Soil and Phytomicrobiome for Plant Disease Suppression and Management under Climate Change: A Review.

The phytomicrobiome plays a crucial role in soil and ecosystem health, encompassing both beneficial members providing critical ecosystem goods and services and pathogens threatening food safety and security. The potential benefits of harnessing the power of the phytomicrobiome for plant disease suppression and management are indisputable and of interest in agriculture but also in forestry and landscaping. Indeed, plant diseases can be mitigated by in situ manipulations of resident microorganisms through agronomic practices (such as minimum tillage, crop rotation, cover cropping, organic mulching, etc.) as well as by applying microbial inoculants. However, numerous challenges, such as the lack of standardized methods for microbiome analysis and the difficulty in translating research findings into practical applications are at stake. Moreover, climate change is affecting the distribution, abundance, and virulence of many plant pathogens, while also altering the phytomicrobiome functioning, further compounding disease management strategies. Here, we will first review literature demonstrating how agricultural practices have been found effective in promoting soil health and enhancing disease suppressiveness and mitigation through a shift of the phytomicrobiome. Challenges and barriers to the identification and use of the phytomicrobiome for plant disease management will then be discussed before focusing on the potential impacts of climate change on the phytomicrobiome functioning and disease outcome.

Stump removal and tree species composition promote a bacterial microbiome that may be beneficial in the suppression of root disease.

Stumping is an effective forest management practice for reducing the incidence of Armillaria root-rot in regenerating trees, but its impact on the soil bacterial community has not been ascertained. This study investigated the long-term impact of stumping and tree species composition in a 48-year-old trial at Skimikin, British Columbia, on the relative abundance, diversity and taxonomic composition of bacterial communities by sequencing the v4 region of 16S rRNA gene using the Illumina Miseq platform. A total of 108 samples were collected from the forest floor (fermented (F) and humus (H) layers) and mineral soil (A (0-10 cm) and B (10-20 cm) horizons) of 36 plots (half each stumped or unstumped) that were planted with pure stands and admixtures of Douglas-fir, western redcedar and paper birch. Bacterial α-diversity in the B horizon declined with stumping whereas ß-diversity was affected both by tree species and stumping treatments, with fir and birch supporting distinct bacterial communities. All horizons of stumped plots of birch and its admixtures were significantly enriched with potential plant growth-promoting bacteria. In conclusion, stumping along with planting birch alone or in admixture with other species promotes a bacterial microbiome that appears beneficial in the suppression of root disease.

Long-term effects of stump removal and tree species composition on the diversity and structure of soil fungal communities.

Stump removal is a common forest management practice used to reduce the mortality of trees affected by the fungal pathogen-mediated root disease, Armillaria root rot, but the impact of stumping on soil fungal community structure is not well understood. This study analyzed the long-term impact of stumping and tree species composition on the abundance, diversity and taxonomic composition of soil fungal communities using internal transcribed spacer (ITS) marker-based DNA metabarcoding in a 48-year-old trial at Skimikin, British Columbia. A total of 108 samples were collected from FH (fermented and humus layers), and soil mineral horizons (A and B) from stumped and unstumped plots of six tree species treatments (pure stands and admixtures of Douglas-fir, western red-cedar and paper birch). Fungal α-diversity in the A horizon significantly increased with stumping regardless of tree species composition, while ß-diversity was significantly affected by stumping in all the horizons. We also observed that the relative abundance of the saprotrophic fungal community declined while that of the ectomycorrhizal fungal community increased with stumping. In conclusion, increase in ectomycorrhizal fungal associations, which are positively associated with tree productivity, suggests that stumping can be considered a good management practice for mitigating root disease and promoting tree regeneration.