Abstract
Gymnosporangium Haraeanum SYD. is one of the oriental species of pear rust fungi and passes its life-cycle developing O and I on the Japan pear and III on the Juniper-tree (Juniperus chinensis and J. chinensis var. procumbens).
In the later stage of the pear rust disease we, not infrequently, observe the occurrence of natural infections wholly devoid of aecia. Following study was made to find out how such things happen and to clarify the sexual behaviour in the fungus.
Numerous monosporidial infections were obtained by sowing the sporidia sparsely on the pear leaves. Every monosporidial infection formed spermogonia, which excreated spermatia-containing nectar. At the end of about eleven weeks after inoculation, a large majority of such infections remained sterile, i. e. failed to produce aeciospores when they were kept in isolated condition; while the better part of those infections to which nectar containing spermatia had been transferred from several of the other infections began to develop aecia within about sixteen days after the treatment and then they behaved themselves normally in the course of the production of aeciospores. When a few monosporidial infections occurred close together, mixing of nectar resulted from coalescence of them and in such cases also aecia were produced in high percentage. Filtered spermogonial nectar which was free from spermatia had no such function as to induce the formation of aecia. Thus, the spermatia of the fungus have a role of supplying to the mycelia of monosporidial infections the opposite sex element. The fungus is heterothallic in a broad sense.
