The seed gall nematode, Anguina agrostis, feeds and reproduces within the developing ovaries of bentgrass seeds and overwinters in seed galls as anhydrobiotic juveniles. These dormant juveniles can survive within the seed gall for many years. In this dehydrated state, they are more tolerant to extreme environmental conditions than are their hydrated counterparts. Nematodes in seed galls were exposed to various high temperatures (80 to 160°C) for time intervals of 5 to 30 min. Survival decreased as time and temperature increased. Remarkably, these nematodes survived exposure to 155°C for 5 min, higher than that recorded for any other metazoan. In contrast, seed galls that had been stored at room temperature and humidity for 5 yr also survived exposure to extreme temperatures; however, their survival rates were not as high as those for freshly collected galls. Juveniles within the seed gall were coiled and grouped together conforming to the shape of the seed gall. The gross morphology of the cuticle of the juveniles was very smooth and relatively undistorted by the shrinkage from the loss water from their body tissues. Wherever the nematodes were cut with a razor blade, a small amount of their contents oozed out of the opening and coalesced with that of other nearby specimens and appeared gel-like. Elucidation of the mechanisms that enable these nematodes to remain viable after exposure to extreme heat remains a mystery. Understanding the changes that occur in these nematodes as they rehydrate and return to life from an ametabolic state may have major impacts on the life sciences, including insights into the answer of the age-old question: “What is life?”
Agrostis stolonifera; ametabolic; Anguina agrostis; anhydrobiosis; desiccation; extremist; feeding site; host-parasitic relationship; longevity; survival; thermal death point; morphology; thermophilic
Previously we showed in laboratory studies that the fungivorus nematode, Aphelenchoides hylurgi, was attracted to and fed upon the chestnut blight fungus, Cryphonectria parasitica, from American chestnut bark cankers and was a carrier of biocontrol, white hypovirulent C. parasitica strains. In the present field study, we recovered Aphelenchoides spp. in almost all (97.0 %) of 133 blight canker tissue assays (three 5-g samples each) from four eastern states. High mean population densities (227 to 474 nematodes per 5 g tissue) of Aphelenchoides spp. were recovered from cankers in Virginia, West Virginia, and Tennessee but not from New Hampshire (mean = 75 nematodes per 5 g tissue). Overall, most canker assays yielded population densities less than 200 nematodes per 5 g tissue. All of 12 very small or young cankers yielded a few to many Aphelenchoides spp. Regression analysis indicated greatest recovery of Aphelenchoides spp. occurred in the month of May (r = 0.94). The results indicate that Aphelenchoides spp. appear to be widespread in blight cankers on American chestnut trees and could play a role in biocontrol of chestnut blight.
biocontrol; chestnut blight; fungivorus nematodes; hypovirulence
Multiple images of a whole nematode specimen were taken with a high power oil-immersion objective lens and joined together to form one high-resolution megapixel, mosaic photomicrograph of the entire specimen, with the use of a relatively new mounting technique made with a 4% water agar pad. The agar pad kept the specimen nearly level and lateral, and when amended with 10 mM sodium azide, this mounting technique gradually paralyzed the nematode in a natural pose to enable production of sharp, clear images. The individual photographs were joined together and merged into one very large, seamless image. These montaged images will be useful for teaching because the student has access to a virtual specimen that is mounted in the correct orientation, imaged with a research grade microscope, and preserved in a narcotized, living condition. Such specimen images can also serve as representatives of type and voucher specimens without the deterioration typical of real types. The files can be copied and viewed with a computer almost anywhere and at any time, rather than using a more cumbersome, limiting, and expensive microscope.
agar pad technique; digital micrographs; mounting nematodes; type specimens; voucher specimens
Individual nematodes were isolated from American chestnut blight-controlled cankers to determine if they were carriers of biocontrol (hypovirulent) isolates of the chestnut blight fungus, Cryphonectria parasitica. These hypovirulent isolates have a white fungal colony phenotype due to infection by the virus CHV1. Of 1,620 individual Aphelenchoides hylurgi isolated, 29.4% carried propagules of the blight fungus and 8.2% of these yielded white hypovirulent isolates. In attraction and movement tests in Petri plates, A. hylurgi moved 2 cm over 24 hr to mycelial discs of white hypovirulent C. parasitica and pigmented C. parasitica strains in nearly equal numbers. After 2 days of nematode movement to fungal colonies on agar in Petri plates and 21 days of nematode growth, large numbers of A. hylurgi were extracted from both white hypovirulent and pigmented C. parasitica strain colonies. Lower numbers of A. hylurgi were extracted from excised young American chestnut blight cankers that were inoculated with A. hylurgi and incubated for 22 days. A. hylurgi inoculated on the surface of an excised American chestnut canker moved within 24 hr to the small, spore-bearing C. parasitica reproductive structures (stromata) on the canker surface. The results indicate that A. hylurgi may play a role in the spread of hypovirulence on American chestnut trees.
