Wilt of Koa (Acacia koa)

Acacia koa is the second most abundant overstory tree in mid to upper elevation Hawaiian forests.  Not only is this tree extremely important from an ecological standpoint, it was highly significant in ancient Hawaiian cultural practices and ceremony, being the tree sought after for constructing large dugout oceangoing canoes.  Koa is also distinct among endemic species in being of commercial importance as a timber source.  Its wood has a distinctive, deep grain and coloring that make it highly prized for furniture, paneling, veneer work, and smaller woodworking crafts.

A number of rust fungi attack koa that are discussed in detail on the "rust" page.  Other koa diseases have been summarized, most of which are of lesser overall importance to the survival of the tree.  However, a virulent wilt disease has become prominent in recent years that appears to threaten koa stands in areas such as the Mauna Loa Strip region of Hawai‘i Volcanoes National Park (HVNP).  The disease is not limited to older, environmentally stressed, or senescent trees as are some forest diseases, but is capable of killing otherwise young, vigorous trees.

I had earlier (Gardner, 1980a) identified a seedborne vascular wilt fungus, Fusarium oxysporum f. sp. koae, as the cause of a seedling wilt of koa, and suggested that the fungus might also be responsible for the rapid death of older trees.  Fusarium oxysporum is a large, widely distributed species of vascular wilt fungi of highly significant economic importance to agriculture.  The species contains numerous specific forms, or "formae specialis" (f. sp.) which resemble one another morphologically, but are distinguished by their pathogenicity to individual hosts, or groups of closely allied host species.  As their name implies, these fungi are typically soilborne and  systemically invade the vascular system through the roots, causing a rapid and irreversible wilt.  Development of resistant varieties of crop and ornamental plants is the most practical approach to the control of these diseases in plants under cultivation.  No means of control is known for plants in a natural setting.

My graduate student, Robert Anderson, is currently investigating this possibility.   He is finding positive preliminary correlation between dying trees and the ability to recover F. oxysporum and grow it in culture.  We have demonstrated the pathogenicity of the recovered F. oxysporum to koa demonstrated in seedling inoculation tests in which plants become wilted and die.  The disease is radiating from certain active centers, causing rapid wilting and death of trees of all sizes.  Former, apparently inactive sites of koa decline also have been identified within mature stands of koa in HVNP. 

         Dying koa trees in upper elevation (5,000-6,000 ft.) forests of Hawai‘i Volcanoes National Park.

   A wilting koa sapling showing rapid onset of symptoms.  Previously vigorous koa trees of all sizes may show symptoms of decline and death.  

  Fusarium oxysporum in pure culture, showing the purple coloration typical of this species.

     Production of chlamydospores (resting spores) of F. oxysporum f. sp. koae by macroconidia (left), and in an old culture.

       Results of inoculation tests demonstrating the pathogenicity of F. oxysporum f. sp. koae to koa.  Roots of inoculated seedlings (severely wilted) were dipped in an aqueous spore suspension and replanted in soil, whereas control seedlings (healthy appearing) were root-dipped in water.  Typically, plants developing symptoms of the wilt disease do not recover, and eventually die.

   Although the vascular wilt Fusaria typically enter their hosts through the roots, they do not cause a  rot of the roots themselves, as do other types of root pathogens, as is shown by the healthy root systems of these wilting seedlings.  From the vasular system of the roots, the fungus becomes distributed systemically throughout the vascular system of the above-ground portions as well.

Although the koa dieback at sites in the Mauna Loa Strip region of Hawai‘i Volcanoes National Park, as illustrated above, has received the most attention, other incidents of koa death have been reported elsewhere around the state.  The cause, or causes, for this phenomon have not been investigated in most instances.  Therefore it is not known at present whether a single disease is repsonsible, or whether several different factors are involved.  A fungus, identified as F. oxysporum, was cultured from internal wood sections of a large branch of such a dying tree on a homeowner's property in the Mt. Tantalus region of Honolulu.  The fungus was cultured and demonstrated to be pathogenic to koa in root-dip inoculation tests on seedlings as described above.  Although it appears to have the same affinity to koa as does Fusarium oxysporum f. sp. koae in Hawai‘i Volcanoes National Park (Gardner, 1980a), it is not yet known whether the fungus from Tantalus actually represents the same fungus.

   A cross section of a large branch of a koa tree growing on a homeowner's lot on Mt. Tantalus, Honolulu.  Fusarium oxysporum was cultured from the log (arrows).  The fungus was demonstrated to be pathogenic to koa, and thus the probable cause of death, in koa seedling inoculation tests.