Brown Root Rot Disease in American Samoa

Phellinus noxius causes root and lower stem rot of woody plants throughout the South Pacific region. Its hosts include rubber, mahogany, cacao, and many timber, fruit and landscape trees. Though endemic to the tropics, we found no reports describing brown root rot disease in native forests, exclusively. Incidence, distribution and host range of P. noxius were measured in primary and secondary rainforests on Tutuila Island, American Samoa. P. noxius was recorded in 19 of 20 strip transects and 1.2 ha established plots and in all vegetation types, infecting 37 tree species in 30 genera and 22 families. Species most affected were Myristica fatua, Dysoxylum samoense and Hibiscus tiliaceus3⁄425, 16 and 10 percent, respectively. Of 62 infection centers, 33 contained the same tree species and 13 were dominated by a single species. The fewest infections were recorded at primary montane and ridge top sites, the most in secondary valleys. Disease incidence was influenced more by human disturbance than by vegetation type, topography, stem diameter, stem density, or soil type. Regenerating secondary valley sites appeared to lack the species richness of mature sites, had the highest disease incidence, and the largest and highest number of infection centers. This agrees with other host/pathogen associations, such as Douglas-fir/P. weirii and hardwood/P. noxius plantations, where disease incidence and spread was higher in species poor than in species rich stands.