The causal agent of anthracnose of Rhododendron in Sweden and Latvia

• Anthracnose caused by Collectortrichum is a severe foliar disease in rhododendron plantations in Sweden and Latvia. Isolates of this pathogen were collected and characterised based on morphological and molecular criteria, Out of 37 isolates examined, two with falcate spores were identified as C. dematittin. The remainder of the isolates had straight cylindrical conidia, which were too variable to be reliably assigned to C. glocosporioides (syn. C.azaleae) the reported causal agent of anthracnose of Rhododendron spp. These isolates were compared with the reference strains of C. gloeosporioides, with holo- and paratypes and with reference isolates of C. acutatum. The light and scanning electron microscopy examinations revealed that conidial shape was not a reliable criterion for separation of these two taxa and for the identification of our isolates. The two internal transcribed spacers (ITS1 and ITS2) and the 5.8S rRNA gene were PCR amplified and sequenced from all the rhododendron isolates and the reference strains, including the holo-and paratype materials. In addition, a segment of mitochondrial small subunit ribosomal DNA (mtSSU rDNA) and a fragment of a p-tubulin gene from several representatives of isolates of rhododendron as well as the reference strains were amplified and sequenced. The sizes of the amplified fragments among the isolates studied were heterogeneous only for mtSSU rDNA, which allowed separation of C. acutatum from C. glocosporioides. Parsimony analysis of the individual and combined nucleotide sequence data sets were concordant and indicated that all isolates originating from rhododendrons belonged to C. acutatum. The grouping of the isolates was further supported by bootstrap analysis. Thus, C. acutatum was identified as the causal agent of anthracnose of Rhododendron in Sweden and Latvia. To our knowledge, this is the first report of C. acutatum and C. dematium on Rhododendron. This study is also the first to determine the genetic status of the holotype of C. acutatum and also one of the paratypes on the basis of ITS sequences in comparison with world-wide living isolates of this taxon.

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