Abstract
The epidermal cell of fruit and leaf of satsuma mandarin (Citrus unshiu), which was penetrated by the hypha of Diaporthe citri (Faw.) Wolf, granulated and turned brown in color within 1 and 2 days after inoculation. Further granulation and browning of cells occurred around the initially browned epidermal cell. A group of necrotic cells consisted of 3 to 5 cell layers was then formed in a semi-spherical shape around the initially damaged epidermal cell. Subsequently, each cell in one or two layers adjacent to the necrotic cells became a pair of cells through a cell division. The cell division was repeated to make up a callus consisting of 10 to 12 cell layers around the necrotic cells. Thereafter, periderm was formed between the necrotic cells and the callus. Thus the melanose spot was finally formed by the three parts of a group of necrotic cells, a callus and a periderm, and invaded tissue was demarcated histologically. This phenomenon means self-protecting reaction responsed to the stimulation of citrus epidermal cell with penetration of D. citri, and melanose spot was formed as a mechanical barricade tissue by the self-protecting reaction. Star melanose and scar developed also in a similar fashion to that of melanose spot, consisting of a group of necrotic cells, a callus and a periderm. It seems that the self-protecting reaction started by response to the local damages resulting in dead cells of citrus by penetration of pathogens, permeation of drugs and mechanical injurys.
