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.
Flavonoids have diverse roles in plants, including defence against plant pathogens and regulation of local auxin transport. Flavonoids have been shown to be produced in feeding sites of root-knot nematodes induced in a leguminous plant, and it has previously been suggested that they may be responsible for manipulation of local auxin levels that underlie early feeding site development. Here we show that flavonoids are also produced in developing syncytia induced by Heterodera schachtii and in galls induced by Xiphinema diversicaudatum in a non-leguminous plant, Arabidopsis thaliana. We further investigated whether flavonoids are required for normal feeding site development by screening mutant lines of A. thaliana, defective in various parts of the flavonoid biosynthetic pathway, with H. schachtii. None of the lines showed a reduced capacity to support nematode infection and some showed a statistically significant increase in the numbers of female nematodes that developed. These data suggest that flavonoids are produced as part of the defence response to nematode infection rather than being an integral component of the mechanisms used by nematodes to induce feeding sites.
The white potato cyst nematode, Globodera pallida, was recently found in the USA and was identified using molecular and morphometric assessment. This paper examined some biological characteristics of the Idaho population of G. pallida. The samples of the original cysts showed that they had low rates of juvenile hatch and, consequently, low rates of reproduction when compared with glasshouse-reared populations. Virulence tests using sources of partial resistance from Solanum vernei and S. tuberosum ssp. Andigena CPC 2802 compared the Idaho population to European and South American populations of G. pallida. The Idaho population showed a virulence pattern comparable to other European populations in the Pa2/3 virulence group. The overall level of reproduction of the Idaho population on the resistant cultivars was the lowest of all the populations tested, suggesting that it is not highly virulent. These results are compatible with molecular characterisation, which indicates that this population is similar to the European gene pool.
Four techniques, i.e., perineal patterns, isozymes, specific sequence characterised amplified region-polymerase chain reaction (SCAR-PCR) and random amplified polymorphic DNA (RAPD), were compared for the identification of species of root-knot nematode (RKN) from Libya. The RAPD technique proved superior for species diagnosis. A population from Massa that could not, because of atypical patterns, be identified using either perineal patterns or isozyme phenotypes, showed amplification patterns consistent with those of M. incognita. A new esterase phenotype is described for this population. Single egg mass lines from two other locations, Elhamma and Durnah, were identified as M. javanica, whereas those from Aun Mare were identified as M. incognita. RAPDs were used to examine the relationships between the Libyan isolates and isolates of Meloidogyne spp. from elsewhere. This technique revealed differences in the Libyan isolates of M. incognita that distinguished them from isolates from other geographic origins. The work demonstrates the potential to utilise the RAPD technique in an integrated programme to control RKN.
The resistance responses present in 15 wild Solanum species to the potato cyst nematode Globodera pallida were investigated, focusing on hatch, invasion, development, sex determination and fecundity. A wide range of resistance responses was observed among the species, which impacted at different points in the life cycle of G. pallida. For most species a reduced rate of development and subsequent shifts towards an increased male to female ratio, indicative of a poor nutritional host, were observed. However, S. acaule ssp. acaule and S. canasense appeared to possess unique resistance responses, and may comprise valuable sources of new genes for incorporation into potato breeding programmes.
Two homologues of map-1, which encodes a putative avirulence factor, were found in a single egg mass line of the root-knot nematode Meloidogyne javanica that was virulent to the Mi-resistant gene. The main difference between the two encoded proteins of these homologues, MJ-MAP-1 and MJ-MAP-2, was the position of a 13 amino acid repeat region. Genes encoding two related but novel proteins, MJAP-1 and MJAP-2 (290 and 283 amino acids, respectively, including a potential signal secretion peptide), were also isolated from M. javanica. They have high similarity to MAP at the C-terminus. MJAP-1 and MJAP-2 differ from each other in the number and position of a seven-amino-acid repeat and in five other amino acids. The mjap genes are expressed in the subventral pharyngeal glands of second-stage juveniles of M. javanica, and transcription analysis in different developmental stages showed expression in the juvenile stage but not in eggs or adult females. Both mjap-1 and mjap-2 were expressed in both Mi-virulent and avirulent lines of M. javanica.
Following the discovery of Globodera pallida in the Uzhhorod region (Zakarpats'ka oblast') of Ukraine, two populations from this region were further examined. Firstly, their virulence was assessed in relation to two sources of resistance to G. pallida and to other G. pallida populations from Europe and South America. The results showed that the Ukrainian populations were very similar in their virulence to the other European populations. One of the test host genotypes was a new cultivar derived from Solanum tuberosum spp. andigena CPC2802, which proved to be more highly resistant than previously available partially resistant cultivars. The second experiment was a comparison of the mitochondrial Cytochrome B gene from the Ukrainian populations with other European and South American populations. The data indicated that the Ukrainian populations were similar to other European populations.
- The mitochondrial DNA region between the COII and lRNA genes and the 63 base pair tandem repeat region have been used to differentiate and characterise Meloidogyne spp. In this study these regions have been amplified from M. mayaguensis, M. javanica, M. arenaria, M. incognita and M. hapla. Meloidogyne mayaguensis produces a unique product of 705 bp from the COII and lRNA region. Also, a product of 322 bp was produced from the 63 bp repeat region of M. mayaguensis unlike M. javanica, M. arenaria, and M. incognita that exhibit hypervariability in this region. Meloidogyne mayaguensis is a widely distributed root-knot nematode with the potential to cause great economic damage. These molecular diagnostics can be used for accurate identification of M. mayaguensis and can be used to monitor the occurrence and spread of this species, and to provide quarantine services tools to limit its dispersal.
Trichodorids are plant-ectoparasitic nematodes, causing economically important diseases in many crops. In this study, we characterised ribosomal DNA (rDNA) of four trichodorid species from two different genera (i.e., Paratrichodorus macrostylus, P. pachydermus, Trichodorus primitivus and T. similis). Sequence data from the 18S and 5.8S genes showed high similarity; in contrast, sequence data from the internal transcribed spacer regions (ITS) were highly variable. Sequence divergence between the two genera included many insertions and deletions. Trichodorus primitivus and T. similis ITS regions of 1537 and 1303 bp, respectively, are the longest reported to date in the Phylum Nematoda. Further exploration of the sequence data indicated perfect and non-homologous repeats which were confined to the middle of the ITS regions.
This is the first report of endospores of Pasteuria penetrans and P. nishizawae isolates binding to juveniles of Meloidogyne chitwoodi and M. fallax. The patterns of endospore attachment to M. chitwoodi and M. fallax compared with the related M. hapla suggest that there are differences amongst these three temperate root-knot nematode species. Intraspecific variability in attachment of P. penetrans to juveniles of M. chitwoodi was demonstrated.