Aspects of Biology and Development of Xiphinema americanum and Related Species

Identification of Xiphinema americanum-group nematodes is based on relatively subtle morphological and morphometric differences, many of which overlap. The significance and importance of these separations cannot be assessed without a basic understanding of the biological differences between species. Currently, information is accumulating on Xiphinema biology, development, and genetics that will help to confirm or refute the current systematics of species in this group. Recently, it was demonstrated that Xiphinema species pass through either three or four juvenile stages before becoming adults. This new and fundamental information divides the genus and the X. americanum group into subgroups based on their developmental evolution and provides new insight into the taxonomy and systematic positions of the species.     

Morphological Variation among 23 Xiphinema americanum Populations

Morphometrics of 23 United States populations of Xiphinema americanum sensu lato, sharing the characteristics of an offset lip region and conoid tail, were examined and analyzed statistically by canonical discriminant analysis (CDA). Specimens were collected from Arkansas, Georgia, Tennessee, Mississippi, Florida, Oklahoma, California, and North Dakota. Eleven measurements and body ratios obtained from female specimens were used in the analysis. Xiphinema americanum, X. bricolensis, X. californicum, X. citricolum, X, intermedium, X. tarjanense, and X. thornei, and one undescribed species were identified among the 23 populations. Three groups -- X. americanum-group, X. californicum-group, and X. intermedium-group (X. intermedium and X. tarjanense) -- were formed and four populations belonging to four different species were separated consistently from these groups in CDA scatterplots of the 23 populations. Composition of the groups was somewhat related to the geographical origins of the populations in the groups. A population from California had morphometrics intermediate between X. americanum and X. californicum. Separation between the X. americanum-group and X. californicum-group in the CDA scatterplots was not as distinct as that between them and the X. intermedium-group or between any of the three groups and the four single outlying populations.     

Morphometric Evidence for Three Juvenile Stages in Some Species of Xiphinema americanum sensu lato

One to two hundred nematodes from each of seven Xiphinema americanum-group populations were measured to determine the range of stylet and body lengths for juveniles and adults. First-stage juveniles were identified by the position of the replacement odontostyle (i.e., the tip of the replacement odontostyle overlapped the base of the odontophore). Nematodes were identified as second stage if the functional odontostyle was the same length as the replacement odontostyle of the first stage. Subsequent stages were similarly identified by establishing the range of corresponding replacement and functional odontostyle lengths. In all populations examined, this procedure created natural divisions that clearly grouped nematodes by stylet and body length. Presumably these groups identified all juvenile and adult stages. Populations of X. americanum, X. rivesi, and X. californicum from the United States had three juvenile stages, but a population of X. pachtaicum from Bulgaria had four juvenile stages.     

Vector Capability of Xiphinema americanum sensu lato in California

Seven field populations of Xiphinema americanum sensu lato from California''s major agronomic areas were tested for their ability to transmit two nepoviruses, including the prune brownline, peach yellow bud, and grapevine yellow vein strains of tomato ringspot virus and the bud blight strain of tobacco ringspot virus. Two field populations transmitted all isolates, one population transmitted all tomato ringspot virus isolates but failed to transmit bud blight strain of tobacco ringspot virus, and the remaining four populations failed to transmit any virus. Only one population, which transmitted all isolates, bad been associated with field spread of a nepovirus. As two California populations of Xiphinema americanum sensu lato were shown to have the ability to vector two different nepoviruses, a nematode taxonomy based on a parsimony of virus-vector relationship is not practical for these populations. Because two California populations of X. americanum were able to vector tobacco ringspot virus, commonly vectored by X. americanum in the eastern United States, these western populations cannot be differentiated from eastern populations by vector capability tests using tobacco ringspot virus.     

Morphological Comparisons Between Xiphinema rivesi Daimasso and X. americanum Cobb Populations from the Eastern United States

Though in the past Xiphinema americanum has been the most commonly reported dagger nematode in the eastern United States, our studies revealed the presence in Pennsvlvania of a previously unrecognized and unreported species related to X. americanum, Morphometric data and photomicrographs establish the identity of this form as X. rivesi and show expected variations in populations of this species from various locations. Similar data and illustrations are given for X. americanum populations from Pennsylvania and other areas, showing variations and relationships. Xiphinema rivesi is widely distributed in the fruit producing area of south-central Pennsylvania and is also reported herein from raspberry in Vermont and apple in Maryland and New York. This species is frequently found in fruit growing areas of Pennsylvania associated with tomato ringspot virus-induced diseases and is also found associated with corn, bluegrass sod, and alfalfa.     

