Date of Award


Level of Access Assigned by Author

Open-Access Thesis

Degree Name

Master of Science (MS)


Plant, Soil, and Environmental Sciences


Robert Larkin

Second Committee Member

David Lambert


Rhizoctonia solani is an important pathogen of potato capable of reducing tuber yield and quality. Integrated, sustainable methods including biocontrol and effective crop rotations are necessary for control of this pathogen. Twenty-eight potential biocontrol organisms were tested for efficacy against R. solani on potato in a series of greenhouse trials. Organisms tested consisted of field isolates of Paenibacillus polymyxa, Pseudomonasfluorescens, Penicillium sp., Trichoderma sp., and Rhizoctonia zeae; known biocontrol isolates including Laetisaria arvalis, Verticillium biguttatum, Cladorrhinumfoecundissimum, and Stilbella aciculosa; and commercial products of Bacillus subtilis (Kodiak), Trichoderma virens (SoilGard), and Trichoderma harzianum (Rootshield). Different formulations and rates of several fungal isolates and the efficacy of combinations of effective antagonists were also investigated. None of the treatments, including a chemical control (azoxystrobin), effectively controlled stem canker and black scurf in all trials. However, B. subtilis GB03, R. zeae LRNE17E, S. aciculosa 112-B, and the chemical control were most effective in reducing stem canker severity (40 to 49 % reduction) relative to the infested controls over all trials. L. arvalis ZH- 1, R. zeae LRNE17E, and the chemical control reduced black scurf 54 to 60 % relative to the infested control. Other treatments also significantly reduced stem canker and black scurf, however they were slightly less effective. Different rates of biocontrol organisms provided varying disease reductions with higher rates usually providing the best control. One combination of biocontrol organisms, B. subtilis and T virens, demonstrated somewhat better control of stem canker than each organism alone, suggesting that this approach may provide improved biocontrol efficacy. Greenhouse trials were also conducted to evaluate the effects of selected rotation crops including barley, two ryegrass varieties (common and "Lemtal"), clover, potato, and combinations of barley with ryegrass or clover, on populations of R. solani and Rhizoctonia disease. Potato and clover preceding potato resulted in higher disease severity of stem canker than most other rotations, whereas Lemtal ryegrass reduced stem canker severity. In addition, all ryegrass treatments resulted in substantially higher populations of R. zeae. Field trials evaluating selected biocontrol treatments in combination with different rotations were conducted at two locations in Maine. Potatoes were treated with Laetisaria arvalis, Trichoderma virens, or Bacillus subtilis and planted as subplots following rotation crops of barley and ryegrass, barley and clover, or potato. Treatment effects on shoot emergence, disease levels, tuber yield, soil microbial communities, and fatty acid methyl ester (FAME) profiles were determined. The barleylryegrass rotation significantly reduced incidence and severity of stem canker and increased tuber yield at one location. Efficacy of the biocontrol treatments varied by rotation and location, with L. arvalis and T. virens reducing black scurf severity and incidence in some rotations. L. arvalis and T. virens also increased some aspects of tuber yield at one location. General fungal and bacterial populations and fatty acid profiles demonstrated distinct differences in microbial community characteristics among rotation crops and biocontrol treatments. Significant crop by biocontrol interactions were observed indicating that biocontrol can be enhanced within beneficial rotations, leading to greater reductions of Rhizoctonia disease of potato.