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.
- Two year old koa seedling dead of koa wilt in a plantation in Hamakua.
- Two year old koa seedling showing a canker caused by the disease.
- Same tree showing typical staining of sapwood.
- Infected koa tree showing typical partial canopy dieback.
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 2007a,b; Shiraishi 2011). Shiraishi isolated a pathogenic strain of Fusarium oxysporum and proposed the name Fusarium oxysporum f. sp. acaciae. Shiraishi also recovered Fusarium pseudocircinatum from diseased koa and showed it to be pathogenic to koa seedlings. Some strains of F. oxysporum are quite virulent while others do not seem to cause disease (Dudley and others 2007a,b).
Koa wilt disease may be caused by a combination of different fungi and is probably affected by the environment of the affected tree.
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. Fusarium species are widely present in koa seedlings grown in nurseries but these may be non-pathogenic strains of the fungus (James and others 2007).
- Stump of infected tree showing stain in the cambium.
- Infected tree showing stain in the roots.
- Infected tree showing stain in the sapwood.
- Chlorosis (yellowing) of an infected koaia on the left as compared with healthy green foliage on the right.
- 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.
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.
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.
- An infected tree along Saddle Road showing canopy dieback...
- bark cankers...
- and staining of the sapwood.
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 occasionally but seldom been found in seed of infected trees. In general, it is best to use locally collected seed from superior individual trees whenever possible. To avoid infection in the nursery, growers should sterilize planting containers if they are re-used. Seedlings should be raised in soil-less media, although Fusarium inoculum may also be present even in soil-less media or may be introduced to the nursery in wind-blown dust (James and others 2007). 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.
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. Some second-generation progeny of trees grown in disease-rich environment at a University of Hawaii experiment station have nonetheless shown low mortality.
Researchers at the Hawaii Agriculture Research Center surveyed koa trees state-wide to determine distribution of koa wilt/dieback disease across the four main Hawaiian Islands: Kauai, Maui, Oahu and Hawaii. Koa trees and seedlings infected by Fusarium oxysporum were found on all of the major islands in forest tree seedling nurseries and natural and plantation forests. More than 500 isolates of F. oxysporum were obtained. Isolates were tested for virulence on koa seedlings in controlled greenhouse inoculation tests and ten highly virulent isolates have been identified for use in screening selected koa families for disease resistance (Dudley and others 2007). More than 300 koa families have been evaluated for their potential resistance to F. oxysporum in greenhouse tests. Most of the seed lots came from wild populations. However, several seed lots were from survivors of family level progeny trials at the HARC’s Maunawili Field Station. All seed lots were open-pollinated. A composite of five virulent isolates of F. oxysporum were used for inoculation (Dudley and others 2007). Seedling mortality among seed lots varied widely, from 4% to 100%. These initial results indicate that natural resistance to F. oxysporum is low within native koa populations. Vegetative propagation methods for koa have been developed to produce planting stock from resistant trees. Continued screening of additional koa families for pathogen resistance, retesting putative resistant families, and developing koa seed orchards with disease-resistant stock are either on-going or planned.
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. Genetic markers for different strains of Fusarim oxysporum will allow rapid identification of pathogenic strains, something that now requires costly and time-consuming greenhouse tests. 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.
Efforts are also underway at the University of Hawaii and the Hawaii Agriculture Research Center to discover whether specific genetic markers exist for resistant koa trees which would allow for identification of resistant seed trees in the field. One possibility is that resistant koa trees produce higher levels of chitinase, which can inhibit fungal infection by degrading chitin in the fungal cell wall. High levels of chitinase in other leguminous trees have been shown to inhibit fungal growth.
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.
- Natural koa stands along Mauna Loa Strip Road. Some koa trees show yellowing of leaves and defoliation tested positive for Fusarium oxysporum.
References on koa wilt
Adamski DJ, NS Dudley, CW Morden, and D Borthakur. 2012. Genetic differentiation and diversity of Acacia koa populations in the Hawaiian Islands. Plant Species Biology. DOI: 10.1111/j.1442-1984.2011.00359.x
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., and R. Hauff. Undated. Forest Health Alert: Koa Wilt Fact Sheet.
Dudley, N. S., R.L. James, R. A. Sniezko, P. Cannon, A. K. Yeh, T. C. Jones, and M. F. Kaufmann. 2010. Developing resistant koa – early results, from disease survey to seedling resistance testing in Hawai‘i. In: Cram, M., ed. Proceedings of the 7th meeting of IUFRO Working Party 7.03.04 Diseases and Insects in Forest Nurseries. July 13 – 17, 2009, Hilo, Hawaii. USDA Forest Service Southern Region Forest Health Protection Report 10-01-01.
Dudley, N., R.L. James, R.A. Sniezko, and A. Yeh. 2007a. Pathogenicity of four Fusarium species on Acacia koa seedlings. Forest Health Protection Numbered Report 07-04, USDA Forest Service Northern Region, Missoula, Montana.
Dudley NS, RL James, RA Sniezko, and A Yeh. 2007b. Investigating koa wilt and dieback in Hawai‘i: pathogenicity of Fusarium species on Acacia koa seedlings. Native Plants Journal 8(3):259–266.
Dudley, NS, TC Jones, RL James, RA Sniezko, P Cannon, and D Borthakur. 2015. Applied disease screening and selection program for resistance to vascular wilt in Hawaiian Acacia koa. Southern Forests 77(1): 65-73.
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.
James, RL, NS Dudley, and A Yeh. 2007. Colonization of Hawaiian Acacia koa seedlings with Fusarium species. Forest Health Protection Report 07-07. USDA Forest Service Northern Region Missoula Montana
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.
Martinez Morales R, TW Idol, and JB Friday. 2011. Assessment of Acacia koa forest health across an elevation gradient in Hawai‘i using fine resolution remote sensing and GIS. Sensors 11(6): 5677-5694.doi: 10.3390/s110605677
Nelson, S.C. 2006. Grafting of Acacia koa Gray onto young Acacia seedlings. Native Plants Journal 7(2): 137-140.
Rushanaedy I, TC Jones, NS Dudley, RJF Liao, R Agbayahni, and D Borthakur. 2012. Chitinase is a Potential Molecular Biomarker for Detecting Resistance to Fusarium oxysporum in Acacia koa. Tropical Plant Biology. DOI: 10.1007/s12042-012-9108-7
Shiraisi, A., J.F. Leslie, S. Zhong, J.Y. Uchida. 2012. AFLP, Pathogenicity, and VCG Analyses of Fusarium oxysporum and Fusarium pseudocircinatum from Acacia koa. Plant Disease 96:1111-1117.
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.