Epsilon Open Archive

Abstract

Clover rot (caused by Sclerotinia trifoliorum) and its influence on the persistence of red clover (Trifolium pratense) was studied in this thesis. Twenty-one red clover cultivars, of various types, were examined in both field and controlled environment experiments. Field experiments were performed at six sites in different climatic regions. The prevalence of root rot, caused by a broad soil-inhabiting group of secondary invading fungi, including Fusarium spp, was also studied. Both diseases were found to be prevalent throughout the country and disease severity was unaffected by latitude. The late flowering clover cultivars grown in southern Sweden exhibited lower levels of mortality due to clover rot than the earlier heading cultivars. In addition, lower root rot disease indices were recorded for the late flowering cultivars. Tetraploid cultivars grown in northern Sweden exhibited lower levels of mortality due to clover rot than diploid ones. This pattern was not found in southern Sweden or in the controlled environment experiments, where conditions were optimised for clover rot infection. In contrast, tetraploid ones had greater root rot disease indices than diploid cultivars in most cases. Studies of more than 250 Sclerotinia trifoliorum isolates revealed a high level of genetic variation. In laboratory tests, twenty of the isolates exhibited different capacities to cause disease. Fungal strains from northern Sweden generally caused more plant death, but aggressive strains were also found in southern Sweden. The fungus Coniothyrium minitans was shown to control clover rot infection in cultivar SW Torun at a northern site with heavy natural soil infestation.