Morphological Variation in Xiphinema spp. from New York Orchards

Xiphinema specimens were collected from orchards in southeastern, northeastern, and western New York. Total length, distance of vulva from anterior end, spear length (odontostyle plus odontophore), body diameter at vulva, tail length, anal body diameter, and length and diameter of hyaline tail tip were measured on fixed, glycerol-infiltrated adult females. Most specimens were identified as X. americanum or X. rivesi, but one western New York population was identified as X. californicum (a new record for New York). Multivariate analyses indicated that, with one exception, western New York populations of both X. americanum and X. rivesi were smaller and slimmer than their eastern counterparts. Regional differences were generally larger than differences attributed to host species.     

Seasonal Population Fluctuation of Xiphinema americanum and X. rivesi in New York and Pennsylvania Orchards

The population fluctuation and composition of Xiphinema americanum (sensu stricto) and X. rivesi were studied in a New York apple orchard (only X. americanum present), a Pennsylvania apple orchard (both X. americanum and X. rivesi present), and a Pennsylvania peach orchard (X. americanum, X. rivesi, and X. californicum present). Few clear trends in population fluctuation or composition were observed. The adult female was the predominant stage in most sample periods, and the reproductive period was limited to late spring and early summer. Only a few of the females at any sample period were gravid. All stages were present throughout the year, and all stages overwintered. Eggs in soil were not monitored. In the Pennsylvania apple orchard, X. americanum and X. rivesi were easily separated by morphological characteristics; however, the two species did not display differences in population structure or composition. The predominance of adults, the relatively low reproductive rates, and the association of these species with stable habitats suggest that the life strategies of X. americanum and X. rivesi are K-selected as opposed to r-selected.     

Transmission of Nepoviruses by Xiphinema americanum-group Nematodes

The transmission of North American nepoviruses by putative species belonging to the Xiphinema americanum-group is reviewed. Xiphinema americanum sensu stricto, X. californicum, and X. rivesi each transmit cherry rasp leaf (CRLV), tobacco ringspot (TobRSV), and tomato ringspot nepovirus (TomRSV), and X. bricolensis is a vector of TomRSV. The apparent lack of specificity in the transmission of North American nepoviruses by X. americanum-group species markedly contrasts with the specific associations between European nepoviruses and their vector nematode species. Two complementary projects are described examining the taxonomic identity of putative species in the X. americanum-group, their morphological and genetic relationships, their ontogeny, and their ability to transmit viruses.     

Effect of Three Plant Species on Population Densities of Xiphinema americanum and X. rivesi

A taxonomic revision of the Xiphinema americanum species complex has necessitated a reexamination of the host range of species in the complex before recommendations can be made with confidence on the likelihood that specific crops will be damaged. Toward this end, populations of X. americanum and X. rivesi collected from apple orchards in eastern and western New York state were evaluated after 3 months in pots planted with cucumber, apple, or dandelion seedlings. Eastern and western New York populations of both nematode species declined on cucumber but increased to similar final densities on apple and dandelion.     

Biotin-Avidin ELISA Detection of Grapevine Fanleaf Virus in the Vector Nematode Xiphinema index

The value of biotin-avidin (B-A) ELISA for the detection of grapevine fanleaf virus (GFLV) in Xiphinema was estimated with field populations and greenhouse subpopulations. Samples consisted of increasing numbers of adults ranging from 1 to 64 in multiples of two. Tests with virus-free X. index populations reared on grapevine and fig plants as negative controls did not reveal a noticeable effect of the host plant. ELISA absorbances of virus-free X. index samples were greater than corresponding absorbances of X. pachtaicum samples. Differences occurred between two X. index field populations from GFLV-infected grapevines in Champagne and Languedoc. In most tests, 1-, 2-, 4-, and 8-nematode samples of virus-free and virus-infected populations, respectively, could not be separated. Consequently, B-A ELISA was not a reliable method for GFLV detection in samples of less than 10 X. index adults, but comparison of the absorbances obtained with increasing numbers may allow differentiation of the viral infectious potential of several populations.     

