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Koa Wilt
by J. B. Friday and Nicklos Dudley
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Koa wilt is a serious, often fatal disease of the native Hawaiian koa, Acacia koa. Trees affected with the disease rapidly lose their canopies and may die within a few months. Young trees less than 15 years old seem to be affected more often than old trees, and the disease is more often seen on trees planted below 2,500 feet elevation than on trees growing in the forest at higher elevations. Both koa and koaia (Acacia koaia ) are susceptible to koa wilt.
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Two year old koa seedling dead of koa wilt in a plantation in Hamakua.
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| Two year old koa seedling showing a canker caused by the disease.
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Same tree showing typical staining of sapwood.
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Infected koa tree showing typical partial canopy dieback.
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Symptoms
Most trees showing koa wilt symptoms have been shown to be infected with some species of the vascular fungus Fusarium, a disease-causing fungus which is found all around the world (Gardner 1980; Anderson and others 2002). The soil-borne fungus enters through the trees' roots and migrates up through the stem and branches, where it blocks vascular tissue (conducting tissue) and chokes off water supply to the leaves. The leaves then wilt and turn brown. The first sign of infection is usually a yellowing or wilting of leaves on a single branch or part of the tree's canopy. If the branch is cut, usually a dark stain is present in the sapwood. Infected roots may also show dark staining. Sometimes the cambium, the thin layer of growing tissue between the sapwood and the bark is also stained. Sometimes the bark splits and the trees ooze sap, and sometimes infected trees are attacked by stem borers. If all the conducting tissue in the stem is blocked, the sap may ferment in the stem and burst out, creating a foamy exudate that looks and smells like beer. Small infected trees may gradually lose their crown until the entire tree dies. In badly infected plantations, 90% of the trees may die over the course of several years. Older and larger trees may be able to wall off or "compartmentalize" the infection and retain a partial live crown. Some trees are apparently resistant to the disease, while others may simply have avoided becoming infected yet.
There are many other diseases of koa, and not all sick koa trees are infected with koa wilt. Many planted koa trees do not grow well because the seedlings were left too long in the pots and the roots were deformed, the roots were bent when the seedlings were planted ("j-rooted"), or the seedlings or the shallow roots were damaged by weed whackers or animals. In lava rock soils, especially pahoehoe, or soils with a hardpan, koa trees may die back because the roots do not have the space to keep growing to keep the canopy supplied with water.
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Infected koa tree showing sap oozing out of holes caused by borers. The borers are probably a secondary pest attacking trees already stressed by the fungus.
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Range
Koa wilt has been observed on the Hawaii Island, Maui, Oahu, and Kauai. While koa wilt symptoms and Fusarium oxysporum have been found in koa trees along Mauna Loa Strip Road at elevations up to 7,000 feet (Anderson and others 2002), most diseased trees are found below 3,000 feet elevation.
Chlorosis (yellowing) of an infected koaia on the left as compared with healthy green foliage on the right.
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Cut stem of an infected koa tree showing typical staining.
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The fungus
Fusarium oxysporum is a soil-borne fungus which is widely present in Hawaiian soils and is the putative cause of koa wilt (Gardner 1980, Anderson and others 2002). The fungus may be a new introduction to Hawaii and may have come in on an ornamental acacia, possibly Formosa koa (Acacia confusa). Genetic tests showed little diversity in Fusarium oxysporum samples taken from diseased koa (Anderson and others 2004). Koa trees are probably initially infected through the roots and not through wounds or insects. The black twig borer, Xylosandrus compactus, and other ambrosia beetles such as Xyloborus spp. transmit another species of fungus, Fusarium solani (Daehler and Dudley 2002). This fungus causes wood staining and may be a serious problem in seedlings. It has been found in many mature trees showing symptoms of koa wilt. Other disease-causing species of Fusarium have also been found in koa and in soil around infected koa trees, including F. semitectum , F. subglutinans, F. lateritium, F. graminearum, F. proliferatum, and F. equiseti (James 2004, 2005). Some isolates of F. solani, F. semitectum, and F. subglutinans have been shown to be pathogenic in koa seedlings (Dudley and others 2007). Some strains of F. oxysporum are quite virulent while others do not seem to cause disease (Dudley and others 2007).
Koa wilt disease may be caused by a combination of different fungi and is probably affected by the environment of the affected tree.
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Stump of infected tree showing stain in the cambium.
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Control
As with most forest diseases, there is no “cure” for koa wilt. Frequent applications of fungicides to forests would be impractical and not cost-effective. It is important to avoid bringing soil or plant parts from infected sites to areas where the disease is not present. Shoes, tools, and truck tires used in infected sites should be well cleaned after leaving the site. Tools such as axes, chainsaws, and machetes should be sterilized after use on infected trees. Fusarium oxysporum has been found in seed of infected trees, so seed and seedlings from infected sites should never be planted outside the site. In general, it is best to use locally collected seed from superior individual trees whenever possible. Seedlings should be raised in soil-less media. Koa growers planting trees in highly susceptible areas below 2,500 feet elevation should expect high mortality. In areas with no local koa populations, planting trees from several different seed sources may increase likelihood of finding resistant trees.
