Date of Award

Fall 2021

Project Type


Program or Major

Plant Biology

Degree Name

Master of Science

First Advisor

Anissa Poleatewich

Second Advisor

Cheryl Smith

Third Advisor

Rebecca Sideman


As by-products of the forest industry, wood fibers (WFs) are affordable and sustainable raw materials that have risen in popularity in the last decade to help meet the demand for soilless substrates. Wood fibers are high in hemicellulose and cellulose content yet have lower lignin content than Sphagnum peat, coconut coir, and tree bark-based growing medium components. Researchers have demonstrated that wood is more microbially active than peat and that WFs from pine and poplar have significantly greater fungal diversity and abundance than peat and coir, which can have higher bacterial diversity than WFs. However, the effects of WF-associated microbial communities on disease suppressiveness are unknown. While other organic substrates such as peat, fresh and composted hardwood and pine barks and wood chips, and other composts have been assessed for natural disease suppression and their interactions with biopesticides, WFs have not. Given increasing demand for affordable, sustainable soilless substrates like WF, our study sought to provide growers with information on WFs under disease pressure and in a realistic greenhouse production system, with the ultimate aim to better inform growers on how to incorporate WFs into their own production operations. Two bioassays were developed for Rhizoctonia Damping-Off of Radish (growth chamber) and and Rhizoctonia Crown and Root Rot of Chrysanthemum (greenhouse).

In the first part of this study, WF-Sphagnum peat-based growing media were examined for natural disease suppressiveness or conduciveness to Rhizoctonia damping-off in comparison to a general peat-perlite potting mix. Specifically, I examined whether potting mixes differing in WF processing type (disc refined, extruded or hammer milled) and the percent WF content (v/v) significantly differed from each other in conduciveness or suppressiveness to Rhizoctonia damping-off in radish. No differences in natural disease suppression across WF were found regardless of processing type or percent WF content. Moreover, radishes grown in WF blends had consistently lower damping-off disease severity than radishes grown in peat: perlite controls, suggesting that compared to peat:perlite blends, WFs are naturally suppressive to R. solani.

Using both bioassays, the efficacy of Rootshield®WP, a registered Trichoderma-based biofungicide reference treatment was examined. Previous studies on composted hardwood and pine barks found a correlation between carbon content and the efficacy of specific Trichoderma spp. biocontrol strains, with efficacy of these strains generally increasing with decreasing cellulose content. This suggested that WFs’ high hemicellulose and cellulose content could potentially influence Rootshield’s efficacy. To test this hypothesis, I assessed whether potting mixes differing in the percent WF content (v/v) significantly affected Rootshield®WP efficacy against Rhizoctonia solani in both radish (damping-off) and chrysanthemum (crown and root rot). Plants grown in WFs and infested with R. solani across experiments had significantly lower disease severity than plants grown in peatlite, despite similar health in control plants. Across experiments, there were no instances of percent WF blend significantly affecting Rootshield application, suggesting that there is no synergistic effect of WF content and Rootshield efficacy on disease severity. While Rootshield application was non-significant across substrate treatment, there were observable differences in aboveground plant biomass and root growth, highlighting opportunity for future work to more fully capture the effects of Rootshield.

Our results suggest that WF blends offer more protection against the soilborne pathogen R. solani than peatlite blends and do so across the plant production cycle. This research provided industry professionals with insight on WF substrate performance in terms of disease management and offered much needed guidance to growers seeking to integrate WF-amended substrates into their operations. Biopesticides and WFs are increasingly integral components of sustainable controlled environmental agriculture (CEA)—with biopesticides as part of IPM and WFs as renewable substrates. While interactions in greenhouse production systems are complex, studies that focus on a few aspects of this complexity will improve our overall understanding of the functional phytobiome and give growers improved working knowledge of the tools that are integral to sustainable crop production, like WFs and registered biopesticides. Given that the composition of WF-associated microbial communities has not been characterized with recent molecular and biochemical methods, future work should examine these unique communities and their interactions with different model plants, biocontrol agents, and soilborne pathogens. However, such studies must be complemented by pilot-scale greenhouse trials under practical conditions to show growers the economic, safety, and environmental merits of incorporating sustainable products like WFs and biopesticides into their production systems.