Laboratory methods for work with plant and soil nematodes

1) Effects of cereal cyst (Heterodera) and rootlesion (Pratylenchus) nematodes on wheat are difficult to identify and control. Symptoms are non-specific and easily confused with stress from nutrient deficiency, drought, or disease. 2) Multiple species of Heterodera and Pratylenchus are capable of damaging wheat. Identification of species is difficult and procedures based on comparative morphology can be unreliable. Identification of species is now assisted by molecular tools. 3) Heterodera species discussed in this chapter reproduce only on hosts within the Poaceae, and individual species are highly heterogeneous for virulence to specific host genotypes. Many pathotypes occur within H. avenae, and the same is anticipated for H. filipjevi and H. latipons. No pathotypic variation within either P. neglectus or P. thornei has been reported on wheat, but both species multiply in a wide range of monocot and dicot hosts. Mixtures of Heterodera species or pathotypes, and Pratylenchus species, may occur within individual fields. 4) Field sanitation is important because these nematodes multiply on many weed species and volunteer cereals. Cereal cyst nematode can be controlled by rotating w\heat with a noncereal, a resistant cultivar, or with weed-free fallow. Root-lesion nematode is best managed by rotating resistant and tolerant wheat cultivars with other poor hosts. 5) Resistance and tolerance are genetically independent, and cultivars resistant or tolerant to one species are not necessarily resistant or tolerant to another species. Root-lesion nematode resistance is quantitative and cereal cyst nematode is controlled by single-gene resistance. Molecular markers have been developed to identify genes and quantitative trait loci for resistance in seedlings. 6) Molecular tests to identify and quantify nematodes in commercial soil testing laboratories will allow more effective surveys of populations. Greater collaboration is needed between research institutions, organizations, and countries.

Nematodes which Challenge Global Wheat Production

Both cereal cyst (Heterodera avenae) and root-lesion nematode (Pratylenchus spp. ) are associated with loss of grain yield in south-eastern and western Australia. Cereal cyst nematode has been a major pest since the 1930s but losses have declined significantly in the last 15 years due to more effective rotations and the widespread adoption of well adapted cereal cultivars with nematode resistance — strategies that were developed during several decades of research by pathologists, nematologists, agronomists and plant breeders. Research efforts are now focussed on reducing crop losses associated with root-lesion nematode. Rotations that minimise losses from the nematode have been identified, but a better-resourced plant breeding program is required to develop resistant cereal cultivars adapted to environmental conditions across the region. This paper reviews 40 years of research on the epidemiology and management of these two important pests.

Managing nematode pests in the southern and western regions of the Australian cereal industry: continuing progress in a challenging environment

The cereal cyst nematode (CCN) Heterodera avenae, is a significant pathogen of wheat. The wild grass Aegilops variabilis Accession No.1 has been found to be resistant to pathotypes of CCN; at least two genes transferred to wheat, designated as CreX and CreY, are involved in the resistance response. The CreY gene may be the same as Rkn-mn1, which confers resistance to root knot nematode (RKN) Meloidogyne naasi. The objective of this work was to pyramid the two CCN resistance genes in a wheat background through marker-assisted selection. As a first step, molecular markers flanking CreX were identified. The completely linked RAPD marker of Rkn-mn1 (CreY), OpY16-1065, previously obtained, was converted into a SCAR. All these dominant markers were used to incorporate in the same genotype the two Ae. variabilis chromosome segments carrying the two genes for resistance. CCN bioassays with the Ha12 pathotype showed that the level of resistance of the pyramided line was significantly higher than that of CreX and CreY single introgression lines, but lower than that of Ae. variabilis. This study thus illustrates the utilization of molecular markers in breeding for host resistance.

Marker-assisted pyramiding of two cereal cyst nematode resistance genes from Aegilops variabilis in wheat

Importance of cereals and wheat nematodes in the world is revised. Distribution of cereal nematodes, species and pathotypes includes root lesion, cereal cyst nematodes and other cereal parasitic species. Life cycle, symptoms of damage and yield losses are also revised for root knot, stem and seed gall nematodes. Integrated control of cereal nematodes and some chemical, biological and cultural practices, including grass free rotations and fallowing with cultivation, are discussed. The effects of time of sowing, crop rotations and cultivation of resistant/tolerant varieties are also revised.

