High nitrogen supply is known to affect the morphology of ectomycorrhiza (ECM). Changes include
wall thickenings of outer epidermal and cortical cells upon contact by hyphae. However, wall thickenings, containing lignin and callose, are known plant responses induced by pathogenic fungi. Therefore, thin longitudinal sections of non-ectomycorrhizal and ECM roots of Picea abies - Hebeloma crustuliniforme grown at low (7.6 mM) or high ammonium concentrations (44.9 mM) in growth pouches were stained for callose with aniline blue and observed with a fluorescence microscope. Callose was not located in non-ectomycorrhizal rootlets nor in ECM rootlets grown at low ammonium concentrations. However, callose was located in the outer walls of epidermal and cortical cells of ECM grown at high ammonium concentration, supporting the hypothesis that, under these conditions, ECM symbioses are disturbed and that plants respond with similar changes induced by pathogenic fungi.