Wood-Fiber Amendments Shift Soilless Substrate Microbiomes and Suppress Globisporangium ultimum Root Rot in Greenhouse-Grown Chrysanthemums

Global growth in soilless plant production systems, coupled with an increase in the soilless production of many traditional field-grown horticultural crops, has substantially increased demand for soilless substrates. This demand, as well as increasing interest in reducing industry dependence on peat, has prompted growers to incorporate non-peat materials into their substrates, such as engineered wood fibers (WFs). While WFs have shown potential to enhance the suppressiveness of peat against Rhizoctonia solani, the breadth and mechanisms of this suppression remain unclear. Existing evidence suggests WF effects on substrate microbiomes may play a role: other substrate amendments, such as straw, can increase microbiome diversity, and more diverse microbiomes can be disease suppressive. Here, we evaluated the effects of three engineered pine WF-amendments on disease suppression and microbiome composition using Globisporangium ultimum root rot of chrysanthemum (Chrysanthemum × morifolium) as a model system. The three WF-amendments used in this study were processed using either disc refinement, screw-extrusion, or hammer-milling. Results indicate that hammer-milled and extruded WFs reduced Globisporangium root rot in chrysanthemums and maintained chrysanthemum growth equal to grower standard media in the absence of Globisporangium. 16s rRNA and ITS amplicon sequencing data of substrate samples revealed that fungal and bacterial alpha diversity did not increase in any WF-amended substrates compared to peatlite standards and was not correlated to disease severity in most treatments. However, some microbiome composition differences were driven by WF amendment type, suggesting that WF-driven community turnover could relate to disease suppression.