Acacia koa (koa), a native tree in Hawaii, suffers from a dieback caused by Fusarium oxysporum. Pathogenicity tests, vegetative compatibility group (VCG) tests, and amplified fragment length polymorphisms (AFLP) analyses were conducted on Fusarium isolates recovered from diseased koa. Koa seedling mortality with individual strains ranged from 0 to 85%, with 42% of the strains killing no seedlings. Thus, strains of F. oxysporum recovered from dying koa trees may or may not be virulent. In addition to F. oxysporum, F. pseudocircinatum strains were isolated from diseased koa; however, they were either nonvirulent or had weak virulence. This is the first report of F. pseudocircinatum in Hawaii. The 46 strains of F. oxysporum and F. pseudocircinatum were grouped into 16 VCGs, but 86% of the highly virulent strains belonged to VCG 2. In AFLP analyses, strains from the same VCG generally clustered with one another. Identification of the same set of strains using VCG, AFLP, and pathogenicity tests showed that the highly virulent strains are genetically close and that high virulence toward koa is not a property of all strains of F. oxysporum. Thus, VCG 2 with the corresponding AFLP data is a significant biological entity for which we propose the name F. oxysporum f. sp. acaciae to reflect its virulence on koa.
Black locust (Robinia pseudoacacia L.) is an economically and ecologically important tree in Japan. The species is widely used for afforestation because of its symbiotic relationship with nitrogen-fixing rhizobia. In this study, genetic diversity of rhizobia isolated from black locust nodules and spatial distribution of their genotypes were examined. From a coastal forest, six black locust saplings including the whole root systems were collected and positions of nodules on the roots were recorded. Bacteria were isolated from each nodule and then genotyped by Polymerase Chain Reaction (PCR)-Restriction Fragment Length Polymorphism (RFLP) and sequencing analysis of the 16S rRNA gene. Dendrogram analysis based on PCR-RFLP and sequencing analysis indicated that Mesorhizobium species dominantly colonized black locust roots in this forest, occupying at least 77% of living nodules. Positions and PCR-RFLP genotypes of the nodules revealed that rhizobia isolated from neighbouring nodules tended to have the same genotype in some cases. Statistical analysis has supported this fact. The clustering distribution of nodules in the same RFLP groups should be considered to contribute to the large genetic diversity of rhizobia.
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