Dieback of Fraxinus excelsior : biology of ash dieback and genetic variation of the fungus Hymenoscyphus pseudoalbidus
Abstract: Ash dieback caused by Hymenoscyphus pseudoalbidus (anamorph Chalara fraxinea) is a disease that has emerged during the past twenty years. It was first observed in Poland and has expanded over most of the distribution area of European ash (Fraxinus excelsior) in Europe. This thesis comprises four scientific papers. The first reports the production of a phytotoxin, viridiol, by the fungus, and shows that it causes necrotic spots on ash cotyledons. The second paper describes a working tool, a species-specific DNA primer situated in the internal transcribed spacer (ITS) region of the ribosomal DNA, which can be used to detect the fungus in diseased host material and confidently identify fungal cultures. In the third paper, microsatellites and arbitrary primed PCR were used to investigate the genetic population structure of the fungus. The genetic variation was evenly distributed in the whole European population, implying a high level of gene flow. Double alleles in apothecia compared with single alleles in mycelia cultures strongly indicated sexual reproduction. H. pseudoalbidus was clearly distinct from the native closely related Hymenoscyphus albidus and it is unlikely that the disease is derived from a H. albidus ancestry. The low number of microsatellite alleles per locus indicated a recent founder effect in H. pseudoalbidus. In the fourth paper disease development was surveyed in 261 naturally infected ash trees over a 32 month period. A clear seasonal pattern was demonstrated, with lesion activity and growth rate peaking during the summer. A substantial proportion of the lesions ceased to develop, often when the lesion reached a branch base; however the rate at which new lesions emerged was greater than the rate at which lesions entered a resting phase. During the course of the survey a third of the trees died and only a few seedlings remained healthy. In addition, the lower temperature limit for the fungus in culture was estimated to be 0.5°C.
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