VARIABILITY IN PATHOGENICITY OF MELAMPSORA-ALLII-POPULINA EXPRESSED ON POPLAR CULTIVARS

A collection of 42 isolates of Melampsora allii-populina was establish ed. Fourteen isolates were collected from various poplar cultivars ove r several French localities and 28 isolates from the alternate hosts ( Allium spp., Arum spp. and Muscari comosum) obtained from nature or af ter inoculation. These isolates were inoculated in the laboratory on a large range of poplar cultivars belonging to several species. No isol ate proved pathogenic on all cultivars and many qualitative interactio ns were observed between isolates and cultivars. This is the first evi dence for the existence of physiological races within this fungus. No link was found between pathogenicity on poplar and on the alternate ho sts. Among the inoculated cultivars, at least three were susceptible t o all isolates (Beaupre, Candicans and Robusta), whereas the following showed qualitative reactions to the inoculated isolates: Altichiero, Carpaccio, Cima, Fritzi Pauley, I 154, Isieres, Luisa Avanzo, NL 2842, Rap and Spijk. Race-specific resistance to M. allii-populina (i.e. re sistance to some races of the pathogen, but not to the others) has bee n found in two North American species (Populus deltoides and Populus t richocarpa) which have never coevolved with this fungus. The results a re discussed in comparison with Melampsora laricipopulina.