Pathogenicity of Four Fusarium Species on Acacia Koa Seedlings

Fusarium isolates obtained from diseased koa plants, rhizosphere soil and seeds/seedcoats may or may not be pathogenic on young seedlings under greenhouse conditions. This includes isolates of F. oxysporum, the putative cause of koa wilt/dieback disease in Hawaii. We tested ten Fusarium isolates, comprising four different species, for their pathogenic potential on Acacia koa seedlings under greenhouse conditions. Tested isolates were obtained in Hawaii from either diseased Acacia koa seedlings, soil adjacent to seedling roots, or seeds/seedpods. All tested Fusarium isolates completely colonized seedling root systems and became systemic, spreading to above-ground plant tissues (stems, branches, and leaves). Virulence was quantified on the basis of production of disease symptoms (mortality, wilting, foliar chlorosis or necrosis) and effects on seedling height, diameter and root volume. Of the five tested F. oxysporum isolates, one exhibited high virulence, another was non-pathogenic, and the other three were moderately-virulent. One tested F. solani isolate was quite virulent, whereas the other was only slightly virulent. One isolate of F. subglutinans was non-pathogenic and the other tested isolate was moderately virulent on inoculated seedlings. The one tested isolate of F. semitectum displayed moderate virulence. Pathogenic screening of many more isolates, particularly those classified within the F. oxysporum species complex, will be necessary to identify pathogens that can be effectively used to screen families of Acacia koa for potential resistance to the wilt/dieback disease that is seriously impacting this important Hawaiian tree species.