In 1921 Dr. F. L. Stevens spent nearly 5 months in Hawaii as a Bernice P. Museum fellow of Yale University compiling a comprehensive list of fungi in the Hawaiian Islands. The information was later (1925) published as Bishop Museum Bulletin 19 entitled Hawaiian Fungi. In this work, Stevens devoted particular attention to the Uredinales, listing each species according to its status, whether thought to be endemic, indigenous, or recently introduced, and to presumed region of origin of members of the latter group.
Stevens recorded 39 species of rust fungi, a number he considered notably small. This total included eight endemic (i.e., unique to Hawai‘i) and 10 "probably indigenous" species (i.e., native to Hawai‘i but found elsewhere as well). The paucity of species is thought to be a factor of the remoteness of Hawai‘i from sources of colonization rather than due to any unfavorable conditions for their development. Stevens also remarked on the infrequency of aecial states among the rusts present in Hawai‘i compared to occurrence of aecia in other locations, and stated that it appeared probable that the rusts had not come to Hawai‘i wind-borne, separate from their hosts, but most likely arrived with their hosts.
Few additional records were added until the Hawaiian rust fungi were reinvestigated in the 1980s to the present. Although several species have been added to the rust flora as a result of these recent investigations, the overall scarcity of rusts relative to the native vascular plant flora remains notable. Currently, of the 92 species known in Hawaii, 24 are recognized as native, 14 of which are probably endemic and 10 indigenous (Gardner, 1994a). Most of the endemic species are members of the form-genus Uredo or are reduced telial forms, being microcyclic or demicyclic.
Some of the species with telial states are unusual in their germination and nuclear behaviors. One of the unusual features is the production of swollen regions in the germ tube which represent apparently vestigial, permanently attached basidiospores. These structures germinate immediately to produce an extended germ tube. Thus, a fully germinated teliospore has a basidium with what appear to be lengthy branches. The "branches" function to penetrate the host and establish infection.
A second unusual aspect of Hawaiian rusts is the production of 4-nucleate basidiospores. The "typical" nuclear cycle of rust fungi includes a single diploid nucleus resulting from karyogamy followed by meiosis in the basidium. The four haploid nuclei are separated into individual cells by septation of the basidium, and each nucleus migrates into a basidiospore, resulting in four uninucleate basidiospores. Depending on the species, the basidiospore may remain uninucleate, or the haploid nucleus may undergo a mitotic division, resulting in a binucleate basidiospore. Although tetranucleate basidiospores have been reported in several rust fungi elsewhere, these usually occur as exceptional, abnormal cases caused by failure of the basidium to become properly septate following meiotic division, resulting in two haploid nuclei in the basidiospore. Each of these then undergoes a mitotic division, resulting in a basidiospore with four nuclei. In contrast, production of tetranucleate basidiospores (or morphological variations substituting for basidiospores, as described above) appears to be normal behavior in the nuclear cycles in six of the eight extant endemic Hawaiian rust fungi with known telial states. Significantly, in these species the tetranucleate condition results directly from meiotic division of a diploid nucleus rather than from mitotic division of haploid nuclei following meiosis. In three Hawaiian rusts, a single, detaching basidiospore is produced containing the four meiotic nuclei. Detachable basidiospores are not produced in the remaining three species. In the genus Endoraecium, with two endocyclic species, no basidiospores are formed, but teliospores germinate directly to produce penetration hyphae in which meiosis occurs. In Atelocauda koae, meiosis takes place in or near a vestigial basidiospore, in the broad hypha of the germinating teliospore. The four resulting nuclei are not separated by septa, but migrate together into the narrower, extended hypha which represents the germ tube of the "basidiospore" and serves as a penetration hypha.
The native rust fungi contribute to the many examples of unusual development and behavior characteristic of many organisms that have developed in insular systems in isolation from continental counterparts. Some of these examples are discussed below.
Rusts on Acacia koa (koa) (Fabaceae)
Rust on Alyxia oliviformis (maile) (Apocynaceae)
Annotated list of rusts and smuts in Hawai‘i