The global importance of the cereal cyst nematode (Heterodera spp.) on wheat and international approaches to its control.

The Cereal Cyst Nematodes (CCNs) are a group of several closely related species which have been documented to cause economic yield loss on rainfed wheat production systems in several part of the world including North Africa, West Asia, China, India, Australia, America and several countries in Europe. The most commonly reported species is Heterodera avenae, however there are at least two other species H. filipjevi and H. latipons are implicated. It is well appreciated that plants under water and nutrient stress suffer greater yield loss. Control of CCNs requires maintaining nematode populations below economic thresholds. Chemicals are not environmentally sustainable or economic and the major emphasis on control has been with host genetic resistance applied with other integrated pest managent options. Unfortunately due to the number of species and pathotype variation genetic control of Cereal Cyst Nematode with plant resistance is complex. Turkey is one of the top ten wheat producers in the world and has identified these nematode as a major biotic constraint in their rainfed wheat systems. In 2001 a new joint intiative was established between CIMMYT International, the Turkish Ministry of Agriculture and (Ukurova University in Adana to understand i) the distribution of cereal nematodes on wheat; ii) assess the economic importance and improve our understanding of the population dynamics iii) culture, screen and assess known sources of resistance and identify new sources to both groups of nematodes; iv) integrate new sources of resistance into bread wheat cultivars for Turkey and International germplasm using conventional and molecular tools; v) investigate other integrated control options such as rotation and different wheat management strategies and finally vi) capacity build scientists to work in this important area. Some highlights of this work will be presented and the newly formed ICCNI - International Cereal Cyst Nematode Initative introduced.

Molecular Polymorphism and Morphometrics of Species of the Heterodera avenae Group in Syria and Turkey.

Molecular characterization of the three most common cereal cyst nematode species of the Heterodera avenae group (H. avenae, H. filipjevi, and H. latipons), originating from various locations in major cereal-cultivating areas in Syria and Turkey, showed distinct restriction fragment patterns of the ITS-rDNA following PCR amplification and RFLP digestion with four endonucleases (Hae III, Hinf I, Ita I, and Pst I). Genetic dissimilarity within H. avenae group populations increased in comparison with H. avenae and other species; it was 0.164 with H. filipjevi and 0.354 with H. latipons populations. No intraspecific polymorphism was observed within H. latipons or H. filipjevi populations. Principal component analysis revealed contrasted correlations among 12 morphological parameters of cysts and juveniles of the three Heterodera species that separated them and distinguished differences within populations of H. latipons. Our results showed a clear separation of the three cyst nematode species on cereal using a conventional method for classification and molecular tests, and confirmed the congruence between genetics and morphological traits.

Genetic diversity among a complex of cereal cyst nematodes inferred from RFLP analysis of the ribosomal internal transcribed spacer region.

This study examined the restriction polymorphism (RFLP) of the nuclear ribosomal DNA in Heterodera avenae, H. filipjevi, H. mani, H. latipons, and the taxonomically unclear Gotland strain in order to establish a molecular characterization and phylogenetic relationships in the complex of cereal cyst nematodes (CCN). The internal transcribed spacer (ITS) and 5.8S rDNA were amplified by PCR from a single female or a cyst of 27 different geographic isolates of the CCN complex and one population of H. schachtii, used as outgroup. The amplified product was 1.2 kb long and 14 of 15 enzymes produced restriction fragments for each isolate. Relationships between populations were determined from UPGMA analysis based on distance values calculated from RFLP data. Digestions with TaqI clearly differentiated H. avenae, H. latipons, and a group composed of H. filipjevi and the Gotland strain. Six endonucleases (HaeIII, HinfI, ItaI, PstI, TaqI, and Tru9I) produced the same restriction pattern with H. filipjevi and the Gotland strain, and both were clearly separated from H. avenae with PstI. Restriction sites have revealed a mixture of the species H. latipons and H. avenae, and possible infraspecific variation in H. avenae. The inferred phylogenetic relationships of species in the CCN complex are in agreement with their morphological characterization.

Genetic variation in natural populations of the cereal cyst nematode (Heterodera avenae Woll.) submitted to resistant and susceptible cultivars of cereals.

The purpose of the present work was to study the genetic characteristics of cereal cyst nematode (Heterodera avenae) populations re-established after the long-term use of resistant oat cultivars in field conditions. Population features were analyzed through fitness components and variation in enzymatic polymorphism (esterase and malate dehydrogenase loci). The longest (6 year) use of the same resistance genes (oat cv Panema) at high frequency (Rotation IB) led to the selection of a resistance-breaking pathotype and to a decrease in viability which suggested either a founder effect or a lower reproductive potential for the new pathotype. Analysis of esterase allelic frequencies led to the conclusions that: (1) the genetic constitution of this pathotype was different from the reference population maintained on the susceptible host (oat cv Peniarth), and (2) that the esterase locus may develop a disequilibrium linkage with loci involved in virulence Nematodes overcoming the resistance of cv Panema did not differ from H. avenae species following RFLPs in ribosomal DNA . Random mating was recorded at the whole-field level but not always at the single-plant level, suggesting that cultivation practices such as annual ploughing could play a major role in homogenizing subpopulations developed in the vicinity of a plant. These phenomena showed that the long-term use of highly effective resistance could provok marked genetic modifications in populations. These risks should be taken into account when deciding strategies for optimal use of resistance genes in nematode management programs.

Interactions between Heterodera avenae and Pratylenchus neglectus on Wheat.

In a long-term field experiment, differential population densities of Heterodera avenae were produced by frequent cropping with resistant (cv. Panema) or susceptible (cv. Peniarth) oat. The two oat cultivars were equally good hosts of Pratylenchus neglectus in a glass house experiment with field soil. On wheat crops grown after oats in field experiments, P. neglectus population densities in roots were higher in plots where H. avenae had been controlled than in plots with moderate infestations (40 H. avenae eggs/g soil). The field observations indicated that the reduction in population densities of P. neglectus coincided with the development in roots of sedentary stages of the cyst nematode. Evidence for an indirect effect of H. avenae on P. neglectus was found in vitro in a split-root experiment. In the same field, grain yields of two wheat cultivars susceptible or resistant to H. avenae, but both susceptible to P. neglectus, was not reduced by P. neglectus. Alternation of H. avenae resistant and susceptible cultivars is a possible way of exploiting the inverse relationship between these nematodes, whilst controlling cyst nematode -populations in intensive cereal production systems.

Partial Resistance to Heterodera avenae in Wheat Lines with the 6M v Chromosome from Aegilops ventricosa.

Lines of wheat with the 6M(v) chromosome from Aegilops ventricosa display partial resistance to both pathotypes Hal2 and Ha41 of Heterodera avenae. With either pathotype, the effect of this alien chromosome on cyst production, size, and fecundity was expressed in resistance tests. Partial resistance of five 6M(v)(6D) substitution lines varied according to the intrinsic cyst-forming capacity of the nematode pathotypes and the recipient germplasms. Such partial resistance can be utilized in wheat breeding lines for integrated management of the cereal cyst nematode.