Assessing linseed (Linum usitatissimum) resistance to Alternaria linicola using a detached cotyledon assay

Summary Levels of resistance to the seed‐borne pathogen Alternaria linicola in linseed and Linum accessions were characterised using an in vitro detached cotyledon bioassay. The response of four marketed varieties to seven isolates showed that differences between the isolates accounted for the majority of the variance. There was no significant interaction between varieties and isolates so that levels of response to the isolates showed a consistent trend. In a more comprehensive study, 102 different linseed and Linum accessions were challenged with an aggressive and a non‐aggressive isolate. Differences between accessions were significant (P > 0.001). Much of the material gave a moderate response to the two isolates although there was a continuous distribution of resistance from susceptible to resistant. The response of a sub‐set of tested material to the two isolates in a whole plant assay was positively correlated to the scores achieved in the in vitro bioassay, indicating that the resistance response of material could be predicted by the in vitro bioassay. Since large isolate‐line interactions with respect to resistance scores were not observed in either test, resistance appears to be polygenically determined. The results suggest that an improvement in the resistance of linseed to A. linicola would be possible and beneficial.