Study on Rosellinia necatrix Isolates Causing White Root Rot Disease in Taiwan

White root rot is a serious soil-borne disease of several woods and crops. Recently, white root rot of tea shrubs and ornamental trees has increasingly been observed in Taiwan. Thirty-six isolates of white root rot pathogen, showing pear-shape swellings adjacent to the hyphal septa, had been isolated from samples of white root rot collected from Taiwan for about 4 years. The pathogen isolates produced Dematophora anamorph. Conidia of the pathogen were one-celled, hyaline, subglobal, with truncate base, 2.9–5.8 · 1.9– 3.5 lm. Ascospore dimensions were in the range of 37.0–55.0 · 5.4–7.9 lm with a short, longitudinal and straight germ slit, which complied with Rosellinia necatrix. Based on molecular studies, the pathogen isolates collected from Taiwan except R701 were identified as R. nectarix. Isolate R701, which was relatively polymorphic in internal transcribed spacer DNA sequence than other isolates, was temporarily considered as R. necatrix-related pathogenic Rosellinia spp. All the tea cuttings (Camellia sinensis) inoculated with isolates developed typical white root rot symptoms. Pathogenicity tests demonstrated the presence of variation in virulence among the Rosellina isolates. Most of the R. necatrix isolates originating from Acer morrisonense were less virulent than those that originated from other hosts. The pathogenic Rosellinia spp., isolate R701, was also highly virulent to both cultivars of tea cuttings.