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Abstract Sequence data were obtained for the full length of the ribosomal DNA (rDNA) 18S gene of specimens from three populations of Paratrichodorus christiei from the USA and four populations of P. minor from Brazil and Portugal. Alignment of the sequences and their application in a phylogenetic analysis revealed that the populations of P.christiei and P.minor formed a single cluster that differentiated from a second cluster comprising P.anemones and P.pachydermus. Previously, P.christiei had been proposed as a junior synonym of P.minor, but this synonymy had not been universally accepted, with many reports, particularly from the USA, continuing to refer to P. christiei. The results obtained here with the sequences of the rDNA 18S gene from the various populations confirm the validity of the synonymy of P.christiei with P.minor.

Twelve plant-parasitic nematode genera and 25 species were identified in soil and root samples collected from 17 localities in the soybean production areas of South Africa during the 1995/96 season. Predominant endoparasites found included Meloidogyne incognita, M. javanica, M. hapla, M. ethiopica, Pratylenchus zeae and P. brachyurus. Meloidogyne species occurred in 91% of all root samples, P. zeae in 87% and P. brachyurus in 33%. The occurrence of the three predominant endoparasites was not restricted to sandy soil, but they were also prevalent in soils with high clay contents. Other endoparasitic nematode species were Pratylenchus crenatus, P. teres, P. neglectus, P. thornei, Rotylenchulus parvus and Ditylenchus africanus. Nematodes found in soil included Helicotylenchus dihystera and Scutellonema brachyurus, which occurred in 78 and 71%, respectively, of all soil samples. Other nematodes found in soil samples were Helicotylenchus pseudorobustus, H. digonicus, H. microcephalus, Scutellonema truncatum, Rotylenchus unisexus, Mesocriconema sphaerocephalum, Paratrichodorus minor, Longidorus pisi, Xiphinema vanderlindei, X. elongatum and Tylenchorhynchus goffarti. Highest prominence values (PV) for Meloidogyne species were recorded on cultivars Prima, Bakgat, Tamboti, Hutcheson and Knap and the lowest on cultivars PAN812, SCS1, Nyala, Gazelle and A7119. Eight nematode genera and 19 species extracted during this study are reported for the first time in association with soybean in South Africa.

Adult and juvenile characters were used to identify Paratylenchus nanus Cobb, 1923 and Paratrichodorus minor (Colbran, 1956) Siddiqi, 1974 from soils under pasture in New Zealand. In glasshouse tests of 15 pasture plants, common in New Zealand, all good hosts (ratio of final to initial population >1) of P. nanus were grasses, namely Dactylis glomerata , Lolium multiflorum and L. perenne (Neotyphodium endophyte-infected and-free), all new host records for this species. Good hosts of P.minor included those listed for P.nanus and Festuca arundinacea (endophyte-free) , Poa annua , Trifolium pratense , T. repens and T. subterraneum. Except for T. repens, these are all new host records for P.minor in New Zealand.

Populations of both Paratrichodorus minor and Paratylenchus nanus in a grazed pasture in Waikato, New Zealand were assessed in single 2.5 cm diam. core samples over six successive seasons. Neither seasonal periodicity nor a consistent vertical distribution pattern was observed in the P.minor population. P.nanus had distinct spring and summer population maxima and was more abundant at 0-10 than 10-20 cm depth. P.minor but not P.nanus abundance was significantly correlated with rainfall and an activity index combining rainfall and soil temperature for the week before sampling. Significant correlations among P.minor, P.nanus and concentrations of some soil nutrients seemed to reflect soil nutrient interactions with other biotic factors such as plant growth. There was no evidence for competition between these two nematodes. Host root resource, spatial aggregation and differences in morphology that indicate niche differences are discussed as possible reasons for the observed lack of competition.

Using published records for the widespread, polyphagous, plant-parasitic nematode Paratrichodorus minor and the 'match climates' function of the computer model CLIMEX, we assessed the impact of a 2°C increase in mean annual temperature. Best current distribution records are from eastern Australia and South Africa (26 and 24 respectively). CLIMEX demonstrates a shift in the range of P. minor in Australia and Africa in response to climate warming. The actual changes are dependent on local interactions between topography and climate. With the available data it is reasonable to consider that similar changes in range would occur elsewhere on the globe. The new distribution would be affected by other environmental conditions and any climate-related changes in them.

Observations are reported on the fine structure of the cuticle of Paratrichodorus minor, P. nanus, P. rhodesiensis and Trichodorus eburneus. The ultrastructure of the cuticle was similar in Paratrichodorus and Trichodorus species; six layers were observed, the innermost layer consisting of three multilaminate units. The anatomy of the cuticle in Trichodorids was quite different from previous observations in other orders of Adenophorea. Two sets of intracuticular canals were observed as well as a peculiar type of junction between cuticle and somatic muscles. These results confirm previous studies on the anatomy of trichodorids and support the withdrawal of the family Trichodoridae from Dorylaimida. Siddiqi's (1983) classification of the trichodorids under the order Triplonchida should be considered with attention.

The effects of Tylenchorhynchus agri and Paratrichodorus minor on infection of creeping bentgrass roots by Meloidogyne naasi were studied in greenhouse experiments. High populations of both ectoparasites reduced M. naasi infection by inhibiting root growth and consequently decreasing the availability of penetration sites. There was evidence that T. christiei also was antagonistic to M. naasi through competition for feeding sites.

Tree growth, fruit production and population dynamics of three root-knot nematodes, Meloidogyne incognita, M. arenaria and M. javanica were evaluated during six years in a peach planting. The treatments were: 1) preplant soil fumigation with methyl bromide at 450 kg/ha + mycorrhizae at planting, 2) root-knot nematode + Dactylella oviparasitica (S isolate), 3) root-knot nematode + soil from an established peach orchard, and 4) root-knot nematode alone. These four treatments were evaluated under normal and high irrigation regimes. Methyl bromide treatment improved tree growth, fruit production, and nematode control most. D. oviparasitica did not improve growth. The high irrigation regime counteracted the detrimental effects of root-knot nematode without reducing the nematode population. After 6 years root-knot nematode numbers were declining in all treatments except those previously free of the nematode.

The relative importance of the effect of soil temperature, soil texture and host species on the increase and the pathogenicity of a population of Paratrichodorus minor from Israel was studied. Populations increased fastest at 26° C, followed by 30°, 22° and 18°, approaching the host-specific equilibrium density after 90 days. In the presence of a host, populations thrived better in sand and sandy loam than in clay, but were obviously sensitive to moisture fluctuations. Host species affected population increase more than soil temperatures and soil types. In fallow, survival was better in clay and sandy loam than in sand. In vitro, P. minor fed vigorously on epidermal cells in the elongation and meristematic zone of wheat and eggplant roots, causing browning and collapse of the epidermis and cessation of growth. After detaching itself from the feeding site, the nematode left a small tube attached to the cell wall. In pathogenicity trials, two good hosts - eggplant and cotton - showed different responses: growth of eggplant was drastically inhibited at an inoculation level of 200 nematodes per plant, while populations of up to 800 nematodes per plant stimulated growth of cotton roots and tops. Infested eggplant seedlings exhibited a severely thinned and shortened lateral root system and stunted tops.