biocontrol; chestnut blight; fungivorus nematodes; hypovirulence spread; white hypovirulent strains
Random amplified polymorphic DNA (RAPDs) were used to investigate the intraspecific variability among 19 geographic isolates of Globodera tabacum solanacearum from eight counties in Virginia and one county in North Carolina. Globodera tabacum tabacum, G. t. virginiae, and the Mexican cyst nematode (MCN) were included as outgroups. Six primers were used and 119 amplification products were observed. Each primer yielded reproducible differences in fragment patterns that differentiated the isolates and species. Hierarchical cluster analysis was performed to illustrate the relatedness among isolates and species. The average Jaccard's similarity index among isolates of G. t. solanacearum was 74%, possibly representing greater variation than that reported in the literature across different pathotypes of the potato cyst nematode, Globodera pallida, in studies where RAPD were also employed. The RAPD markers described here may be useful for the development of specific primers or probes that could improve the identification of TCN populations. Such improvements in the characterization of TCN genotypes would facilitate the effective deployment of existing and future resistant cultivars to control these economically important pests.
DNA fingerprinting; genetic variation; Globodera tabacum solanacearum; RAPD; tobacco cyst nematode; virulence
Meloidogyne haplanaria n. sp. is described and illustrated from specimens parasitizing peanut in Texas. The perineal pattern of the female is rounded to oval with a dorsal arch that is high and rounded except for striae near the vulva, which are low with rounded shoulders. The striae are distinctly forked in the lateral field, and punctations often occur as a small group near the tail tip and singly within the whole perineal pattern. The female stylet is 13-16 µm long and has broad, distinctly set-off knobs. The excretory pore opens 40-118 µm from the head, approximately halfway between the anterior end and the metacorpus. Males are 1.2-2.4 µm in length and have a high, wide head cap that slopes posteriorly. The labial disc and medial lips are partially fused to form an elongated lip structure. In some specimens the labial disk is distinctly separated from the lips by a groove. The stylet is 17-22 µm long and has wide knobs that are rounded and distinctly set off from the shaft. Mean second-stage juvenile length is 419 µm. The head region is not annulated, and the large labial disc and crescent-shaped medial lips are fused to form a dumbbell-shaped head cap. The stylet is 9-12 µm long and has rounded, posteriorly sloping knobs. The slender tail, 58-74 µm long, has a distinct, inflated rectum and a slightly rounded tip. The hyaline tail terminus is 11-16 µm long. The isozyme phenotypes for esterase and malic dehydrogenase do not correspond to any other recognized Meloidogyne species. Tomato and peanut are good hosts; corn and wheat are very poor hosts; and cotton, tobacco, pepper, and watermelon are nonhosts.
esterase phenotype; malate dehydrogenase phenotype; scanning electron microscopy; taxonomy
Penetration and development of juveniles of tobacco cyst nematode (Globodera tabacum solanacearum) on a resistant (NC567) and a susceptible (K326) cultivar of flue-cured tobacco (Nicotiana tabacum L.) were determined in root zone chamber experiments. More vermiform juveniles developed into a swollen shape at 22, 27, and 31 °C than at 17 °C. Development of flask-shaped nematodes appeared to be similar across tested temperatures (17, 22, 27, and 31 °C). General patterns of penetration and development of juveniles in both resistant and susceptible cultivars were similar under all temperatures tested. More vermiform, swollen, and flask-shaped nematodes were found in roots of K326 than in those of NC567. Development from swollen to flaskshaped nematodes appeared to be similar between the two cultivars, although more vermiform juveniles developed into swollen nematodes on K326 than on NC567. Differences in resistance between the two cultivars remained stable across tested temperatures.
