Rusts on Alyxia oliviformis (maile) (Apocynaceae)

Uromyces alyxiae is a microcyclic rust known only in the telial state.  Its host, known commonly as maile in Hawai‘i, is an endemic forest shrub or liana prized in Hawaiian culture for the fragrantly-scented bark of its stems.  It is widely used for decoration and leis at festive occasions.  The rust was described by Stevens (1925) as an endemic species, and it is still considered as such.  However, collections and reports of similar, as yet little studied, rusts on other species of Alyxia in the Pacific suggest that our understanding of these rusts is not complete as discussed below.  Infection is visible as clusters of telia on hypertrophied spots on under surfaces of leaves and on gall-like growths on stems.  Telia on leaves are accompanied by sunken, yellow spots on upper surfaces.  Recently, a similar rust was found on Alyxia ruscifolia in Queensland, Australia, and was described as a variety of U. alyxiae (as U. alyxiae var. australiensis) (Tierney and Gardner, 1992).  The discovery of the Australian rust further illustrates the biogeographic connection between the native rust floras of Australia and Hawaii discussed under Acacia rusts.

Undersurface of maile rust-infected leaves.

  Distinct, sunken yellow spots on the upper leaf surfaces corresponding to telia on hypertrophied tissue on the lower surface are typical of maile rust.

.  Closeup of maile rust telia occurring as clusters on hypertrophied tissue on the leaf undersurface. Maile rust occurs only in the telial state.

Maile rust on a leaf midrib.

  Teliospores of maile rust (Uromyces alyxiae var. alyxiae) from Hawaii.

  Maile rust occurring as a stem gall.

Following karyogamy in the teliospore, the diploid nucleus migrates into the basidium where meiosis occurs.  A single detachable basidiospore is formed at or near the end of the basidium into which the four meiotic nuclei migrate, resulting in a tetranucleate basidiospore (Gardner, 1987).

A pair of haploid nuclei (n + n condition) in a young Uromyces alyxiae teliospore.

  A single diploid nucleus in a mature teliospore following karyogamy.

Upon germination of the teliospore, a single terminal basidiospore is produced.  The function of the basidiospore is modified as compared with the function of "typical" basidiospores since it contains all four meiotic nuclei.

Although the basidiospore of maile rust is deciduous, it frequently begins to germinate prior to release from the sterigma by production of a narrow germ tube (small arrow).  Note the lengthy, sinuous basidium (large arrow) arising from the apically germinating teliospore.

      Diploid nuclei migrating into developing germ tubes (basidia) following karyogamy in the teliospore.

   The series of four meiotic nuclei in germ tube (basidium) (arrows) prior to formation of the basidiospore.

   Following meiosis, the four meiotic nuclei migrate toward the developing basidiospore.

   The four meiotic nuclei within the basidiospore.

Uromyces alyxiae var. australiensis was found on leaves and stems of Alyxia ruscifolia in the Bunya Mountains of Queensland, Australia:

Undersurface of an infected leaf.  Although the telia are associated with hypertrophied tissue, they occur singly rather than clustered in a group as with the Hawaiian variety.  The ragged edges of torn host epidermal are prominent surrounding the telia.  Telia may occur on the upper leaf surface as well as the lower, whereas telia of the Hawaiian rust typically occur only on the lower surface with corresponding sunken, yellow spots as shown above.

  Teliospores of U. alyxiae var. australiensis are similar to those of U. alyxiae var. alyxiae, but are somewhat larger and more robust appearing.  The apical wall thickening is prominent.

Another, microcyclic, obviously closely related rust, Puccinia alyxiae, is known on Alyxia sp. from New Caledonia.  As suggested by its placement in the genus Puccinia, the teliospores of this rust consist of two cells, as opposed to the single-celled teliospores of the genus Uromyces. However, despite this difference, these genera are often considered to be closely related to one another.

Telia of Puccinia alyxiae on leaf undersurfaces of a herbarium specimen of the Alyxia sp. host from New Caledonia.

      Two-celled teliospores of P. alyxiae.

References