Ryan P. Lynch

Date of Award


Level of Access Assigned by Author

Campus-Only Thesis

Degree Name

Master of Science (MS)


Plant, Soil, and Environmental Sciences


Robert Larkin

Second Committee Member

Eric Gallandt

Third Committee Member

David Lambert


Soilbome pathogens are persistent problems in potato production systems in the northeastern U.S. Chemical treatments have shown some reductions in many soilbome diseases; however, their use may not provide long-term crop protection and are not always practical. Effective and consistent control of some soilbome diseases has not been clearly demonstrated. Previous research has reported significant decreases in multiple soilbome diseases when Brassica crops (members of the family Brassicaceae), are included in the rotation cycle. There are multiple mechanisms of action in which Brassicas may suppress soilbome diseases. Present endogenously in Brassica tissues, glucosinolates (sulphur-containing molecules) degrade to form compounds toxic to multiple organisms. This toxicity has been shown to decrease the abundance of pathogens, therefore leading to lower disease levels. Another mechanism through which Brassica rotation crops suppress diseases may be the influences on soil microbial communities. These changes to the structure of soil microbial communities may include population shifts of different groups of organisms, changes to the proportions of specific classes of organisms, increases to certain organisms that may suppress pathogens and disease, and changes to the overall diversity and functioning of the soil microbial communities. The objectives of this project were to evaluate the efficacy of selected Brassica crops as well as different management strategies in controlling soilborne potato diseases and their effects on soil microbial community characteristics. Multiple 2-year crop rotations were established on several research and commercial field sites with histories of soilborne disease. Rotations included up to six different Brassica crops (with varying amounts of glucosinolate content) that were either incorporated as green manures or left as crop residues in the first year and followed by potato the second year. We expected that disease levels would negatively correspond with the relative glucosinolate content of each Brassica variety/species. Significant differences were observed among the rotation crops in tuber diseases and soil microbial community characteristics. However, although some differences in soilborne diseases among rotations were observed, consistent disease control was not achieved with single-year Brassica crop rotations. Perennial ryegrass and a proprietary mustard blend with a high glucosinolate concentration showed reductions of multiple soilborne diseases, including powdery scab. These results indicate that certain rotations were more capable of reducing specific diseases, and also suggest that factors other than relative glucosinolate content are important to controlling soilborne diseases. Multiple techniques were used to characterize and evaluate the responses of soil microbial communities to the presence of different plant species, including soil dilution plating, community-level physiological profiling, and fatty acid methyl ester profiling. We expected that rotations that had the most differences between specific measures (such as activity levels, diversity and abundance of specific individual fatty acids) will have significant differences in their disease reduction capabilities. Some rotations were consistently associated with high levels of specific microorganisms. Perennial ryegrass was associated with mycorrhizal fungi, whereas canola tended to increase fungal levels. The results suggest that effects of crop rotations on soil microbial community structure may have significant impacts over an extended period of time, therefore potentially influencing soilborne diseases in subsequent potato crops.

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