## 2 New Sources of Genetic Resistance to Nematodes of Small-Grain Cereals  
Genetic resistance remains a desirable means of controlling soilborne pathogens. New mapping approaches are enabling the screening of populations with a genetically wider range of individuals. For instance, populations for association mapping can include accessions from global collections (diversity panel) or accessions enriched for the trait of interest based on geographic origin (focused identification of germplasm strategy, or FIGS) [1; 50]. Mapping approaches now use single-nucleotide polymorphisms, or SNPs (90K iSelect SNP genotyping assay), or hybridization of host DNA to short DNA sequences on an array that represent a nonredundant set of genes located on all 21 wheat chromosomes (diversity array technology, or DArT, platform) [51; 52]. A DArT array for barley has also been developed [53]. Genome sequences of individuals from the mapping population (genotyping-by-sequencing) have been used to obtain SNPs at a high density for better mapping resolution [54]. In some cases, the genome sequence of the host can be used to select candidate resistance genes based on physical proximity to a mapped locus or marker. The new mapping approaches, suitable for elite lines and adapted cultivars, enable the screening of a wider variety of genetic resources and avoid the problems associated with inter-specific crosses. An excellent review of genetic resistance to cereal cyst nematodes in wheat and barley is available [20]. Resistance to _Meloidogyne graminicola_ in rice is important because all cultivars generally are susceptible to this nematode. Genetic resistance from _Oryza glaberrima_, an African cultivated rice, and _O. longistaminata_, a perennial wild rice relative, have been introgressed into adapted cultivars, although expression of resistance is attenuated in progeny of the latter [55]. Nevertheless, _O. glaberrima_ confers nematode arrest at different stages in the infection process, providing different mechanisms of resistance for deployment. Recently, 332 accessions from a rice diversity panel, comprised of elite lines and landraces from 82 countries and representing five rice subpopulations [56], was screened for resistance to _M. graminicola_ under controlled conditions [3]. Strong resistance was manifest in two accessions as absence of gall formation and very few females after _M. graminicola_ infection. A GWAS approach revealed 11 QTL associated with gall number. Five of the QTL, located on Chr 3, 4, and 11, co-segregated with eight SNP markers. The authors concluded that two loci on Chr 3 and Chr 11 were close to previously reported loci. Another source of resistance to _M. graminicola_ was identified in _O. sativa_ cultivar Zhonghua 11, from the Chinese Academy of Agricultural Sciences rice breeding program [55]. This cultivar displayed reduced reproduction, fewer feeding sites and underdeveloped giant cells, compared to susceptible cultivar IR64. Resistance functioned in both flooded and dryland(aerobic) field conditions, and appeared to be dominant in F1 progeny of a cross between Zhonghua 11 and IT64. Zhonghua 11 shows promise as a source of an effective and readily transmissible trait, although, as pointed out by the authors, further testing is required. Native sources of resistance to cereal cyst nematodes include the _Cre_ genes from wild relatives of wheat or adapted cultivars. These have been deployed and combined for resistance against _Heterodera avenue_[14]. The wheat DArT array was used to discover markers for new nematode resistance loci in 107 spring wheat genotypes from the CIMMYT semi-arid wheat screening nursery [14]. Genotypes grown in this nursery are scored for yield under low or high rainfall (depending upon the year) in soil containing nematodes, root rot pathogens, and other biotic challenges. The study revealed three DArT markers for four novel _H. avenue_ resistance QTL. In a separate study, resistance to _H.