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Cross Inoculation Tests with Phellinus noxius Isolates from Nine Different Host Plants in the Ryukyu Islands, Southwestern Japan

Brown root rot, caused by Phellinus noxius, leads to problems in management of shade, ornamental, and windbreak trees in subtropical regions of the world, and it has been causing serious problems in Japan since 1988. To identify the pathogenicity, host specialization, and virulence of the pathogen, cross inoculation tests were carried out using isolates of the fungus obtained from nine different tree species. P. noxius was pathogenic to all of the nine tree species tested, and it killed inoculated seedlings. Among the 810 trees inoculated with the fungus, a total of 141 trees (17.4%), including all nine species, were dead within 110 days after inoculation. The first symptom of infection was rapid wilt with discoloration of leaves 20 to 30 days after inoculation, and then the plants quickly declined. Only one isolate of P. noxius that was tested caused significantly higher mortality in its original host than in other species, suggesting that the pathogen has little to no host specificity. However, the fungus did show variation in virulence, with the isolates originating in Bischofia javanica and Casuarina equisetifolia causing significantly higher mortality than those collected from other host species. More isolates of P. noxius from the same host species and/or from various countries should be studied to understand host specialization and virulence of the pathogen.