CHANGED PATTERNS OF RESISTANCE IN A POPULATION OF LINUM-MARGINALE ATTACKED BY THE RUST PATHOGEN MELAMPSORA-LINI

1 Various resistance phenotypes of L. marginale can be identified acco rding to their responses to nine distinct pathotypes of the rust fungu s M. lini. The relative frequency of these phenotypes was monitored on an 8-m x 8-m permanent plot established at Kiandra in southern New So uth Wales, Australia. 2 Over the period 1986-92 there was a marked cha nge in the resistance structure of this population. This change was pa rticularly associated with a major epidemic of rust occurring in the s ummer of 1989. The dominance of the host population by three resistanc e phenotypes prior to the epidemic was subsequently lost. 3 A similar change in resistance structure was detected in random samples taken in 1981 and 1991 from a different part of the same population. The chang es in resistance in both the random-sample and the permanent plots wer e of no obvious adaptive value. 4 The distribution of the commonest re sistance phenotypes on the permanent plot originally showed distinct p atterns of aggregation which disappeared after the 1989 epidemic. 5 It is possible that the changing frequencies of particular resistance ph enotypes could be explained by linkage between resistance genes and ot her traits that are under more intense selection, Alternatively, the c hanging frequencies could result from the combined effects of the pres ence of a pathogen pathotype which is, at least at certain locations, virulent on all host phenotypes, recruitment patterns in the host popu lation and low pathogen transmission efficiencies.