development; flue-cured tobacco; Globodera tabacum solanacearum; Nicotiana tabacum; parasitism; resistance; screening; temperature; tobacco cyst nematode
The tobacco cyst nematode (Globodera tabacum solanacearum) continues to pose a serious threat to flue-cured tobacco production in Virginia and nearby states. Soils were sampled from five uninfested and two infested flue-cured tobacco-producing locations. Twenty-three edaphic factors were characterized to determine if any were correlated with G. t. solanacearum reproduction. Comparisons were also made between pasteurized and natural soils to determine if biological suppression of G. t. solanacearum reproduction might be occurring in currently uninfested areas. Differences in G. t. solanacearum reproduction were noted among the soils, but results were inconsistent across the three trials conducted in this study. Only soil pH correlated significantly with nematode reproduction, and then only in one of three trials. Globodera tabacum solanacearum reproduced with similar efficiency in natural and pasteurized soils.
cyst nematode; flue-cured tobacco; Globodera tabacum solanacearum; nematode; Nicotiana tabacum; soil edaphic factors; suppressive soil; tobacco cyst nematode
Tobacco cyst nematode (Globodera tabacum solanacearum) isolates were collected from 11 locations in Virginia, 3 in North Carolina, and 1 in Maryland. Isolates from each location were maintained and increased on flue-cured tobacco, Nicotiana tabacum cv K326. Plants of flue-cured tobacco cultivars K326 (susceptible) and NC567 (resistant) were each inoculated with 6,000 G. t. solanacearum eggs/plant. Tests were conducted over one (6 weeks) or two (14 weeks) generations of the nematode. Shoot and root weights and the number of nematodes present within a 1-g subsample of feeder roots were recorded at completion of the tests. Nematode counts were categorized by nematode life stage (vermiform, swollen, pyriform, and adult). Although significant differences in nematode development were detected among isolates, differences were not consistent across experiments. Results indicate similar virulence among G. t. solanacearum isolates on resistant and susceptible flue-cured tobacco cultivars. Therefore, plant breeders may effectively use a single G. t. solanacearum isolate when screening tobacco germplasm for resistance.
cyst nematode; flue-cured tobacco; Globodera tabacum solanacearum; nematode; Nicotiana tabacum; reproduction; resistance; tobacco cyst nematode
The effects of infection by tobacco cyst nematode (Globodera tabacum solanacearum) on growth of flue-cured tobacco cultivars NC 567 (resistant) and K 326 (susceptible) were evaluated in the field in 1993 and 1994. Infection by G. t. solanacearum suppressed number of leaves, plant height, and fresh weight of leaves and feeder roots. Correlations between weekly egg densities of G. t. solanacearum collected from soil and host growth during 11 weeks after transplanting (WAT) were often inconsistent between cultivars and years. However, consistent correlations were obtained between root weight and egg densities collected 9 WAT, as well as between leaf weight from susceptible K 326 and nematode egg densities 6 WAT. Leaf and feeder root weights were significantly correlated with the area under the curve for all nematodes per gram of feeder root for K 326 in 1993 and for both cultivars in 1994. Reduction in feeder root weight by G. t. solanacearum was similar for the resistant and susceptible cultivars. Reduction in fresh leaf weight by G. t. solanacearum was twice as great (P ≤ 0.07) for K 326 as for NC 567 in 1994. Incorporating nematode resistance into germplasm possessing improved yield and quality traits should produce cultivars more acceptable to growers.
