Nonhost plant species as donors for resistance to pathogens with narrow host range I. Determination of nonhost status

Summary In crop species where disease and pest problems are serious, a common strategy is to attempt to introduce genes for resistance from more or less related (wild) species by means of interspecific crosses. Little attention has been given to the question, whether such a donor species should be a host or a nonhost if the target pathogen is a highly specialized micro-organism.In this paper it is discussed that, apart from obvious host and obvious nonhost relationships, various intermediate categories are to be discerned.Aspects like small numbers of accessions, taxonomic problems, environmental conditions and the age of the inoculated plants further hamper clear-cut verdicts whether a plant species is a host or nonhost.It is argued that, at least in the case of powdery mildew and rust fungi, histological observations are helpful in determining whether a predominant resistance of a plant species is based on avoidance, on pre-haustorial or on post-haustorial defence mechanisms. The former two mechanisms are typical of nonhost relationships, the latter one is the predominant mechanism of major genic host resistance, although exceptions occur. The questions for plant breeders would be which mechanism would provide the most effective protection of the crop and which would be better suitable for transfer to crop species.