Antoon T. Ploeg

RNA-2 of tobacco rattle virus encodes the determinants of transmissibility by trichodorid vector nematodes.

Pseudorecombinant isolates produced from an efficiently transmitted and a non-transmitted isolate of tobacco rattle tobravirus were tested for their transmissibility by trichodorid vector nematodes. Isolates with RNA-2 originating from the non-transmissible isolate were not transmitted by the vector, whereas isolates with RNA-2 originating from the efficiently transmitted isolate were transmitted efficiently. It is therefore concluded that the factor determining vector transmissibility is located on the RNA-2 genome segment of TRV. Because the serological properties of the isolates also correlate with transmissibility, it is likely that the virus particle protein is involved in the transmission process.

Glasshouse studies on the effects of time, temperature and amendment of soil with broccoli plant residues on the infestation of melon plants by Meloidogyne incognita and M. javanica

The effects of heating, over a range of temperatures and for increasing periods of time, and of adding finely chopped broccoli leaves to soil infested by Meloidogyne incognita and M. javanica on nematode infestation of melon, were studied in glasshouse experiments. There was a significant interaction between the effects of soil temperature, the period for which this temperature was maintained and broccoli amendment. At the lowest temperature tested (20°C), adding broccoli to the soil had very little effect on nematode infestation or galling of melon. Increasing the temperature of the broccoli-amended soils to 25, 30 or 35°C dramatically reduced infestation and galling compared to that in non-amended soils. Within this range, the effect of broccoli-amendment occurred sooner at higher temperatures. Heating the soil to 40°C for 10 days generally eliminated nematode infestation and root-galling, irrespective of whether broccoli had been added. Fewer egg masses were obtained from melon roots grown in broccoli-amended soils than from those grown in non-amended soils, even when roots exhibited similar degrees of galling. The results suggest that the temperature and time treatments using soil solarisation to control M. incognita or M. javanica could be reduced in soils amended with broccoli residues.

Damage to melon (Cucumis melo L.) cv. Durango by Meloidogyne incognita in Southern California

The relationship between increasing Meloidogyne incognita inoculum density and the growth of melon plants in pots was well described by the Seinhorst model. The estimated tolerance level was not affected by the age of the plant at time of exposure to the nematodes, but minimum yields increased significantly when nematode inoculation was delayed until 2 weeks after seeding. Similarly, in field grown plants, melon fruit yields decreased with increasing pre-plant nematode levels. Tolerance levels estimated from the field study were one second stage juvenile (J2) per 200 g soil sampled before planting, and the estimated yield loss at high nematode densities was 65%. The yield loss resulted primarily from fewer fruits being harvested per plant, rather than from the fruits being smaller. This may be due to a redirection of plant nutrients towards sustaining the nematodes developing in the root systems, inhibiting fruit set. The results indicate that preventing immediate access of the nematodes to very young seedlings may prevent severe yield losses, but may also result in high populations at harvest.

Acquisition and subsequent transmission of tobacco rattle virus isolates by Paratrichodorus and Trichodorus nematode species

Isolates of the PRN serotype of tobacco rattle virus (TRV) were transmitted with different efficiencies by the nematode vectorParatrichodorus pachydermus. Virus isolates which belonged to other serotypes were not acquired and/or transmitted by this vector, nor were PRN serotype isolates which had been obtained from naturally infected potato plants and maintained by mechanical transmission in the glasshouse for several years. PRN serotype TRV isolates from the Netherlands or from Scotland were equally well transmitted by initially virus-freeP. pachydermus populations from either country. Allowing a naturally viruliferous nematode population access for 3 weeks to uninfected or TRV-infected roots resulted in an increased proportion of the trichodorid population transmitting TRV.

