Verticillium dahliae , the causal agent of verticillium wilt, is the cause of high losses in a number of crops, especially nursery trees. Existing methods for quantification of V. dahliae microsclerotia in the soil were compared. The distinction of V. dahliae and V. tricorpus was studied on two semi-selective media. The morphology was highly dependent on the medium. Discriminating morphological characteristics were successfully used to identify isolates. There were no virulence differences on trees between the two V. dahliae VCGs that occur in the Netherlands. Biological soil disinfestation reduced V. dahliae in the soil by 85% and Pratylenchus fallax nematodes by 99%, through the creation of anaerobic conditions. The relationships between soil inoculum densities and verticillium wilt in Acer platanoides and Catalpa bignonioides showed that up to 5% diseased plants occurred at 1-2 detected microsclerotia per g soil. Diseased plants often recovered, but had a higher chance of becoming diseased again.