cyst nematode; damage function; flue-cured tobacco; fosthiazate; Globodera tabacum solanacearum; multiple-point model; nematode; Nicotiana tabacum; plant disease loss; plant growth; resistance; tolerance
Meloidogyne petuniae n. sp. is described and illustrated from specimens parasitic on petunia (Petunia hybrida L.) in Brasilia, Brazil. The perineal pattern of the female is elongate to ovoid with a high, squarish arch and widely spaced, coarse striae. The stylet of the female is 12.9-16.5 µm long and has three small, rounded knobs that are distinctly set off from the shaft. Each knob is marked by a deep longitudinal indentation posteriorly and anteriorly. In SEM the base of the shaft appears to be divided into six distinct ridges. The excretory pore opens about 15.4-53.6 µm from the head end. Males are approximately 0.8-2.2 mm long. Most specimens have a high and narrow head cap, but in some the head cap is narrow and low. The stylet of the male is 21.1-26.0 µm long and has small, rounded knobs, set off from the shaft, but not indented as in the female. Second-stage juveniles are 353-464 µm long; the labial disc is fused with the medial lips to form a dumbbell-shaped head cap; the medial lips are indented posteriorly; and the head region is marked with one to two irregular annulations. The stylet is 9.2-10.8 µm long and has rounded, posteriorly sloping knobs. The tail is slender, approximately 46.4-57.2 µm long, and has a short hyaline terminus, 10.3-13.5 µm long. The somatic chromosome number is 2n = 41 and the esterase phenotype is VS1-S1, with S1 being a weak band. The malate dehydrogenase phenotype is N1, which is unique for this species. Petunia, tomato, tobacco, pea, and bean are good hosts; pepper, watermelon, and sweet corn are poor hosts; and peanut, cotton, and soybean are non-hosts. Galls produced by this species are smaller on petunia than on tomato.
Brazil; host range; Meloidogyne petuniae; nematode; new species; petunia; root-knot nematode; scanning electron microscopy; taxonomy
Stimulation of hatching of a tobacco cyst nematode (Globodera tabacum solanacearum) by root exudates from resistant NC 567 and susceptible K 326 cultivars of flue-cured tobacco, Nicotiana tabacum, was investigated. Root exudates were collected by soaking seedlings in deionized water for 2 hours at 22 °C in the dark. Fifteen mature and uniformly sized cysts were exposed at 15, 20, or 25 °C to undiluted root exudate, root exudate diluted 1:1 or 1:3 with deionized water, or deionized water alone. Hatched juveniles were counted and removed at weekly intervals during 42 and 53 days of exposure in experiments conducted in 1994 and 1995, respectively. Root exudates from both susceptible cultivar K 326 and resistant cultivar NC 567 stimulated more hatching than deionized water at 25 °C in 1994, and at all three tested temperatures in 1995. In 1994, dilution of root exudates 1:3 reduced stimulation of hatching at 25 °C compared to undiluted exudate. Hatching at 25 °C was similarly stimulated by exposure to undiluted root exudate and exudate diluted 1:1. In 1995, both dilutions reduced stimulation of hatching by root exudates at all the temperatures.
flue-cured tobacco; Globodera tabacum solanacearum; hatching; Nicotiana tabacum; resistance; root exudate; temperature; tobacco cyst nematode
Meloidogyne trifoliophila n. sp. is described from white clover collected at Ames Plantation, Fayette County, Tennessee. The perineal pattern is rounded, with long, smooth striae and rounded arch, and without distinct lateral lines or perivulval striae. The female stylet is 12.6-15.5 μm long, the excretory pore is level with or up to one stylet length posterior to the stylet knobs, and the vulva is subterminal. The posterior terminus is weakly protuberant. The male lateral field is composed of approximately eight repeatedly broken or forked incisures. The male stylet is 17.0-18.9 μm long, the stylet knobs are rounded and sloping, gradually merging with the shaft, and the head region consists of one large annule. Second-stage juveniles are 357-400 μm long, with a stylet length of 11.9-13.6 μm and one head annule. The tail tapers to a slender tip. This new species is similar to M. graminicola and M. triticoryzae but differs from them in perineal pattern and lateral field morphology, and numerous morphometric characters.
clover; clover root-knot nematode; Meloidogyne graminicola; Meloidogyne trifoliophila; Meloidogyne triticaryzae; nematode; new species; root-knot nematode; scanning electron microscopy; taxonomy; Tennessee; Trifolium spp.
Meloidogyne konaensis n. sp. is described from coffee from Kona on the island of Hawaii. The perineal pattern of the female is variable in morphology, the medial lips of the female are divided into distinct lip pairs, and the excretory pore is 2-3 stylet lengths from the base of the stylet. Mean stylet length is 16.0 μm, and the knobs gradually merge with the shaft. The knobs are indented anteriorly and rounded posteriorly and the dorsal esophageal gland orifice (DEGO) is long, 3.5-7 μm. The morphology of the stylet of the male is the most useful diagnostic character, with 6-12 large projections protruding from the shaft. One medial lip may be divided into distinct lip pairs. A large intestinal caecum often extends nearly to the level of the DEGO. Mean juvenile length is 502 μm, mean stylet length is 13.4 μm, and mean tail length is 58 μm. The tail may be distinctly curved ventrally and the phasmids are located in the ventral incisure about one anal body width posterior to the anus.