Research work
Koa wilt was only discovered in 1980 (Gardner 1980) and became a widespread issue with expanded koa plantings on former agriculture lands in the 1990s. Some trees in badly affected stands have grown well and shown no signs of the disease (Shi and Brewbaker 2004). Work is underway at the Hawaii Agriculture Research Center , the USDA Forest Service, the Hawaii Division of Forestry and Wildlife, and the University of Hawaii College of Tropical Agriculture and Human Resources to identify resistant trees and learn if seed collected from these trees will produce resistant seedlings.
Genetic analysis of the fungus will reveal how diverse the problem is and whether it is a recent introduction or something that has been in Hawaii all along. Analysis of infected stands should discover whether koa wilt disease is caused solely by Fusarium oxysporum or by a combination of pathogens or other factors. Incidence of the disease is also being mapped so that growers can avoid areas where the disease is most likely to occur.
Another possibility is that koa might be protected from disease by being grafted onto resistant rootstocks, either of koa or of related species. Koa has been successfully grafted onto rootstocks of Acacia koa, Acacia mangium, and Acacia confusa (Nelson 2006). Evaluation of grafted trees is ongoing.
New flushes of juvenile leaves often occur below infected branches or stems.
A koa tree growing on top of a rock along Saddle Road. The trees's roots cannot keep up with the growth of the tree and the canopy is dying back.
Koa grafted onto rootstock from Acacia mangium. The tree is 20 months old.
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Infected tree showing stain in the sapwood...
...and in the roots.
An infected tree along Saddle Road showing canopy dieback...
bark cankers...
and staining of the sapwood.
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Natural koa stands along Mauna Loa Strip Road. Some koa trees show yellowing of leaves and defoliation tested positive for Fusarium oxysporum.
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References on koa wilt
Anderson, R.C., T.C. Bentley, and J.C. Correll. 2004. Population diversity of Fusarium oxysporum. f. sp. koae, the pathogen of Koa dieback in Hawaii. Phytopathology 94:S4 (abstract).
Anderson, R.C., D.E. Gardner, C.C. Daehler, and F.C. Meinzer. 2002. Dieback of Acacia koa in Hawaii: ecological and pathological characteristics of affected stands. For. Ecol. and Mgmt. 162: 273-286.
Daehler, C.C. and N. Dudley. 2002. Impact of the black twig borer, and introduced insect pest, on Acacia koa in the Hawaiian Islands. Micronesica Suppl. 6: 35-53.
 Dudley, N., R.L. James, R.A. Sniezko, and A. Yeh. 2007. Pathogenicity of four Fusarium species on Acacia koa seedlings. Forest Health Protection Numbered Report 07-04, USDA Forest Service Northern Region, Missoula, Montana.
Dudley, N., and R. Hauff. Undated. Forest Health Alert: Koa Wilt Fact Sheet.
Gardner, D.E. 1996. Acacia koa: A review of its diseases and associated fungi. University of Hawaii Department of Botany. In: Koa: A Decade of Growth. Hawaii Forest Industry Association (HFIA), Hilo.
Gardner, D.E. 1996. Wilt of koa. University of Hawaii Department of Botany.
Gardner, D.E. 1980. Acacia koa seedling wilt caused by Fusarium oxysporum f. sp. koae, f. sp. nov. Phytopathology 61, 1377-1381.
James, R.L. 2005. Pathogenic comparison of Fusarium isolates from diseased Hawaiian Acacia koa. Nursery Disease Notes No. 164, USDA Forest Service Northern Region. Full-text article, no figures.
James, R.L. 2004. Fusarium colonization of seeds, seedpods, and diseased seedlings of Acacia koa from Hawaii. Nursery Disease Notes No. 159, USDA Forest Service Northern Region. Full-text article now available .
Nelson, S.C. 2006. Grafting of Acacia koa Gray onto young Acacia seedlings. Native Plants Journal 7(2): 137-140.
Shi, X and J.L. Brewbaker. 2004. Genetic improvement of Hawaii's premier hardwood, Acacia koa. Forest Genetic Resources. 31:36-40. FAO/Rome.
Tenbrink, V.O. and A. Hara. 1994. Xylosandrus compactus. In: Crop Knowledge Master, UH CTAHR.
Other references on koa
Elevitch, C.R., K.M. Wilkinson, and J.B. Friday. 2005. Acacia koa (koa) and Acacia koaia (koaia), Version 1. In: C. R. Elevitch (ed). Species Profiles for Pacific Island Agroforestry. Permanent Agriculture Resources (PAR), Holualoa, Hawaii.
Whitesell, C.D. 1990. Acacia koa A. Gray. In: R.M. Burns and B. Honkala. Silvics of North America Vol. 2: Hardwoods. USDA Forest Service Handbook No. 654, 17-28. USDA, Washington, DC.
Wilkinson, K.M., and C.R. Elevitch. 2003. Growing Koa: A Hawaiian Legacy Tree. Permanent Agriculture Resources, Holualoa, Hawaii.
Wilkinson, K.M., and C.R. Elevitch. 2003. Propagation protocol for production of container Acacia koa Gray plants; Permanent Agriculture Resources, Holualoa, Hawaii. In: Native Plant Network. University of Idaho, College of Natural Resources, Forest Research Nursery.
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Last updated on 12/5/2007
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