpublished data).
Nematicides:
Nematode control can be achieved with nematicides (Nemacur - phenamiphos).
In Prunus spp., preplant and postplant nematicides have been important for management of BC and PTSL. Historically, the postplant nematicide of choice was 1,2-dibromo-3-chloropropane (DBCP) but, following withdrawal of that chemical from the market, there have been some successes with phenamiphos (Nemacur 3). In North Carolina, annual fall applications of phenamiphos reduced ring nematode populations and tree death due to P. syringae (Ritchie, 1984; Ritchie, 1989; Ritchie and Clayton, 1981). Nematicides have been more effective in reducing PTSL in orchards on Nemaguard rootstock than on the more tolerant Lovell rootstock (Zehr et al., 1976).
Ferris et al (2004) found that spring and fall nematicide applications to orchards on both Lovell and Nemaguard rootstock reduced the the nematode degree-days experienced by the trees by about 70%. Spring and summer applications of phenamiphos appeared to be most effective in moderating the rate of increase of M. xenoplax dosage. That conclusion agrees with observations from many other orchards (M. V. McKenry, unpublished) and is consistent with reports from the southeast U.S. that applications of phenamiphos in the spring and fall are more effective in reducing ring nematode populations than applications in the fall alone (Ritchie, 1989).
Resistance:
Host plant resistance has been difficult to find for this nematode in grapevines. Small plot work with one population of M. xenoplax indicated that 'Freedom', 'Harmony' and 'Schwarzmann' rootstocks population levels only half as high as those on V. vinifera cultivars; however, in a field trial in northern California, 'Freedom' was among the best hosts while 'Harmony' was not.
Host Plant Resistance, Non-hosts and Crop Rotation alternatives:
Orchard Management:
Attention to soil fertility and irrigation to reduce additional stresses on plants. Careful pruning to avoid crop overload stress and prolonged pruning wounds. Avoidance of root injury during cultivation. Drip irrigation may help to relieve or remove plant stress, thus increasing plant tolerance of nematode feeding.
The following management plan is a modification of a 10-point plan used in peach production in the southeastern US. The objectives of such plans are to integrate stress relief and nematode management components.
Before planting, it is necessary to evaluate the crop history of the site to determine whether hosts of nematode species of concern (including Meloidogyne spp., Mesocriconema xenoplax, Pratylenchus vulnus and Xiphinema americanum) have been grown there. Since many of these nematodes have a wide host range, it is unlikely that all can be avoided, but it may be possible to avoid several. It would also be important to know of any growth patterns in previous crops that indicate distribution characteristics of the nematodes or of their damage. Additional clues to nematode distribution or potential damage will be provided by assessing soil texture uniformity and patterns in the field, soil pH, profile characteristics, and nutrient status.
To the greatest extent possible, roots remaining from a previous crop should be removed from the soil to reduce nematode and virus reservoirs. The soil should be prepared to reduce stress on the plants to a minimum, which may enhance their ability to tolerate any subsequent nematode stress that develops. This process may include subsoiling or deep plowing to disrupt restrictive layers. Based upon the site evaluations mentioned above, an irrigation system should be designed to ensure uniformity and control of delivery, since minimizing moisture stress on trees may affect their tolerance of nematode stress. From information on the susceptibility of trees to diseases, such as bacterial canker in the presence of ring nematodes under certain environmental conditions, it may be necessary to prepare the site through several of the following procedures: adjust nutrient and micronutrient status based on soil analysis; raise the soil pH above 6.5 throughout the soil profile; increase levels of organic matter in sandy soils; and improve soil structure.
If populations of potentially damaging plant-parasitic nematodes are found during the site evaluation process, it may be necessary to select another site. Alternatively, combinations of tactics will be assembled to reduce population levels of plant parasites, preferably while conserving beneficial species. One tactic, often costly, is to allow the passage of time with the field in a fallow condition since nematode populations decline in the absence of food. Another approach is to implement a rotations system before planting the orchard and to avoid planting after crops which may have supported damaging species. In addition, antagonistic plants and residues, may be used so that nematode populations are suppressed by "allelochemicals." Several cultural approaches provide some degree of control for certain nematodes: populations may be suppressed in anoxic conditions created by flooding the field, and physical disturbance of soil by repeated cultivation suppresses some nematode species. Where beneficial organisms are reduced by these approaches, it may be necessary to re-introduce or augment them prior to planting.