Distribution of Xiphinema americanum and Related Species in North America

All species of the Xiphinema americanum-group and their synonyms are listed. The North American species reported are listed by state or province. Among these species, X. rivesi has the most widely reported distribution. Six species (X. diffusum, X. floridae, X. laevistriatum, X. luci, X. shell, and X. tarjanense) have been reported from only Florida. The reports of X. pachtaicum, X. sheri, and X. luci did not include morphometrics and need to be confirmed; X. brevicolle from California was identified before Lamberti and Bleve-Zacheo described 15 new species in 1979 and similarly needs to be confirmed. Because of the proliferation of species in this group, reports of X. americanum (sensu stricto) before 1979 are questionable. Extraction techniques for longidorids are discussed.     

Cryofracturing for Scanning Electron Microscope Observations of Internal Structures of Nematodes

Nematodes were prepared for scanning electron microscopy by cryofracturing in ethanol and then by critical-point drying in carbon dioxide. Cross sections of Caenorhabditis briggsae and Xiphinema americanum showed the arrangement of the intestine, ovaries, muscle cells, and some layers of the cuticle. The technique is complementary to transmission electron microscopy and facilitates the interpretation of results from thin sections.     

Factors Affecting Population Trends of Plant-Parasitic Nematodes on Rangeland Grasses

The effects of environmental conditions on population trends of plant-parasitic nematodes were studied in experimental plots of five wheatgrasses in the western Utah desert. In a 3-year (1984-86) field study, soil water and temperature affected the population trends of the ectoparasites, Tylenchorhynchus acutoides and Xiphinema americanum, and the migratory endoparasite, Pratylenchus neglectus, on Fairway crested wheatgrass, Agropyron cristatum; ''Hycrest'' crested wheatgrass, A. cristatum X A. desertorura; ''Rosana'' western wheatgrass, Pascopyrum smithii; ''Oahe'' intermediate wheatgrass, Thinopyrum intermedium; and RS-1 hybrid (Elytrigia repens X Pseudoroegneria spicata). The largest soil populations of these nematode species were collected in 1984 under good plant-growth conditions. A reduction in nematode populations occurred in 1985 and 1986, possibly because of low soil-water conditions. There was a positive relationship between high soil water and maximum population densities of T. acutoides in the spring and fall of 1984, and between low soil water and minimum population densities of the nematode in 1985 and 1986. Pratylenchus neglectus populations were affected by soil water, although to a lesser degree than the ectoparasitic nematodes. Population densities of the three nematode species were significantly lower in the drier years of 1985 and 1986 than in 1984. Nematode populations were greater at the lower soil depths in the fall than in the spring or summer.     

Plant-parasitic Nematodes in Loess Toposequences Planted with Corn

Populations of plant-parasitic nematodes in an Iowa cornfield were studied along north- and west-facing toposequences. Samples were collected monthly during the growing season. The greatest biomass for Xiphinema americanum occurred at the footslope on the north face. Paratylenchus microdorus had its greatest biomass at the summit position, generally more in the west- than in the north-facing slope. Pratylenchus spp. in the roots peaked at the toeslope in the north-facing slope, but at the footslope in the west-facing slope. Helicotylenchus pseudorobustus peaked at the backslope and the toeslope along the north- and west-facing slopes, respectively. Diversity, as computed for each plot by the Shannon-Weiner diversity index, was highest at the backslope in both toposequences.     

Seasonal Fluctuations in the Spatial Distribution of Nematode Populations in a California Vineyard

Distribution of Xiphinema americanum and four Meloidogyne spp, was studied in a vineyard over a 13-mo. period. The X. arnericanum population was concd in the upper 60-cm of undisturbed soil in the vine row, whereas the Meloidogyne species were distributed both in and between rows and to greater depths, similar to the distribution of the root system. Samples for assessment of X. americanum densities had least variation when taken in the vine row from the upper 60-cm of soil. Sampling error is reduced in Meloidogyne populations by sampling within 40 cm of the vine both within and/or between rows.     

