Doctoral Dissertations

Date of Award


Degree Type


Degree Name

Doctor of Philosophy


Entomology, Plant Pathology and Nematology

Major Professor

Heather Kelly

Committee Members



Nematodes and diseases can limit field crops and hemp production in Tennessee. Therefore, seven research objectives were undertaken to improve the management of the crops: First, the prevalence and density of plant-parasitic nematode populations were evaluated across many field crop acreages in Tennessee through a survey from 2018 through 2020. Second, the virulence phenotypes of soybean cyst nematode (SCN), (Heterodera glycines [HG]), populations were assessed in Tennessee fields using the HG types test. Third, the impact of the virulent SCN HG type on soybean nitrogen fixation activity and growth was evaluated on different soybean genotypes in the greenhouse. Fourth, the impact of different densities of Macrophomina phaseolina [Mp] microsclerotia on charcoal rot disease severity and soybean damage were assessed. Fifth, the impact of soybean varieties on Mp density in soil and plant tissue was evaluated using greenhouse tests. Sixth, the causative pathogen of greenhouse hemp powdery mildew in Tennessee was identified using PCR and DNA sequencing. Finally, multiple fungicides were screened for the control of hemp powdery mildew and leaf spots in the greenhouse and field, respectively.

Fourteen nematode taxa occurred across samples. The most prevalent nematodes across samples occurred at 83% (spiral, Helicotylenchus spp.), 29% (lance, Hoplolaimus), 25% (SCN, Heterodera glycines), 23% (lesion, Pratylenchus), 20% (reniform, Rotylenchulus), 19% (stunt, Tylenchorhynchus), and 13% (root-knot, Meloidogyne). Reniform, RKN, SCN, and lesion nematodes occurred at high densities in samples. All tested SCN populations were virulent (HG-types, 2.5.7, and 7), and HG-type had varying effects on soybean NF and growth among susceptible cultivars, but no effect on the resistant cultivar. Macrophomina phaseolina density of about 327 CFU/gram of soil caused charcoal rot disease on a susceptible soybean. Moderately resistant soybean cultivar suppressed Mp densities in soil. Golovinomyces ambrosiae was identified as the pathogen causing hemp powdery mildew. All tested fungicides suppressed powdery mildew, with Bonide sulfur, Cinnerate, Exile, MilStop, Regalia, and Sil-Matrix being the most effective fungicides.

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Table 2.1

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