Acacia koa: A Review of its Diseases and Associated Fungi

Acacia koa (koa), among the most prominent overstory species of native Hawaiian forests, is affected by a number of diseases, including those caused by rust fungi, wood-rotting fungi, root-infecting fungi, and diseases categorized as seedling blights, foliar infections, and vascular wilts. A number of fungi occurring saprophytically on koa substrates are also known. Symptoms of other apparent diseases and disorders of unknown origin, or whose cause has not been clearly demonstrated, are often manifest as leaf spots or other foliar abnormalities. Endemic pathogens which have evolved with their host may be responsible for many of the heretofore observed disease conditions, and as such usually do not threaten koa populations. However, a condition tentatively referred to as "koa decline" may represent an exception. This condition is characterized by slow to rapid wilt and death of apparently healthy, vigorous trees of all ages, occurring in more or less well defined disease centers. This phenomenon is particularly prominent in upper-elevation koa forests on the slopes of Mauna Loa on the island of Hawai'i. Other apparent decline problems on O'ahu and elsewhere in the Islands have been reported, some associated with insects. The relationship of these observations to the koa decline on the Big Island has yet to be determined, and the question as to whether "koa decline" represents a complex of disease conditions or can be more closely attributed to a single, specific cause requires further study. Koa (Acacia koa Gray), endemic to the Hawaiian Islands, is the second most abundant overstory species in upper-elevation forests. It forms nearly pure stands in montane forests such as those on the slopes of Mauna Loa between approximately 4000 and 7000 ft (1230 and 2150 m) (Whitesell 1990). Koa is distinctive in the formation of crescent-shaped phyllodes which, in older trees, assume the function of the pinnately compound true leaves produced in juveni le stages of development. As an endemic tree, koa is of critical ecological importance, forming habitat for numerous native birds, insects, and other flora and fauna. As a nitrogen-fixing 56 leguminous species, koa is thought to account significantly for the nitrogen content of otherwise nitrogenpoor volcanic forest soils (Whitesell 1990). Koa is also prominently represented in early Hawaiian legends and culture, and economically is considered the most valuable of the common native timber species. Koa wood, sometimes referred to as "Hawaiian mahogany," can be highly polished to emphasize its deep reddish coloration and wavy grain, and is used for furniture, paneling, and such woodworking crafts as bowls and ukuleles. St. John (1979) recognized three distinct species as representing the genus Acacia in Hawai'i: A. koa Gray, A. koaia Hbd., and A. kauaiensis Hbd. Furthermore. within A. koa three varieties were recognized: A. koa var. koa, found on all major Hawaiian islands; A. koa var.latifolia (Benth.) St. John (=A. koa var. hawaiiensis Rock); and A. koa var. waianaiensis St. John; the latter two restricted to the islands of Hawai'i and O'ahu, respectively. Although the current treatment of A. koa (Wagner et al. 1990) combines all of these forms within A. koa, the distinction among them is still apparently sufficient to delimit the host ranges of host-specific pathogens, such as the rust fungi. Reference to St. John's classification is therefore useful to this discussion. Several observations of diseases and pathogenic fungi occurring on koa have been individually reported in the literature as they have been observed over the years (Raabe et al. 1981), or have not been formally published. It was of interest to assemble such published information, together with as yet unpublished observations of fungi and disease conditions ofkoa, to provide the basis for a discussion of the pathology of this pivotal species. Diseases caused by rust fungi Rust fungi, members of the order Uredinales, are so called because of the often brownish to orange appearance of the spore masses. These fungi produce perhaps the most widespread and prominent diseases of koa, collectively referred to as koa rust. Stevens (1925), originally described a single fungus, Uromyces koae Arth., as the cause of the disease, but five species of rust fungi. Koa:ADecadeq(Grou,th ~ four of which are considered endemic and one indigenous, are currently recognized on koa (Hodges and Gardner 1984, Gardner 1991), each with distinctive symptoms. These include three species now placed in the genus