This chapter focuses on the general principles concerning resistance to cyst nematodes together with detailed information that relates specifically to soyabean, cereal, potato or sugarbeet cyst nematodes. The results of extensive breeding programmes on improving the resistance of some of the most economically important crops against plant parasitic nematodes are also presented.

Populations of beet cyst nematodes Heterodera schachtii vary in aggressiveness and virulence toward sugar beet varieties, but also in traits like host range, or decline rate in the field. Diversity of their essential pathogenicity gene vap1 is shaped by diversifying selection and gene flow. The authors developed a technique to study interpopulation variation and intra-population diversity and dynamics of H. schachtii based on the gene vap1. Degenerate primers were designed to amplify, clone, and sequence this gene from diverse species and populations of cyst nematodes. This resulted in a high diversity of sequences for H. schachtii, and allowed to design non-degenerated primers to amplify a fragment suitable for sequence dependent separation of gene variants in denaturing gradient gel electrophoresis (DGGE). The developed primers span a highly variable intron and part of a slightly variable exon. A marker comprised of the 14 mostly detected gene variants was established for gel-to-gel comparisons. For individual juveniles up to six gene variants were resolved and substantial variation within and among cysts was observed. A fast and easy DNA extraction procedure for 20 pooled cysts was established, which provided DGGE patterns with high similarity among replicate samples from field populations. Permutation tests on pairwise similarities within and among populations showed significant differences among vap1 patterns of field populations of H. schachtii. Similarly, gene diversity as expressed by the Shannon index was statistically different among field populations. In conclusion, the DGGE technique is a fast andcompared to sequencing approaches-inexpensive tool to compare populations of H. schachtii and link observed biological characteristics to genetic pattern.

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Summary Characterising the non-neutral genetic variation within and among populations of plant-parasitic nematodes is essential to determine factors shaping the population genetic structure. This study describes the genetic variation of the parasitism gene vap1 within and among geographic populations of the beet cyst nematode Heterodera schachtii. Forty populations of H. schachtii were sampled at four spatial scales: 695 km, 49 km, 3.1 km and 0.24 km. DGGE fingerprinting showed significant differences in vap1 patterns among populations. High similarity of vap1 patterns appeared between geographically close populations, and occasionally among distant populations. Analysis of spatially sampled populations within fields revealed an effect of tillage direction on the vap1 similarity for two of four studied fields. Overall, geographic distance and similarity of vap1 patterns of H. schachtii populations were negatively correlated. In conclusion, the population genetic structure was shaped by the interplay between the genetic adaptation and the passive transport of this nematode.