Varietal mixture of poplar clones: Effects on infection by Melampsora larici-populina and on plant growth

The effects of poplar varietal mixture on infection levels caused by Melamp sora larici-populina and on plant growth were studied during two consecutiv e growing seasons. The varietal mixture consisted of an Intimate mixture of three poplar clones, Populus x interamericana 'Boelare','Rap' and Populus candicans 'NNT', expressing different race-specific resistances. Infection levels of M. larici-populina were assessed in monoclonal and polyclonal plo ts. Plant heights were determined at the end df each growing season. Infect ion levels in the growing season of the first year were not significantly d ifferent between monoclonal and polyclonal plots at the end of the epidemic . During the second growing season, a naturally occurring complex pathotype , able to infect clones 'Boelare' and 'Rap', invaded some plots prematurely and changed the composition of pathogen population. During this season, fo ur infection assessments were conducted. Infection reduction in polyclonal plots compared with monoclonal plots was observed on 'Boelare' and P. candi cans plants. Nevertheless, no significant difference was observed for growt h between monoclonal and polyclonal plots. Biological factors which may be involved in the reduction of infection levels in polyclonal plots are dilut ion of susceptible individuals, physical barriers caused by resistant plant s and cross-protection. A high auto-infection ratio may have partly hidden the positive effects of the clonal mixture.