Latency- and defense-related ultrastructural characteristics of apple fruit tissues infected with Botryosphaeria dothidea

Apple fruit tissues infected with Botryosphaeria dothiriea were examined by transmission electron microscopy using susceptible cv. Fuji and resistant cv. Jonathan. Immature (green) and mature (red) fruits of cv. Fuji with res tricted or expanding lesions were also examined to reveal subcellular chara cteristics related with latent and restricted disease development. In infec ted susceptible mature fruits, cytoplasmic degeneration and organelle disru ption commonly occurred, accompanying cell wall dissolution around invading hyphae. Cell wall dissolution around invading hyphae in subepidermis was r are in immature, red halo-symptomed cv. Fuji and resistant cv. Jonathan fru its. In infected immature fruits of cv. Fuji, presumably at the latent stat e of disease development, cellular degeneration was less severe, and invadi ng hyphae contained prominent microbody-lipid globule complexes or the depo sition of thin electron-dense outer layer around cell wall of intercellular hyphae. Both mature fruits with red halos and resistant apple fruits forme d cell wall protuberances at the outside of cell walls. In addition, electr on-dense extramural layers were formed in the resistant apple fruits. Aberr ant hyphal structures such as intrahyphal hyphae were found only in resista nt fruit tissues, indicating the physiologically altered fungal growth. The se ultrastructural changes of host tissues and fungal hyphae may reflect th e pathogenesis of apple white rot under varying conditions of apple fruits.