Investigating Koa Wilt and Dieback in Hawai‘i Pathogenicity of Fusarium species on Acacia koa seedlings

Abstract

Fusarium isolates obtained from diseased Acacia koa Gray (Fabaceae) plants, adjacent soil, and seeds and seedpods may or may not be pathogenic on young seedlings under greenhouse conditions. This includes isolates of Fusarium oxysporum, the putative cause of koa wilt and dieback disease (“koa wilt”) in Hawai‘i. We tested 10 Fusarium isolates, made up of 4 different species (F. solani, F. subglutinans, F. oxysporum, F. semitectum), for their pathogenic potential on koa seedlings under greenhouse conditions. All tested Fusarium isolates completely colonized seedling root systems and became systemic, spreading to aboveground plant tissues (stems, branches, and leaves). Virulence was quantified on the basis of disease symptoms (mortality, wilting, foliar chlorosis, or necrosis) and effects on seedling height, stem diameter, and root volume. Results varied, ranging from nonpathogenic to high levels of virulence. Pathogenic screening of many more isolates will be necessary to identify pathogens that can be effectively used to screen families of koa for potential resistance to the koa wilt and dieback disease that is seriously affecting this important Hawaiian tree species.