host range; Meloidogyne species; morphology; nematode; scanning electron microscopy; taxonomy
Plants have evolved a broad array of defense mechanisms involved in disease resistance. These include synthesis of phytoalexin antibiotics and proteinase inhibitors, deposition of cell wall materials, and accumulation of hydrolytic enzymes such as chitinases. Resistance appears to depend on the ability of the host to recognize the pathogen rapidly and induce these defense responses in order to limit pathogen spread. Application of molecular technologies has yielded significant new information on mechanisms involved in pathogen recognition, signal transduction, and defense-related gene activation, and is leading to novel strategies for engineering enhanced disease resistance. We are using these approaches to analyze regulation of 3-hydroxy-3-methylglutaryl CoA reductase (HMGR), a key enzyme mediating the production of terpenoid defense compounds. This enzyme is encoded by four genes in tomato; hmg2 gene expression is specifically associated with responses to pathogen or defense elicitors. Transgenic plants containing DNA constructs that fuse the hmg2 promoter to a reporter gene have been used to analyze both tissue specificity and patterns of defense-related expression. Because this gene is rapidly induced in tissues directly surrounding the site of ingress by a variety of pathogens, it may serve as a valuable tool in engineering new disease-resistance mechanisms.
disease interaction; gene expression; phytoalexin
The availability of interactive multimedia authoring software programs promises to revolutionize the teaching of nematology. These programs integrate text, hypertext, graphics, animations, video, and sound. The user interacts with the information on demand in a nonlinear fashion. Beginning students can limit themselves to the general outlines of the subject, and advanced students can explore the information to the limits of their ability. Use of interactive multimedia does not eliminate the need for effective, enthusiastic teachers but provides a mechanism for the efficient transfer of information. An interactive multimedia presentation that supplements lectures in an introductory course is presented as an example of the application of this technology for teaching nematology.
authoring program; computer; instruction; multimedia; nematology; teaching
A new aseptic culture system for studying interactions between tomato (Lycopersicon esculentum) and Meloidogyne incognita is described. Epidermal thin cell layer explants from peduncles of tomato produced up to 20 adventitious roots per culture in 4-9 days on Murashige &Scoog medium plus kinetin and indole acetic acid. Rooted cultures were transferred to Gamborg's B-5 medium and inoculated with infective second-stage juveniles. Gall formation was apparent 5 days after inoculation and egg production by mature females occurred within 25 days at 25 C in the susceptible genotypes Rutgers and Red Alert. Resistant genotypes LA655, LA656, and LA1022 exhibited a characteristic hypersensitive response. This system provides large numbers of cultured root tips for studies on the molecular basis of the host-parasite relationship.
aseptic culture; host resistance; host susceptibility; Lycopersicon esculentum; Meloidogyne incognita; nematode; southern root-knot nematode; tomato
The description of Pratylenchus macrostylus Wu is amended using specimens collected from Fraser fir and red spruce in the Black Mountains of North Carolina. Measurements of females in North Carolina overlap those of the type series. However, stylet length (21.8-27.8 μm, 24.7 ñ 1.1) is greater in North Carolina specimens, which also have a longer body length and greater C ratio. Heads of the North Carolina specimens are divided into lateral and submedian segments which taper and fuse with oral discs. Males are rare and not important in species diagnosis. Previously described specimens in Japan differed from those in North America in key diagnostic characters of stylet and body length. This discrepancy suggests that the Japanese species may be distinct from the North American.
Abies fraseri; Fraser fir; light microscopy (LM); morphology; morphometrics; Picea rubens; Pratylenchus macrostylus; red spruce; scanning electron microscopy (SEM); taxonomy
light microscopy; method; technique
identification technique; morphology; scanning electron microscopy; stylet; taxonomy
Second-stage juveniles of Meloidogyne incognita were prepared by several different techniques for scanning electron microscopy (SEM). Sequential fixation in the cold (4-8 C) was superior to rapid fixation at room temperature, glutaraldehyde and glutaraldehyde-formalin were better fixatives than formalin alone, and critical point drying with carbon dioxide or Freon gave similar results that were only slightly better than air drying with Freon. Freeze drying sequentially fixed nematodes from 100% ethanol in liquid propane produced the best preserved specimens with the fewest artifacts. Specimens of various free-living and plant-parasitic nematodes were prepared for SEM by freeze drying. This technique was adequate for most genera but unsatisfactory for a few. Although each genus may require a different procedure for optimum preservation of detail, sequential fixation with glutaraldehyde and freeze drying are comparable and often superior to commonly used techniques for preparing nematodes for SEM.
