Melampsoridium - rusts of alders
Fig. 1. Melampsoridium sp., sori of the uredo-form on the lower leaf-surface.
Leaves of alders are commonly infested by rusts belonging to the genus Melampsoridium. In summer, the lower surface of the leaves becomes covered by numerous orange-yellow pustules (uredinia) of the rust producing vast numbers of uredospores (fig.1). In the invasive species M. hiratsukanum there is host alteration with European larch, where another spore type is produced.
On alders of all ages in natural and urban habitats.
Widespread in Europe; very common in Alnus incana-sites, less common on A. glutinosa.
Impacts have generally been low up to now, even for the invasive species M. hiratsukanum, which entered Europe from the North-East during the 1990ies. However, a repeated loss of the majority of leaves already by August can be expected to weaken the trees.
None, if the uredinia are present.