Wettability of Poplar Leaves Influences Dew Formation and Infection by Melampsora larici-populina

The wettability of leaves of 60 poplar (Populus spp.) clones from sections Aigeiros (black) and Tacamahaca (balsam) and their hybrids was assessed by measuring the contact angle of calibrated water droplets on their abaxial leaf surface. The frequency and duration of dew on these clones were recorded in two nurseries. Black poplar leaves had the highest wettability, with relatively flat-shaped water droplets and a short drying time, in both the laboratory and nursery. Conversely, round-shaped water droplets on balsam poplars dried more slowly in the laboratory. In the nursery, dew was more frequent and persisted longer. First-generation intersectional hybrids behaved like balsam poplars, but successive backcrosses with black poplar (P. deltoides) resulted in clones with leaves that were more wettable and possessed less frequent and persistent dew. Infection by Melampsora larici-populina on two hybrid clones ('Robusta' and 'Beaupré'), assessed by the number of uredinia in the laboratory, was dependent on the duration of leaf moisture. Maximum infection occurred when water was present on the leaves for 12 h. Leaf wettability should be considered as an additional trait when breeding poplar for durable resistance to M. larici-populina, as this characteristic is likely to be a useful defense against all pathotypes of the pathogen.