Suppression of Phytophthora on Chamaecyparis in Sustainable Horticultural Substrates Depends on Fertilization and Is Linked to the Rhizobiome

Abstract

Nature management residues (i.e., biomass generated from the management of nature reserves) are promising peat alternatives for horticultural substrates and may have a positive effect on disease suppression because of their microbiological characteristics. Moreover, addition of fertilizer may also affect the rhizosphere microbiome and, accordingly, disease suppression. In this study, we determined the effect of two management residues in horticultural substrates (i.e., chopped heath and acidified soft rush) and two fertilization regimes (i.e., pure nitrogen fertilizer and compound fertilizer) on the suppression of Phytophthora spp. on Chamaecyparis lawsoniana. The bacterial and fungal rhizosphere community was characterized using 16S ribosomal RNA and internal transcribed spacer 2 gene metabarcoding. Soft rush with a compound fertilizer (R2) and chopped heath with a pure nitrogen fertilizer (H1) showed a disease-suppressive effect and showed the largest shifts in microbial community composition compared with peat-based substrates. The disease-suppressive treatments showed differences in their microbial communities. Different genera associated with described biocontrol agents for Phytophthora spp. were found in higher amounts in those treatments. Aspergillus and Trichoderma spp. were highly abundant in H1, while Actinomadura and Bacillus spp. had a high abundance in R2. In addition, the relative abundances of 24 bacterial and 9 fungal genera were negatively correlated with disease severity. Several of those genera, including Bacillus, Chaetomium, and Actinomadura, were significantly more abundant in one of the disease-suppressive treatments. This study shows that disease suppressiveness in sustainable horticultural substrates is dependent on fertilization and can be linked to changes in the microbial rhizosphere communities.