<p>A two-year study was carried out on the ericaceous plant collection of the Botanic Garden of the Jagiellonian University in Cracow and the Rogów Arboretum of the Warsaw University of Life Sciences on the following plants: wild rosemary <em>Ledum palustre</em> L., leatherleaf <em>Chamaedaphne calyculata</em> (L.) Moench and American cranberry <em>Oxycoccus macrocarpus</em> (Ait.) Pursh. Diverse micromycetes species composition was specified on the leaves of tested ericaceous plants. The perpetrators of dying leaves were recognized, among which the dominant role was played by the necrotroph <em>Pestalotiopsis sydowiana</em>. Quantitative and qualitative comparative analyses of micromycetes in both locations were carried out, showing a comparable degree of plant colonization by these fungi in the botanic garden and arboretum. The study may be helpful in explaining the causes of dieback of protected and endangered plants in Polish flora.</p>