Study of koa wilt disease : characterization of acacia koa and fusarium oxysporum

Abstract

Koa (Acacia koa) is a highly valuable tree species in Hawaii. Over the past few decades however, the species has suffered from a severe wilt and dieback disease. The goal of this study is to manage the disease and improve koa plantation health. Fungal species were isolated from koa trees showing typical dieback symptoms, and species of these isolates were identified. Fusarium pseudocircinatum was recovered from the samples and its virulence to koa was confirmed with Koch's postulates. This is the first report of this fungal species in Hawaii. The population structure of F. pseudocircinatum and F. oxysporum collected from the samples were analyzed with pathogenicity tests, vegetative compatibility group (VCG) tests, and Amplified Fragment Length Polymorphism (AFLP) analyses. Forty-six isolates including F. oxysporum and F. pseudocircinatum were grouped into 16 VCGs. Among the highly virulent isolates, 86% belonged to the single VCG group, and these isolated clustered together in AFLP analyses. VCG and AFLP could be used for detection and identification of F. oxysporum strains in soils, and therefore would help koa growers to establish new koa plantations. VCG2 of F. oxysporum is a significant biological entity for which the name F. oxysporum f. sp. acaciae is proposed to reflect its virulence on koa. Pathogenicity tests conducted on 18 koa families revealed genetic variations in wilt tolerance among koa families. Mortality rates of the 18 families inoculated with a highly virulent F. oxysporum f. sp. acaciae isolate ranged from 0 % to 100 %. However, a strong interaction, P < 0.0001, was observed when host-pathogen interaction among five koa families and ten F. oxysporum f. sp. acaciae isolates were tested. This result indicates that each koa family and F. oxysporum isolate behave differently depending on their host family/pathogen combination. When koa mortality data from the field trials and pathogenicity tests were compared, no direct correlation was observed (correlation coefficient =-0.013). Understanding the host-pathogen relationship between koa and F. oxysporum and trends in koa mortality in the field trials will ultimately lead to the establishment of more effective disease screening protocols for koa silviculture in Hawaii.