Effects of amending soil with Tagetes patula cv. Single Gold on Meloidogyne incognita infestation of tomato

The effects of amending soil with either roots or tops of tomato or marigold on subsequent tomato growth and infestation by Meloidogyne incognita was determined in greenhouse pot experiments. Controls consisted of non-amended soil, and pre-cropping for 8 weeks with tomato or marigold previously infested with M. incognita. Amending soil with marigold or tomato tops or roots increased the weight of tomato tops at high M. incognita inoculum densities. Galling and nematode populations were high when tomato followed tomato, but very low when tomato followed marigold. Galling and final nematode population levels were reduced by all soil amendments, but by much less than pre-cropping with marigold for 8 weeks. Although some reduction in nematode infestation can be achieved by amending soil with marigold plant parts, this reduction is not specific to marigold and is unlikely to be of practical use. Die Wirkung einer Bodenverbesserung mit Tagetes patula cv. Single Gold auf den Befall von Tomaten durch Meloidogyne incognita - In Topfversuchen im Gewachshaus wurde die Wirkung von Wurzeln oder oberirdischen Teilen von Tomate oder Tagetes auf das anschliesende Wachstum und auf den Befall mit Meloidogyne incognita untersucht. Als Kontrollen dienten nichtverbesserter Boden oder eine acht Wochen lange Vorkultur von Tomaten oder Tagetes in voher mit M. incognita verseuchter Erde. Eine Bodenverbesserung mit oberirdischen Teilen oder Wurzeln von Tagetes oder Tomate steigerte das Gewicht der oberirdischen Teile von Tomatenpflanzen bei hohen Befallsdichten. Gallbildung und Nematodenpopulationen waren hoch, wenn Tomate auf Tomate folgte, aber nur gering bei Tomate nach Tagetes. Gallbildung und Hohe der Endpopulation wurden durch alle Bodenverbesserungen vermindert, aber deutlich weniger als durch eine achtwochige Vorkultur von Tagetes. Obwohl eine gewisse Verminderung der Nematodenpopulation durch eine Bodenverbesserung mit Teilen von Tagetespflanzen erreicht werden kann, ist diese Verminderung nicht spezifisch fur Tagetes und wahrscheinlich ohne praktischen Nutzen.

Characterisation of populations of Longidorus orientalis Loof, 1982 (Nematoda: Dorylaimida) from date palm (Phoenix dactylifera L.) in the USA and other countries and incongruence of phylogenies inferred from ITS1 rRNA and coxI genes

Needle nematode populations of Longidorus orientalis associated with date palm, Phoenix dactylifera, and detected during nematode surveys conducted in Arizona, California and Florida, USA, were characterised morphologically and molecularly. The nematode species most likely arrived in California a century ago with propagative date palms from the Middle East and eventually spread to Florida on ornamental date palms that were shipped from Arizona and California. This is the first validated continental record of this needle nematode species in the USA and the Americas. The USA populations of L. orientalis contained a small number of males that were not reported in the original description and are herein described. Longidorus orientalis was able to survive for at least 4 years at very low numbers in the warm and humid environment of Florida on date palms imported from California and Arizona. Association of L. orientalis with L. africanus was observed in all of the surveyed sites, indicating that date palm is a host of both nematodes. Phylogenetic relationships of L. orientalis with closely related Longidorus species, in addition to relationships between populations of L. orientalis from the USA, Greece, Iran and Spain, were inferred from the analyses of D2-D3 of 28S rRNA, ITS1 rRNA and partial coxI gene sequences. The PCR-D2-D3 expansion segments of 28S rDNA-RFLP diagnostic profile is provided. Longidorus orientalis populations display a high level of intraspecific variation (up to 15.5%) in coxI mtDNA sequences. Analysis of phylogenetic relationships of nematode populations revealed incongruence of the ITS1 rRNA and coxI mtDNA gene trees, which might be the result of selective introgression of mtDNA through gene flow between previously isolated populations introduced simultaneously into new geographical regions.

