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Presymptom Histopathology of Peach Trees Inoculated with Botryosphaeria obtusa and B. dothidea. A. R. Biggs, Research scientist, Agriculture Canada, Research Station, Vineland Station, Ontario, L0R 2E0, Canada; K. O. Britton, Research assistant, Department of Plant Pathology, University of Georgia, Athens 30602. Phytopathology 78:1109-1118. Accepted for publication 23 February 1988. Copyright 1988 Department of Agriculture. Government of Canada. DOI: 10.1094/Phyto-78-1109.

Peach trees were wounded mechanically in April and July and inoculated immediately with spore suspensions of the fungal gummosis pathogens, Botryosphaeria obtusa and B. dothidea. Samples for histopathological study were taken at 7, 14, 28, and 56 days after inoculation, before the onset of macroscopic symptoms. At 7 and 14 days, there were no differences in lignin and suberin deposition or periderm and callus formation between uninoculated wounds and wounds inoculated with either fungus. Fungal hyphae were visible on the inoculated wound surface of xylem and bark and were colonizing the outer layers of necrotic tissue delimited by the wound reaction. By day 28, new callus tissue was being initially colonized by fungal hyphae located on the xylem surface beneath the nonsuberized ventral callus surface. The nonsuberized ventral region of callus was primarily parenchymatous without a ligno-suberized outer layer and was the focal point for fungal pathogenesis. By day 56, the breakdown of this tissue was directly associated with fungal hyphae in close ventral proximity. Direct fungal penetration, although occasionally observed, was not required to induce gum pocket formation, cambial alteration, and tissue breakdown. The formation of gum pockets was followed by the sloughing of the parenchymatous portion of callus external to callus xylem tissue as the gum pockets expanded. Sloughed tissues usually were colonized by fungal hyphae. Both fungi appeared to incite disease in a similar manner. There were no differences in the types of cells colonized or the extent to which various tissues were colonized, although the frequency of positive isolation of inoculated fungi was higher in April and May than in July and August. Plants wounded in July had a thicker phellem layer in the new periderm compared with those wounded in April, independent of whether or not the wounds were inoculated with Botryosphaeria.