Date of Award

Summer 2019

Project Type


Program or Major

Animal and Nutritional Sciences

Degree Name

Master of Science

First Advisor

Anissa Poleatewich

Second Advisor

Cheryl Smith

Third Advisor

Rebecca Sideman


Oomycetes, such as Pythium, are soil-borne plant pathogens that can cause significant losses in greenhouse crop production due to their swimming zoospores and wide host range. Additionally, the increasing use of substrates that lack microbial diversity in greenhouse production creates a “biological vacuum” that can reduce the substrate’s capacity to resist microbial invasion by soil-borne diseases. The lack of competition by a natural microbial community and the environmental conditions of greenhouse production creates an ideal situation for the use of biopesticides. Biopesticides are commercial products that use beneficial microorganisms (biocontrol agents) to suppress disease and promote plant health. Greenhouse producers can utilize commercial biopesticides in addition to chemical treatments to protect plants from soil-borne pathogens. One barrier to use of biopesticides is the variability of their performance which can be attributed to differences in environmental conditions, such as plant species and substrate materials. Few studies have evaluated the effect of plant cultivar and current substrates on the efficacy of biopesticides to suppress disease in horticulture crops. The objectives of this research were to 1) develop a greenhouse-based assay to study biopesticide suppression of Pythium root rot of greenhouse grown crops, 2) evaluate the effect of plant cultivar on biopesticide efficacy in a tomato system, and 3) evaluate the implications of propagation substrate on biopesticide efficacy in cucumber and calibrachoa systems. For each of these systems, commercially available biopesticides were applied at the label rate twice during propagation. At transplant, plants were challenged with Pythium spp. or a water control. Root rot and root growth were evaluated at 21 days post infection. Findings reveal that the plant cultivars tested did not affect biopesticide efficacy, however a different cultivar panel with greater genetic diversity may affect biopesticide efficacy. There was a significant effect of propagation substrate on disease severity. Plants propagated in coconut coir had higher root disease than those propagated in Oasis®. These findings suggest that the chemical and physical properties of these substrates affect plant susceptibility to disease or pathogen activity, however further research is needed to evaluate this observation. Furthermore, there was a significant interaction between the effects of substrate and biopesticide on root rot severity in which biopesticide efficacy varied by substrate. This result suggests that substrate may affect biopesticide performance, but further research is needed to confirm these results and to understand the mechanisms behind this phenomenon. Finally, in all experiments, the commercial biopesticide Rootshield® WP suppressed root disease compared to the infested water control. These experiments provided initial data for determining the mechanisms driving variation in biopesticide performance and to improve on-farm performance and adoption.