Abstract
The percentage of seed infected with Fusarium oxysporum f. sp. lagenariae Matuo et Yamamoto, was estimated as 2.0-16.8 in the seed lots assessed by sowing them in soil and observing symptom appearance on the seedlings. The seed infection varied according to seed lot originated from different mother fruit. The seeds tested were collected from diseased fruits after allowing their mesocarps to decompose completely in the fields following the routine method practiced by seed collectors for market. The pathogenic fungus was isolated from various parts of infected plant in order to clarify the route by which the fungus moved from diseased stem to the seeds. The fungus was detected consistently from the discolored xylem tissues of stems and peduncles. In unripe fruits, the invasion of the fungus was restricted to the base of peduncle, and no fungus was detected from any other part of the young fruit. In mature fruits borne on the diseased stem, however, brownish discoloration was found in the principal vascular bundles and the fungus was readily isolated from those parts. It was also isolated from tissue sections obtained from the mesocarp including finer vascular bundles as well as from the seeds. The result shows the possibility of direct invasion of the pathogen to the seeds through the vascular bundles. But the level of infected seed resulting from such direct invasion was quite low as a whole. With advance of maturity of diseased fruit, the fungus latent mostly in the discolored vascular bundles multiplied saprophytically within the decaying mesocarp and then penetrated subsequently into seed coats. Some of them penetrated even into embryos. High rate of seed infection, 16.8-46.7%, resulted from decomposition of mesocarps. The fungus was deep-seated in the seed-coats from its epidermis to parenchyma tisssues in the forms of mycelia and chlamydospores. On the contrary, the fungus adhering to the surface of seed coats was very low in its population, and almost all of them died out during its storage for 4 months.