Global Knowledge And Its Application For The Integrated Control And Management Of Nematodes On Wheat

Graminaceous cyst nematodes form a group of eleven valid species including Heterodera avenae, Heterodera filipjevi and Heterodera latipons and constitute major pests to cereals They are widely spread in circum-mediterranean areas where they are presumed to cause yield losses on bread and durum wheat The objective was to document the diversity of these cereal cyst nematodes, in particular samples from Mediterranean regions, in comparison to species which develop on cultivated or wild grasses (H arenaria, H hordecalis, H mani) and on rice or sugarcane (H sacchari) Studies involved PCR-RFLP of ITS and morphometrics of the juvenile and cyst characters UPGMA analysis of molecular data showed that the isolates segregated in two main clusters which seem to represent different evolutionary lineages The H avenae sensu lato cluster (I) contained H arenaria, H avenae, H filipjevi and H mani The second cluster (II) contained isolates of H hordecalis and H latipons Within H avenae sensu lato, H filipjevi was separated from the other related species with significant bootstrap value The differentiation of H arenaria, recognized for the first time based on molecular data, and H mani with few restriction enzymes were the least significant Intraspecific polymorphism allowed differentiation of isolates originating from Australia within H avenae sensu stricto The group H hordecalis–H latipons showed the greatest genetic variability between and within isolates Two representatives of Heterodera sacchari, taxonomically included in the ‘schachtii’ group, were genetically as distant to this group as to the other graminaceous species belonging to either H avenae sensu lato or H hordecalis–H latipons group Results inferred from multivariate analysis applied on morphometrics of the cysts and juveniles showed agreement between genetic and phenotypic classifications This study demonstrates the utility of combined molecular and classical methods to enhance our knowledge about the diversity within the complex of graminaceous cyst nematodes and to establish robust techniques to identify a wider set of nematode species

Genetic and phenotypic diversity in the graminaceous cyst nematode complex, inferred from PCR-RFLP of ribosomal DNA and morphometric analysis

Two sets of experiments in 1998 and 1999 studied the (a)virulence status of 14 populations of Heterodera avenae, and two populations each of H. filipjevi and H. latipons towards the resistance genes and genotypes of Triticeae, presently known to be resistant to H. avenae, the main species of the cereal cyst nematode (CCN) species complex. Susceptible controls were Triticum aestivum cv. Arminda and Triticum turgidum cv. Cham1. Even in controlled experimental conditions, host responses differed between replicates in the same test and between consecutive tests for unexplained reasons. In spite of that, it has been confirmed that several of these CCN populations differ in their intrinsic capacity to reproduce, and this has to be taken into account in resistance screening. Consequently, the (a)virulence status of a CCN population is better established by a qualitative demarcation between resistant and susceptible plant genotypes based on an average of one cyst limit per plant. The matrix inferred from such a classification of host responses of pooled 1998 and 1999 data showed a complex distribution of avirulent and virulent phenotypes in these CCN populations which seems to arise from a mosaic-like evolution. The more or less wide virulence spectrum shown by this complex of populations and species is discussed in relation to their phylogenetic relationships. Correspondence factor analysis of the pooled data demonstrated a geographical demarcation between several populations of H. avenae and the representatives of H. filipjevi and H. latipons for their (a)virulence to either Cre1 or Cre3 genes and T. aestivum AUS4930 genotype. This could reveal local or regional selection of new pathotypes, more particularly in H. avenae. Genetic introgression to improve resistance to these nematodes in both bread and durum wheats is discussed for regional and global applications.

Amor Yahyaoui

Papers

Genetic and phenotypic diversity in the graminaceous cyst nematode complex, inferred from PCR-RFLP of ribosomal DNA and morphometric analysis

Variation in reproductive capacity and virulence on different genotypes and resistance genes of Triticeae, in the cereal cyst nematode species complex