Masters Theses

Date of Award


Degree Type


Degree Name

Master of Science


Entomology and Plant Pathology

Major Professor

Kimberly D. Gwinn

Committee Members

Bonnie H. Ownley, Ernest C. Bernard, Craig H. Canaday


Pythium damping-off has the potential to cause severe loss in greenhouse and field grown tomatoes. Species of Pythium are found in soils from all climates, and capable of surviving for long periods without a host. Infectious structures of Pythium species are motile, and therefore able to travel through irrigation water and runoff. Pythium myriotylum thrives in warm, humid environments such as that of the Southeastern United States, and was thus chosen for this study. Currently, no tomato varieties with resistance to damping-off are available. In addition, the agriculture industry is striving for sustainable and biological methods of control of plant pests and pathogens. Therefore, biological controls that are capable of simultaneously protecting plants from pathogens and pests are needed. To that end, the first part of this investigation for biological control of tomato damping-off involves the seed application of an entomopathogenic fungus, Beauveria bassiana (Balsamo) Vuillemin, along with a commercial plant growth-promoting rhizobacteria formulation (BioYield) that is known to induce systemic resistance in plants to herbivores and pathogens, and a soil amendment with Monarda sp. containing essential oils that are fungicidal to many soilborne pathogens. The objectives of the first study were to determine the following: (i) if herbage of Monarda didyma used as a soil amendment is capable of suppressing damping-off of tomato seedlings; (ii) if conidia of Beauveria bassiana isolates used as seed coatings are capable of suppressing damping-off of tomato seedlings; (iii) if a commercial form of plant growth promoting rhizobacteria used as a seed drench is capable of suppressing damping-off of tomato seedlings; and (iv) if combinations of the herbage and the seed treatments are synergistic or antagonistic.

Results from the first study indicated cultivar specificity with Beauveria bassiana and herbage treatments. Survival was increased in ‘Mountain Spring’ tomato seedlings treated with either B. bassiana 11-98 or BotaniGard when challenged with the pathogen, but no similar effects were observed in ‘Celebrity’ seedlings. There was also an increase in stem diameter in Beauveria-treated ‘Mountain Spring’ that was not seen in ‘Celebrity.’ When ‘Celebrity’ seedlings were grown in media amended with ‘Puerto Purification,’ there was a significant decrease in disease index when challenged with the pathogen. This effect was not observed in ‘Mountain Spring.’ ‘Violet Queen’ had negative effects on ‘Celebrity’ seedling growth, seen as a decrease of survival and increase in disease index. Treatment with PGPR had no significant effects in either cultivar.

The second part of this research investigated dried, ground leaves (herbage) from 16 Monarda varieties as amendments for biological control against Pythium damping-off in tomato. The objectives of this study were to determine the following: (i) if Monarda essential oil constituents could inhibit growth of P. myriotylum in vitro; (ii) if herbage amendments could suppress Pythium damping-off; (iii) if herbage amendments had any adverse or beneficial effects on tomato seedling growth.

When essential oil constituents of Monarda were tested for toxicity against P. myriotylum, thymol and carvacrol inhibited mycelial growth at low and high concentrations (5 and 50 µl, respectively). GC-MS analysis of the herbage used in this study showed concentration of thymol and carvacrol to be variable among varieties. ‘Croftway Pink’ was high in thymol; ‘Sioux’ was approximately equal in thymol, carvacrol, and the sesquiterpene thymoquinone. ‘Mohawk’ had a high concentration of thymoquinone and Rose Geranium had no detectible amounts of thymol, carvacrol, or thymoquinone. Treatments with four of sixteen Monarda varieties were successful in decreasing disease index and increasing survival of ‘Mountain Spring’ seedlings when challenged with the pathogen. ‘Croftway Pink’ dominated the varieties with significantly increased shoot height, stem diameter, and survival, as well as decreased disease index in tomato seedlings. Three other amendments, ‘Sioux’, ‘Mohawk’, and Rose Geranium, had no negative effects on seedling growth and increased seedling survival.

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