Investigating koa wilt in Hawai’i examining Acacia koa seeds and seedpods for Fusarium species

Abstract

We sampled Acacia koa A. Gray (Fabaceae) seeds and seedpods from 4 of the Hawaiian Islands (Big Island, Kaua‘i, O‘ahu, and Maui) for colonization by and contamination with Fusarium spp. (Hyphomycetes). The vast majority of healthy-appearing seeds from storage were not colonized by Fusarium. Stored seeds with superficial fungal mycelium, however, were extensively contaminated by Fusarium. Nearly 80% of the sampled seeds from forest trees with koa wilt disease symptoms had evidence of insect predation. More than 70% of the insect-predated seeds were contaminated by Fusarium; about 60% of healthy-appearing seeds from diseased forest trees were also contaminated. Seedpods were commonly colonized by the same Fusarium species that contaminated seeds. Thirteen different Fusarium spp. were isolated from koa seeds and seedpods. Most species were found at low levels, although F. semitectum, F. subglutinans, and F. solani were frequently isolated. Fusarium oxysporum, the putative cause of koa wilt/dieback disease, was isolated very rarely from either seeds or seed coats. Ecological significance and potential disease roles of Fusarium contaminating koa seeds need to be determined.