Abstract
Background and Objectives: Koa (Acacia koa A. Gray) is an economically, ecologically, and culturally valuable tree species endemic to Hawaii. A vascular wilt disease caused by the fungal pathogen Fusarium oxysporum f. sp. koae Gardner (FOXY) induces high rates of mortality in plantings and threatens native koa forests as well. Landowners are reluctant to consider koa for reforestation purposes in many areas due to the risk of mortality caused by FOXY. Producing seeds with genetic resistance to FOXY is vital to successful koa reforestation and restoration. The Hawaii Agriculture Research Center (HARC), with both public and private partners, operates a tree improvement program to develop wilt-resistant koa populations in Hawaii. The population genetics of koa is poorly understood and seed zones are evolving. Thus, HARC uses provisional seed zones based on genetic and biogeographic variables and has selected wilt-resistant koa populations that are locally found in Kokee, Kauai (eco-regions) of Hawaii. Materials and Methods: To make these selections, virulent FOXY isolates were used in previous seedling inoculation trials to evaluate resistance levels among koa families in greenhouse experiments, and the most resistant families were used in the field trial reported here. Results: In this trial, survival rates two years after planting varied by family, and ranged from 45% to 95%, but all resistant families had greater survival rates than the susceptible control (25%). The trial has been converted to a seed orchard. Conclusions: The higher survival rates of the families are encouraging and seeds coming from the orchard will improve the success of future restoration and reforestation efforts. Within these resistant families it was also possible to make some selections based on height, growth, diameter, and stem form. Thus, the establishment of a wilt-resistant seed orchard results in locally adapted, eco-region specific, disease-resistant koa seed that will allow for the restoration of this iconic species and provide plant material for commercial reforestation opportunities at the landscape level.
Highlights
IntroductionGray) is an overstorey tree species endemic to the Hawaiian archipelago that holds cultural, ecological, and economic significance
Survival within individual seed sources in year 1 ranged from 50% to 95% for the disease-resistant selections
Significant differences in mortality among families in young koa field trials indicate that selecting the appropriate seedling material with disease resistance is a powerful tool for outplanting success
Summary
Gray) is an overstorey tree species endemic to the Hawaiian archipelago that holds cultural, ecological, and economic significance. Significant koa forests are found on four of the major Hawaiian Islands (Hawaii, Maui, Oahu, and Kauai). This tree is of immense cultural importance to native Hawaiians. The very limited supply of commercial-quality trees is a significant limiting factor for the Hawaiian forest industry; in 2019 the total annual value of Hawaii’s koa industry was estimated at $20 to $30 million [2]. J. Applied Forest Tree Improvement; John Wiley. Applied Forest Tree Improvement; John Wiley & Sons: New York, NY, USA, 1984; p. Gray) is an economically, ecologically, and culturally valuable tree species endemic to Hawaii.
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