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Cytology and Histology

Pathogenicity and Histopathology of Botryosphaeria dothidea on Apple Stems. E. A. Brown, II, Former graduate student, Department of Plant Pathology and Plant Genetics, University of Georgia, Athens 30602, Current affiliation of senior author: Department of Extension Plant Pathology, University of Georgia, Athens; F. F. Hendrix, professor, Department of Plant Pathology and Plant Genetics, University of Georgia, Athens 30602. Phytopathology 71:375-379. Accepted for publication 11 August 1980. Copyright 1981 The American Phytopathological Society. DOI: 10.1094/Phyto-71-375.

In vitro Botryosphaeria dothidea grew best at 29 C under fluorescent lights and produced mature pycnidia after 12 days. Apple stem and fruit isolates did not differ in growth and development from 25–35 C. Lesions developed on both wounded and unwounded apple stems inoculated with B. dothidea, but wounded trees became infected more readily and had larger lesions than unwounded trees. Conidia of B. dothidea germinated and germ tubes penetrated pruned and punctured areas on apple stems within 4 and 6 hr, respectively, after inoculation. Conidial germ tubes consistently grew toward injured tissue on stems. In stems wounded by cutting or injured by cold or heat, the mycelium was first associated with the disrupted cortical tissue. Rapid, unobstructed vertical growth occurred once the mycelium was established in the xylem vessels. Hyperplasia of parenchyma cells initially restricted movement of B. dothidea into the xylem of heat-treated trees and resulted in only 56% establishment of cankers. Cankers developed on 100% of cut-wounded and cold-injured apple stems and on 12% of the unwounded, inoculated stems. Proliferation of parenchyma cells restricted infection to the cortex on unwounded stems which became infected via the lenticels.