A histological study of apple roots affected by replant disease

Little is known about the microscopical symptoms of roots from apple trees suffering from replant disease. Roots were sampled from healthy trees and from such diseased trees in four orchards from May until October. Roots were stained for the detection of mycorrhizal infection and other roots were fixed, dehydrated, embedded in paraffin, sectioned, and stained in safranin and fast green in order to elucidate morphological details. Healthy tree roots possessed considerably higher frequencies of mycorrhizal infection than diseased trees during the entire growing season. Arbuscules and hyphae were very common, vesicles were sometimes present, and possible chlamydospores of Glomus radiatum (Thaxter) Gerd. & Trappe were found in several samples. Whereas roots from healthy trees were structurally intact, roots from declined trees had extensive sloughing away of the epidermal and cortical layers and the cortical cells possessed significant amounts of densely stained material. Nematodes and possible hyphae of Rhizoctonia, Phytophthora, and Pythium were found in roots of declined trees. Althugh the stele of these roots appeared unaltered, hyphae were sometimes observed in the vascular elements.