Impact of cellulose-rich organic soil amendments on growth dynamics and pathogenicity of Rhizoctonia solani

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 Rhizoctonia solani. Saprotrophic growth of R.
solani on cellulose-rich materials was tested in vitro. This
confirmed paper pulp as a highly suitable substrate for R.
solani, whereas its performance on wood sawdusts varied with tree species.
In two pot experiments, the effects of amendment of R.
solani-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 R.
solani 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 Cytophagaceae,
responded to paper pulp during the first two weeks and may have
counteracted further spread of R. solani. The results showed that
fungus-stimulating, cellulose-rich amendments have potential to be used
for suppression of R. solani. However, such amendments require a
careful consideration of material choice and application strategy.