Pure Sphagnum peat is known for its good physical and chemical properties for growing container seedlings. However, it may not always provide the optimal water and aeration conditions for container seedlings raised on open fields especially during rainy summers, when water logging may occur. An excess water content in the growth medium favours fungal root infections as the infection by the root dieback pathogen Ceratobasidium bicorne. The use of perlite and mineral sand in formulating growth media mixtures has been in common use for a long time. Recently, use of composts in growth media mixtures has increasingly gained ground because of environmental interests to recycle organic wastes. Compost microflora might also decrease the growth of root pathogens. In this study, these additives were used in peat medium to produce different physical properties and conditions for testing their effect on root dieback and seedling growth.

Scots pine and Norway spruce seedlings were grown from seed in separate containers in a greenhouse in central Finland. In the first experiment containers were filled with light, low-humified Sphagnum peat or mixtures of peat and coarse perlite and/or fine quartz sand. The experiment was repeated with an additional treatment of composted forest nursery waste as a component in the growth medium. When the seedlings were two–month–old, a part of them were inoculated with C. bicorne. The seedlings were examined at the end of the first growing seasons and in the following springs.

The amendments altered the physical properties of the Sphagnum peat. The organic matter content of all the mixtures was lower, and the bulk density higher, than the corresponding values for pure peat. The addition of quartz sand especially increased the bulk density of the growth media. The total porosity and water retention capacity of the peat and peat-compost were higher than those of the perlite or/and quartz sand-mixtures. However, the study showed that none of the growth media limited the pathogenicity of C. bicorne under normal irrigation practice.

Key words: Ceratobasidium bicorne, uninucleate Rhizoctonia, Pythium, Picea abies, Pinus sylvestris, physical properties, growth media.