In Hawaii, koa (Acacia koa) is a valuable tree species economically, ecologically, and culturally. Koa’s natural distribution ranged from lowland to montane areas and dry to wet forests. As Hawaii’s largest native tree, it provides habitat for many native birds, insects and plants, some of which are endangered and is also the primary nitrogen fixing species in native forest ecosystems. Koa is Hawaii’s premier timber tree and is used to produce furniture, musical instruments, bowls, surfboards, and craft wood items. Koa has deep cultural significance to the native Hawaiians and was the focal point of many traditional ceremonies. The resurgence of interest in Hawaiian voyaging and racing canoes using traditional methods has led to a greater public awareness of the scarcity of trees suitable for “canoe koa” and the importance of renewing this depleted resource.
With major land use change and declines in sugarcane, pineapple, and cattle production, there is an opportunity and keen interest in utilizing native koa in reforestation and restoration efforts. However, moderate to high mortality rates in many plantings have impeded past efforts. Currently, many landowners/managers are reluctant to reforest with koa in many eco-regions due to high mortality rates. The primary cause for this mortality is thought to be koa wilt, caused by Fusarium oxysporum f.sp. koae (FOXY). F. oxysporum is a relatively common agricultural and nursery soil-borne fungus, but the origin of virulent strains of FOXY infecting koa in Hawaii is unknown.
Identifying and developing koa populations that are genetically resistant to virulent strains of FOXY may be the key to successful koa restoration and reforestation. Great differences in mortality among seed sources in young koa field trials planted in the 1990’s were the impetus for developing a seedling screening test and investigating naturally occurring genetic resistance to FOXY.
A state-wide survey was conducted to determine distribution of koa wilt/dieback disease across the four main Hawaiian Islands: Kauai, Maui, Oahu and Hawaii. A total of 386 samples were taken at 46 different sites covering approximately 13,830 acres of natural and planted koa forest. Koa trees and seedlings infected by F. oxysporum were found on all of the major islands in forest tree seedling nurseries, natural, and plantation forests. From these samples more than 500 isolates of F. oxysporum were obtained. Of these, 160 isolates have been tested for virulence on koa seedlings in controlled greenhouse inoculation tests. From isolate screening tests, 12 highly virulent isolates have been identified for use in screening selected koa families for disease resistance.
Between 2006 and 2010, more than 250 koa families were evaluated for their potential FOXY resistance 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 FOXY were used for inoculation. Seedling wilting and mortality in the greenhouse was monitored over a 90 day period for each test. Seedling mortality among seed lots varied widely (4 to 100%) and averaged 61.5%. These initial results indicate that natural resistance to FOXY is low within native koa populations.
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.
Specifically, this project will use HARC’s methods to identify resistant koa seed sources for use in reforestation and restoration. This will provide project partners the opportunity to plant genetically adapted, eco-region specific, disease resistant koa seedlings.
Nick Dudley is a Forester and manages the HARC koa wilt resistance project.
Photo credits: Nick Dudley.