Structure of Melampsora larici-populina populations on wild and cultivated poplar

Abstract

Populations of Melampsora larici-populina, causal agent of poplar leaf rust, have been collected during 1992–1994 in three types of sites: natural stands of Populus nigra in the south-east of France; cultivated poplar stands in the northern half of France, including mainly interspecific and exotic hybrids of poplar; and two overseas populations collected from Northern Ireland and Western USA. The pathotype of each rust isolate was determined by inoculating a differential set of poplar clones. The number of virulences per isolate and the frequency of each virulence in each population were compared, according to the sites and to the clones from which the isolates were collected.

Several diversity indices were applied to the populations. The α and Simpson indices were retained since they described best the richness and the evenness, respectively, of the populations, and they were the least sensitive to sample size. The complexity of races was lower in the natural stands of P. nigra than in the cultivated poplar stands. Populations of M. larici-populina collected on Populus × euramericana ‘Robusta’ and P. nigra ‘italica’ (Lombardy poplar) presented a high richness and evenness, confirming the value of these clones for describing race populations. Populations from Western USA presented a very low diversity, which is in accordance with the recent introduction of the pathogen in North America. These results suggest that the race populations of M. larici-populina are mainly influenced by the structure of the host populations.