1980 Volume 46 Issue 5 Pages 607-614
The distribution of the causal fungus, Fusarium oxysporum f. sp. lagenariae Matuo et Yamamoto, in the vascular bundles of the loculus and placenta as well as seed coats of infected fruits was studied with the aim of tracing the direct invasion through the vascular bundles to the seeds. The infected fruits which were borne on diseased stems and did not show any decay of tissues but showed discoloration of the principal vascular bundles in the epicarp caused by the causal fungus penetration were used as materials for the observations. Presence of the causal fungus in the loculus and the placenta was confirmed by placing the tissue sections on solid Komada's medium, a selective medium for quantitative isolation of Fusarium oxysporum, and observing the development of the causal fungus. A distinct vascular bundle extending from the hilum through the raphe was observed in the seed coats. This bundle is known to be one of the anatomical features of the seeds developed from anatropous ovules such as the seeds of bottle gourd. Out of about 1000 seeds harvested from the infected fruits mentioned above, 23 seeds were proved to be infected with the causal fungus. In 6 seeds among the infected seeds, chlamydospores which presumably had originated from the causal fungus were observed in the vessels and in the parenchyma tissues of vascular bundles of the seed coats. Their diameter was 7.6μm in average. While the diameter of the chlamydospores of the causal fungus cultured on apricot medium was 7.5μm. Thus both chlamydospores showed nearly identical shape and diameter. The chlamydospores in the seed coats germinated when the infected seeds were incubated on Komada's medium at 25 C for 24hr. The results mentioned above suggested the possibility of direct invasion of the seeds by the causal fungus through the vascular bundles.