KOA WILT

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. 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).

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.

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.

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 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.

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. 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.