Abstract
Cuttings of poplar hybrid NE-388 (P. maximowiczii Henry × trichocarpa Torr. & Gray) were inoculated with mycelium of Cytospora chrysosperma (Pers.) Fr. Bark samples for histological study were taken at varying times after inoculation from slowly enlarging cankers, blackstem, control wounds, and uninoculated healthy plants. Histological examination revealed that slowly enlarging cankers are characterized by hyphae in the xylem and necrosis of the cambium ahead of visible bark symptoms, by large- and small-diameter hyphae both between and within host cells, and by hyphal aggregations associated with the leading edge of bark colonization. The blackstem symptom is characterized by rapid colonization of the cortex and phloem by large-diameter intercellular hyphae followed by the digestion of cell contents by small-diameter intracellular hyphae. Histochemical tests indicate degradation of pectic substances, loss of cellulose and starch, and deposition of wound gums. Lignification is not involved. Host response mechanisms associated with wounding and pathogen attack are similar. The underlying common denominators are the formation of a nonsuberized impervious tissue, followed by formation of necrophylactic periderm and redifferentiation of new phellogen and cambium.
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