Dark Mildews

The family Meliolaceae includes about 45 genera and well over a thousand species of epiphyllic, parasitic fungi, commonly referred to as the "dark mildews" (Goos and Anderson, 1972).  Although these fungi are widely distributed, they are most prevalent in the tropics.  Accordingly, this family is well represented on the native vegetation of Hawai‘i.  All members of this group are thought to be obligate parasites, requiring a living host for existence.  The disease effects on the host leaves appear to be limited to the tissue covered by the fungal colonies on the leaf surface, however.  These fungi are characterized by a high degree of host-specificity, and the identity of the host, at least to family, is necessary for determination of the identity of the fungus.  Goos and Anderson (1972) compiled a comprehensive list of these fungi in Hawai‘i, citing in turn the comprehensive monograph of this group by Hansford (1961), which they referred to as a "monumental work" and offered this group of fungi as positive evidence that endemic fungi occur in Hawai‘i.  To this I would add that such examples are to be found among the rust fungi on endemic hosts as well (see the "rust fungi" page).

   Colonies of a typical dark mildew on a leaf surface of po‘ola (Claoxylon sandwicense), an endemic member of the Euphorbiaceae.

   A microscopic view of the mycelium of dark mildew on the leaf surface.   The hyphae of this group produce characteristic stubby projections termed hyphopodia (arrows) whose function is uncertain, but which, because of the species-specific shape and structure, serve as taxonomic features.

   Characteristically septate ascospores.  The number of cells in the ascospore, and the shape of the spore itself, serve as taxonomic characteristics used in separating genera within the family Meliolaceae.