The harmful effect of the fumigant Dazomet (DMTT, Mylone 50 D) on the development of mycorrhiza-forming fungi and accompanying growth of nursery stock is usually due to an inadequate pre-seeding detoxification period and/or a prolonged occurrence during this period of temperatures below 10°C. The eradication of the symbionts is occasionally caused by the lateral movement of water causing local high concentrations of the toxic chemical. A low supply of soil organic matter greatly aggravates the vulnerability of the mycorrhiza-formers to damages by the eradicant. During the past many years, a critical demycorrhization of stock in Wisconsin nurseries was observed predominantly in soils with organic matter content of less than 1.5% in the 15-cm surface layer.