critical point drying; fixation; fixatives; freeze drying; methods; morphology; Acrobeles sp.; Belonolaimus longicaudatus; Criconemella sp.; Hemicycliophora sp.; Heterodera glycines; Hoplolaimus sp.; Meloidogyne incognita; M. pini; Mononchus sp.
Sphaeronema sasseri n. sp. is described from Fraser fir and red spruce on Mount Mitchell in North Carolina. Females are distinguished from other species in the genus by body shape, occurrence of body annulations, stylet morphology, head shape, and by several morphometric characters. The nematodes occur in colonies surrounding the bases of lateral and feeder roots, and the infected tissues show a general breakdown of the cortex and bark. The roots appear to be severely damaged by high populations of nematodes. This parasite may be important in the etiology of the slow decline of spruce and fir that has occurred in recent years in the southern Appalachian Mountains.
taxonomy; scanning electron microscopy; spruce decline; fir decline
Meloidogyne pini n. sp. is described from sand pine, Pinus clausa, in Georgia. The perineal pattern of the female has a large cuticular ridge surrounding a deeply recessed perivulval area. The lateral fields are marked by transverse striae. The female stylet is 14.6 μm long, and the knobs are small, rounded, and set off from the straight and narrow shaft. The excretory pore is near the level of the base of the stylet. The labial disc of the male is large, rounded, and fused with the crescent-shaped medial lips. The head region is smooth, the styler is 20.8 μm long, and the cone is more than twice as long as the shaft. The knobs are rounded and set off from the shaft. In the second-stage juvenile, the labial disc, medial lips, and lateral lips form one smooth, continuous, ovoid head cap. Mean juvenile length is 434 μm, stylet length is 12.8 μm, and tail length is 44.4 μm. M. pini n. sp. also parasitizes loblolly and slash pine. Additional morphological details of M. megatyla are presented.
taxonomy; host range; scanning electron microscopy; Pinus taeda; loblolly pine; Pinus elliotti; slash pine; new species
Meloidogyne enterolobii n. sp. is described and illustrated from roots of pacara earpod tree, Enterolobium contortisiliquum (Vell.) Morong, on Hainan Island in China. The perineal pattern of the female is usually oval shaped, the striae are fine to coarse, the dorsal arch is moderately high to high and usually rounded, and the phasmids are large. The stylet knobs in females are divided longitudinally by a groove so that each knob appears as two. The mean distance of the excretory pore to the anterior end in the female is 62.9 μm. Males have a large, rounded labial disc that fuses with the medial lips to form a dorso-ventrally elongate head cap. The labial disc is slightly elevated, and the medial lips are crescent shaped. The second-stage juvenile mean body length is 436.6 μm. The lateral lips are large and triangular in face view. The tail is 56.4 μm long and narrow with a broad, bluntly rounded tip. M. enterolobii n. sp reproduces well on E. contortisiliquum and causes severe damage. Other good hosts include cotton, resistant tobacco 'NC 95,' pepper, watermelon, and tomato.
taxonomy; morphology; host range; scanning electron microscopy
Meloidogyne carolinensis n. sp. is described from cultivated highbush blueberry (cultivars derived from hybrids of Vaccinium corymbosum L. and V. lamarckii Camp) in North Carolina. The perineal pattern of the female has a large cuticular ridge that surrounds the perivulval area, and the excretory pore is near the level of the base of the stylet. The stylet is 15.9 μm long and the knobs gradually merge with the shaft. The head shape and stylet morphology of the male are quite variable. The typical head and four variants, as well as the typical stylet and two variants, are described. The labial disc, medial lips, and lateral lips of second-stage juveniles are fused and in the same contour. The head region is not annulated. Mean juvenile length is 463.7 μm, stylet length is 11.9 μm, and tail length is 42.5 μm.
taxonomy; morphology; new Meloidogyne species; host range; scanning electron microscopy