AbstractIn the Heidelberg area of southwestern Germany declining trees ofAlnus glutinosa(L.) Gaertn. andPopulus tremulaL. as well as stunting shrubs ofCrataegus monogynaJacq. were observed. In all 3 species mycoplasmalike organisms, which appear to be the causal agents of the disorders, were detected by fluorescence and electron microscopy. The diseases of alder and aspen seem to be lethal while the hawthorn apparently is less affected.