Decline symptoms on roots of Quercus robur

A comparative study by light and scanning‐electron microscopy of fine and coarse roots from healthy and declining oaks demonstrated a progressive deterioration of fine roots and mycorrhizal systems on mature trees in decline. Histopathological events such as infection of cortical and vascular root tissues by parasitic fungal hyphae leading to the impairment of woody roots are described. Distinct disease symptoms such as necrotic lesions on small diameter roots and callussing wounds on coarse roots are documented, supporting the hypothesis that Phytophthora spp. are involved in oak decline.