The article determines the species composition of causative agents of plant leaf diseases of the genus Alnus Mill. for the Siberian region, that grow in the arboretum collections and urbanized environment. The author has identified four phytopathogens, their spread and harmfulness are specified; established the confinement of foliar fungal pathogens to the host plant species, its ontogeny phase, and abiotic environmental factors. It had been found a hyperparasite Ramularia uredinis (W. Voss) Sacc. on a causative agent of alder rust (Melampsoridium alni (Thum.) Dietel), for which this phytopathogen is a new host.