Genes for disease resistance from native populations

Genotypes of hosts' resistance cannot be completely revealed by hosts' phenotypes, because incompatibility of host to parasite is epistatic to compatibility. The identifiable part of host genome depends on parasites' genotypes used for screening. Recessive resistance of a heterozyous plant is also unidentifiable. In wild plants heterogeneity prevails and different plants carry different combinations of genes. The same picture is found in the parasites. Genes for resistance are not durable, and additional genes are being mobilized, usually in a wasteful manner, in the search for a source that will resist everything. The use of single relevant cultures reveals single ''building blocks'' of resistance from which a complex resistant genotype may be constructed. Single genes are also needed for more sophisticated gene management: multi-line, gene cycling, deployment and varietal mixtures. The key to identification of single genes is specific selecting cultures, which can be collected or generated by crosses. Genetic analyses of resistance are space- and time-consuming. Computerized methods use hosts' reactions and describe hosts' genotypes, pointing out interesting genotypes for further use and study. Field resistance, shown to be differential, probably obeys similar rules as specific resistance. Centres for the evaluation of disease resistance would improve international co-operation and strategy, and minimize the risk of introduction of new pathotypes. Evaluation centres would accumulate local variation of pathogens and test for resistance that might be needed there and later also elsewhere. Preservation of native germ plasm should be dynamic in nature, where possible in nature reserves and controlled grazing areas. (author)