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Histopathology of Scar Skin Disease of Apple. P. E. Parker, Graduate Assistant, Department of Plant Pathology, University of Massachusetts, Amherst 01002; G. N. Agrios, Associate Professor, Department of Plant Pathology, University of Massachusetts, Amherst 01002. Phytopathology 65:707-713. Accepted for publication 31 January 1975. DOI: 10.1094/Phyto-65-707.

Scar skin disease of apple, a fraft-transmissible disease of unknown etiology, produces characteristic external and internal fruit symptoms. Early symptoms consist of raised, red, pimple-like areas. Red coloration in these areas precedes the appearance of brown necrotic scar tissue, which later replaces the red pigmented tissue in epidermal and subepidermal layers of cells. At certain points on the fruit surface, the tissue appears raised due to hyperplastic activity beneath the epidermis. This results in formation of “pockets” of hyperplastic and, later, necrotic cells underlain by phellogen and periderm. Pressure from these hyperplastic tissues and the underlying periderm causes fracturing of the cuticle. Pocket formation occurs only at scattered points in the affected tissues, while the remaining scar tissue consists primarily of necrotic epidermal cells. The subepidermal layers underlying the necrotic epidermis also become necrotic as the fruit matures. Beneath the necrotic epidermal and subepidermal areas, several layers of periderm form, resulting in the rupture of the cuticle.