Even if citrus fruits are severely affected by melanose, they do not rot. This is considered to be due to some pre-inhibitins or phytoalexins induced in their tissues. The effect of extracts from leaves, twigs, fruit peel and fruit vesicles of citrus plants (satsuma mandarin and natsu-daidai) on pycnospore germination, germ tube elongation and disease development was examined. Among them, the peel extract showed the greatest inhibitory effect on pycnospore germination and germ tube elongation, followed by the extracts from leaf, fruit vesicle and twig in that order. A similar examination was carried out to prove the presence of inhibitory substances in the vascular bundle system of the fruit. The extract from the tissues directly under the disk had the greatest inhibitory effect, followed by that from the stem button. The extract from the vascular bundle in the fruit also had an inhibitory effect. When the inhibitory effects of extracts from exocarps, mesocarps, endocarps and vesicles of natsudaidai fruit were compared with each other, the extracts from vesicle and exocarp had an inhibitory effect at a 100-time dilution, but those from others did not have at the same concentration. The inhibitory effect of vesicle extract against pycnospore germination and germ tube elongation decreased with the period of fruit storage after harvest. Extracts from fruit peel and vesicle inhibited melanose even if they were diluted 100-1, 000 times, and the 100-time dilution also effective in controlling black rot, gray mold, anthracnose, and phoma rot when the fruit was treated with the 100 times diluted extract before inoculation.