Tobacco rattle virus serotypes and associated nematode vector species of Trichodoridae in the bulb-growing areas in the Netherlands

Soil samples from the coastal bulb-growing areas in the provinces of North- and South-Holland and the North-East Polder in the Netherlands were examined for trichodorid nematodes and tobacco rattle virus (TRV) serotypes. At least one of a total of eight species of Trichodoridae, of whichParatrichodorus pachydermus was most prevalent, was found in 93% of the samples from the provinces of North- and South-Holland and TRV, including four serotypes, was obtained from 49% of these samples. In the North-East Polder one of three species of trichodorids, of whichP. teres occurred most frequently, was present in 72% of the samples, and TRV of one serotype was obtained from 28% of these samples. The TRV isolates recovered from these samples reacted serologically with one of four antisera to strains of TRV. Virus transmitted byP. pachydermus reacted to the PRN-, byTrichodorus viruliferus to the RQ-, byP. teres to the N5- and byT. similis, to the TS-antiserum, respectively.

The occurrence and distribution of trichodorid nematodes and their associated tobraviruses in Europe and the former Soviet Union.

Information on the distribution and occurrence of Trichodorus and Paratrichodorus species and their associations with tobraviruses in Europe and the former Soviet Union (FSU) is reviewed. A total of 17 Trichodorus and 13 Paratrichodorus species have been reported and of those, 14 have been reported to transmit virus. Paratrichodorus pachydermus and Trichodorus primitivus are the species most often associated with virus transmission.

Morphological and molecular characterisation and diagnostics of some species of Scutellonema Andrássy, 1958 (Tylenchida: Hoplolaimidae) with a molecular phylogeny of the genus

Scutellonema spp. are widely distributed across tropical and subtropical regions of the world and are associated with numerous agricultural and horticultural crops. Identification of many Scutellonema species is not always reliable, in part because many species share very similar diagnostic characters. In this study, we provide morphological and molecular characterisation of S. brachyurus from the USA and South Africa, S. bradys from Nigeria and three unidentified species from California, USA, New Zealand and Burkina Faso. Morphological descriptions, measurements, light and scanning electron microscopic photos and drawings are given for S. brachyurus. Females of S. brachyurus from the USA (type A) and South Africa (type B) showed a significant variation in the number of sectors and blocks on the lip annuli, ranging from about 4-12 and from 8-20, respectively. Molecular analysis using the D2-D3 of 28S rRNA, ITS rRNA and the COI mtDNA gene sequences revealed two distinct genotypes within S. brachyurus samples: type A (samples from USA, Italy, Korea, Taiwan) and type B (South Africa). Multivariate analyses determined that S. brachyurus from the USA and Taiwan (type A) differed from that from South Africa (type B) mainly in body, tail and DGO lengths, and ratios b′, c′, c and V. Phylogenetic relationships within Scutellonema are given as inferred from the analyses of the D2-D3 of 28S rRNA, ITS rRNA and the COI mtDNA gene sequences. PCR-RFLP diagnostic profiles and PCR with species-specific primers are developed for the studied Scutellonema species.

Studies on an artificially fragmented population of potato cyst nematode Globodera pallida

A series of sub-populations was produced from a population of Globodera pallida by raising single cyst progenies on three susceptible potato cultivars. After completion of each generation, single cysts were taken to produce the next generation. Six generations were completed and then the resulting sub-populations were multiplied on the susceptible cultivar Ddsir6e. A sample of these sub-populations was then used to conduct a virulence test and to investigate the genetic variation among the sub-populations. The virulence test showed that the susceptible cultivar used in the generation of these sub-populations had a marked effect on the general reproductive capability of the sub-populations and that there was a range of variation in virulence on partially resistant clones. A RAPD study revealed that the range of genetic variation among these sub-populations was comparable to that shown between field populations in other studies and that there was a tendency for the sub-populations to be grouped in relation to the susceptible cultivar they were reared on. These results from an artificially fragmented population are discussed in relation to the introduction and spread of potato cyst nematodes in Europe.