In perennial crops, it is sometimes possible to select a rootstock that provides desired characteristics without requiring genetic manipulation of the scion cultivar. Currently, rootstocks such as Nemaguard are available for stone fruits that confer resistance to root-knot nematodes; however, these rootstocks appear particularly sensitive to the ring nematode and associated bacterial canker complex, so they should be selected and used carefully based upon evaluation of the site. Rootstocks are not yet available for resistance to the ring or lesion nematodes, although some sources show promise (Culver et al., 1989; Ramming, 1988). Another approach would be to select planting stock that is tolerant of the presence of certain nematodes. For example, the rootstock "Lovell" appears to confer some tolerance of the ring nematode and bacterial canker complex, although it is susceptible to root-knot nematodes. Often, tolerance is determined by prolonged observational experience; it is difficult to determine experimentally, except in very long and costly trials. As technology advances, it may become possible to create transgenic rootstocks in which genes for resistance to nematodes are derived from other plant species. An example of potential in this area is with the Mi gene from tomatoes, although such transgenic plants have not yet been successfully created. Also interesting is the possibility of introducing a microtoxin gene into a rootstock.
Having established the orchard, using strategies developed appropriate for the site, it will be necessary to employ site-specific orchard management practices that create a superior environment for the trees and, where possible, an inferior environment for plant-parasitic nematode species. Orchard management should attempt to maintain adequate soil moisture for the trees and avoid extreme fluctuations in soil moisture. Avoidance of major and minor nutrient stress on the trees appears important in boosting their tolerance of nematode stress and associated problems; foliar and tissue analyses may be used as a basis for nutrient management. Management of the soil environment to the advantage of the trees, to the advantage of nematode antagonists, and to the disadvantage of nematodes, may include monitoring and adjustment of soil pH and additions of organic matter to the soil. Crop load should be adjusted in accordance with the age and vigor of the trees by appropriate crop thinning and pruning practices. Also, there is considerable evidence to suggest that pruning wounds heal slowly in trees infected with ring nematodes, thereby exposing the trees to bacterial canker infection. Selection of pruning times so that wounds will heal rapidly may be important in some situations.
In the established orchard, it will be important to monitor and manage the status of the plant-parasitic nematode community. Depending on the expected severity or risk of a nematode problem at the site, it will be necessary to assess nematode population levels and spatial pattern, and their probable impact on tree vigor and productivity. As our understanding of rhizosphere biology improves, it may be very important to create and maintain an unfavorable biotic environment for plant-parasitic nematodes by conserving and augmenting biological antagonists in the soil that exploit nematodes as a food source, such as fungi and bacteria, or other nematodes of lower pathogenicity that compete with them, or organisms such as rhizosphere bacteria that have an antibiotic effect. At present, we only have formative ideas about how such an antagonist-conducive environment might be created.
Another important emerging area in management of nematode species in orchards is the use of antagonistic cover crops and their residues. Plants such as marigolds, and other members of the Compositae, are known to release compounds into the soil that are toxic to certain nematodes. Studies by McKenry (1988) suggest that the effect can be enhanced by incorporating the above-ground parts of these plants into the soil and following with an irrigation to move the compounds to the nematode target. However, in many cases such plants are not sufficiently competitive to be grown as cover crops in orchards. Other plant species show promise but, again, further research is necessary.
Finally, as the development of biorational pesticides continues, it is important that we encourage the development of materials that may allow effective nematode management in valuable perennial plantings while constituting minimal environmental and health hazards. With any such materials, it will be advantageous to maintain a balanced and buffered soil environment. Monitoring for, and reintroduction of, beneficial organisms may also be crucial.
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