Atelocauda, A. koae (Arth.) Cumm. & Y. Hiratsuka, A. digitata (Wint.) Cumm. & Y. Hiratsuka, and A. angustiphylloda Gardner, and two species of Endoraecium, E. acaciae Hodges & Gardner and E. hawaiiense Hodges & Gardner. Atelocauda koae occurs commonly on Acacia koa var.latifolia on Hawai'i. and on A. koa var. koa on Kaua'i, O'ahu, and Maui. It has not been reported on Moloka'i, nor has it been found on A. koaia anywhere. Atelocauda koae is confined to young tissue, occurring most frequently on newly developing true leaves, phyllodes, and shoots of saplings, but also found on new shoots of older trees. It is evident as raised, powdery, brown leaf spots, 1-10 mm diameter, occurring singly or in groups, on both leaf surfaces. Heavy infection, in which spots coalesce to form large, irregular powdery blotches, can cause severe distortion of leaves, shoots, and small branches, leading to stem deformation (Hodges and Gardner 1984, Chen et al. 1996). Atelocauda digitata is known from all major islands except Lanai, although its presence on this island in upland forests would not be surprising. On Acacia koa var. latifolia, the conspicuous stages of this fungus cause "witches' -brooms" up to approximately 15 em tall, comprised of clustered, misshapen, abnormally thickened shoots developing from infected branches. The diseased tissue may be covered by powdery, brown spore masses that are easily rubbed off. Removal of witches' brooms is ineffective in controlling the disease, because the fungus invades branch tissues systemically. From one to many brooms may occur in a tree. Less conspicuous stages of A. digitata occur as small, brown, raised sporulating pustules up to approximately 1 mm diameter on the surface of otherwise normal phyllodes. Occasionally such spots are surrounded by larger chlorotic spots, causing them to be more conspicuous. Atelocauda digitata kills branch portions distal to the broom, but entire trees are typically not killed by this fungus alone. On Acacia koa var. koa on Maui and O'ahu A. digitata produces hypertrophy of flower and shoot tissue, resulting in malformation, but it does not produce witches' brooms typical of those on Acacia koa var. latifolia .Hodges and Gardner 1984). Atelocauda angustiphylloda is confined to Acacia koa var. latifolia and is limited in distribution on the island of Hawai'i, but visible in large koa trees on the upper Saddle Road. This rust forms large (up to 1 m tall) witches' brooms comprised of hypertrophied but abnormally reduced, much narrowed phyllodes almost circular in cross section, giving a "shoe-string" appearance (Gardner, in press). As with A. digitata, brooms are covered with powdery, brown spore masses. Although limited in distribution, large numbers of brooms (100+) may occur in certain mature, presumably susceptible trees. Host trees appear to tolerate infection by A. angustiphylloda, but loss of vigor and local tissue death may occur. Endoraecium acaciae, the most common of koa rusts, has been found onA. koa var. koa on Maui, Kaua'i, and Hawai'i, and on A. koaia on Hawai'i, Kaua'i, and Moloka'i. It has not been found on A. koa var.latifolia. This fungus produces conspicuous, profusely branched brooms up to 30 em long comprised of much-reduced, flattened phyllodes covered with spore masses. In apparent contrast to other koa rusts, E. acaciae may cause severe damage to the host, and heavy infections producing multiple brooms may kill the entire tree. Brooms of E. hawaiiense are similar to those of E. acaciae but are smaller, being seldom taller than 12 cm, and relatively inconspicuous. This fungus appears to be limited in distribution, having been found only at certain sites on O'ahu on A. koa var. waianaiensis and A. koa var. koa. An insufficient number of trees heavily infected with this fungus has been found to assess its potential for host damage. The two species of Endoraecium are further separated from one another by differences in spore morphology. Wind-distributed spores of the koa rust fungi are thought to be the principal means of longand shortdistance dispersal, although Leeper arid Beardsley (1973) noted the possibility that the koa psyllid Acizzia uncatoides (Ferris & Klyver) [=Psylla uncatoides (Ferris & Klyver)] may at least in part account for short-distance dispersal where populations of these insects are high. Wherever koa rust diseases are observed, evidence can be found of variability in susceptibility to infection among neighboring individual trees. Koa breeding programs directed at selecting such resistance may be the most effective control approach for rust diseases.