Impact of Cellulose-Rich Organic Soil Amendments on Growth Dynamics and Pathogenicity of .

Cellulose-rich amendments stimulate saprotrophic fungi in arable soils. This may increase competitive and antagonistic interactions with root-infecting pathogenic fungi, resulting in lower disease incidence. However, cellulose-rich amendments may also stimulate pathogenic fungi with saprotrophic abilities, thereby increasing plant disease severity. The current study explores these scenarios, with a focus on the pathogenic fungus . Saprotrophic growth of on cellulose-rich materials was tested in vitro. This confirmed paper pulp as a highly suitable substrate for , whereas its performance on wood sawdusts varied with tree species. In two pot experiments, the effects of amendment of -infected soil with cellulose-rich materials on performance of beetroot seedlings were tested. All deciduous sawdusts and paper pulp stimulated soil fungal biomass, but only oak, elder and beech sawdusts reduced damping-off of beetroot. Oak sawdust amendment gave a consistent stimulation of saprotrophic Sordariomycetes fungi and of seedling performance, independently of the time between amendment and sowing. In contrast, paper pulp caused a short-term increase in abundance, coinciding with increased disease severity for beet seedlings sown immediately after amendment. However, damping-off of beetroot was reduced if plants were sown two or four weeks after paper pulp amendment. Cellulolytic bacteria, including , responded to paper pulp during the first two weeks and may have counteracted further spread of . The results showed that fungus-stimulating, cellulose-rich amendments have potential to be used for suppression of . However, such amendments require a careful consideration of material choice and application strategy.