Age Structure and Community Diversity of Nematodes Associated with Maize in Iowa Sandy Soils

Age structure of nematode populations around maize growing in sandy soils in Iowa was studied at soil depths of 0-15and 15-30 cm for 2 years. Numbers of Longidorus breviannulatus were generally greater at 0-15 cm than at 15-30 cm deep until mid to late season. The decline in numbers of females as the season progressed indicates that fecundity slowed and is evidence of only one generation per year. Peak populations of Pratylenchus scribneri and Xiphinema americanum occurred in late August or early September. Adults of Hoplolaimus galeatus were few in the roots but common in the soil, indicating that fertilization occurred mostly in the soil. Numbers of P. scribneri were generally greater at the lower depth, especially late in the season. Community diversity (H'') was less when nematode biomass was used instead of numbers. Numbers of H. galeatus did not decline over the winter. Numbers of L. breviannulatus, P. scribneri, and X. americanum declined significantly over the winter, but not between spring cultivation and planting.     

Transmission of Tomato Ringspot Virus by Xiphinema americanum and X. rivesi from New York Apple Orchards

Populations of Xiphinema americanum and X. rivesi were collected from apple orchards in eastern and western New York and tested in the laboratory for ability to transmit tomato ringspot virus (TmRSV) to cucumber and dandelion. Populations varied in the frequency with which they transmitted TmRSV, but this variation did not correspond to variation in disease prevalence in the orchard. The lower prevalence of TmRSV-incited disease in apple trees in western New York cannot be attributed to inability of the local Xiphinema spp. to transmit TmRSV.     

Population Densities of Five Migratory Endoparasitic Nematodes in Carrot Disk Cultures

Numbers of nematodes recovered per culture varied greatly among five species cultured on carrot disks. Radopholus similis and Pratylenchus vulnus showed the highest population densities, with 23,400-fold and 16,600-fold increases, respectively, in 90 days. Final populations of P. thornei and Zygotytenchus guevarai were similar but lower than those of R. similis and P. vulnus. The population of P. neglectus increased 74 times. Species with the greatest reproduction in this study reproduce sexually.     

Morphometric and Serologic Comparisons of a Number of Populations of Cyst Nematodes

Thirty-five populations of Heterodera glycines and populations of 15 other Heterodera, Globodera, and Punctodera species were studied morphometrically and some were compared serologically. There was a wide range of each measurement within each nematode population. Except for one soybean cyst nematode population from Indiana, which was a tetraploid and considerably larger than the others, morphometric measurements overlapped. In a discriminant function comparison most of the populations were closely grouped but at least three were rather distinctly separated. Morphometrically H. fici, H. cruciferae, H. schachtii, and H. trifolii were closely associated with H. glycines. Serology indicated a close relationship between H. glycines, H. lespedezae, H. trifolii, H. schachtii, and the Heterodera sp. from Rumex, while H. betulae appeared to be more distantly related.     

Effect of Entomopathogenic Nematodes on Mesocriconema xenoplax Populations in Peach and Pecan

The effect of Steinernema riobrave and Heterorhabditis bacteriophora on population density of Mesocriconema xenoplax in peach was studied in the greenhouse. Twenty-one days after adding 112 M. xenoplax adults and juveniles/1,500 cm³ soil to the soil surface of each pot, 50 infective juveniles/cm² soil surface of either S. riobrave or H. bacteriophora were applied. Another entomopathogenic nematode application of the same density was administered 3 months later. The experiment was repeated once. Mesocriconema xenoplax populations were not suppressed (P ≤ 0.05) in the presence of either S. riobrave or H. bacteriophora 180 days following ring nematode inoculation. On pecan, 200 S. riobrave infective-stage juveniles/cm² were applied to the soil surface of 2-year-old established M. xenoplax populations in field microplots. Additional applications of S. riobrave were administered 2 and 4 months later. This study was terminated 150 days following the initial application of S. riobrave. Populations of M. xenoplax were not suppressed in the presence of S. riobrave.