NATIVE RANGE: East Asia (China, Japan)

FIRST FINDING IN SLOVENIA: before 2010

PATHWAYS: likely transported with fruits; secondary spontaneous spreading

POSSIBLY TO FIND: June-October

DESCRIPTION: Summer fruiting bodies (uredinia) of al-der rust look like small orange pustules on the under-side of leaves, which excrete orange coloured summer spores. These fruiting bodies can be so dense that they cover the entire underside of leaves and cause them to fall off prematurely. This fungus overwinters in the form of summer spores, or as mycelium inside alder buds. In Europe, it rarely forms winter fruiting bodies (telia) which form basidia with basidiospores. These infect larch (Larix spp.) trees on which spring spores develop in the follow-ing season.

HOST PLANTS: The disease is common on grey alder (Alnus incana), rarely also on common alder (Alnus glutinosa).

STATUS: Widespread throughout Europe. In Slovenia spreading, the severity of the infection differs from year to year, depending on the weather (high humidity increases infection pressure).

SIMILAR SPECIES: several related fungi cause rust on alder leaves: Melampsoridium alni, M. betulinum and M. carpini. For a certain identification, either microscopic analysis of summer fruiting bodies is needed or a molecular analysis.

SOURCE: Field Guide to Invasive Alien Species in European Forests