Masters Theses

Date of Award

12-2005

Degree Type

Thesis

Degree Name

Master of Science

Major

Plant Sciences

Major Professor

William E. Klingeman

Committee Members

Bonnie Ownley, Kimberly Gwinn

Abstract

Beauveria bassiana (Balsamo) Vuillemin is an entomopathogenic fungus with an extensive insect host range. The isolate Beauveria bassiana 11-98 also endophytically colonizes tomato and cotton plants. In our research, B. bassiana 11-98 was evaluated for its potential to control insect pests of tomato in laboratory assays. Potato aphids, Macrosiphum euphorbiae (Thomas) exposed to different formulations of B. bassiana 11-98 coniidia in direct contact and contact-while-feeding assays using target concentrations of 1 x 104, 105, 106 and 107 conidia/ml. After exposure for 10 days, aphid mortality did not exceed 40% and rate of reproduction was not affected in any treatment. B. bassiana 11-98 provided insufficient control of aphid populations in our assays.

Foliage feeding tomato fruitworms, Helicoverpa zea (Boddie) were exposed to initial concentrations of 1 x 106 and 107Beauveria bassiana 11-98 conidia/ml in direct contact and consumption of viable conidia assays. Larvae in all assays were observed for mycosis 5, 10 and 30 days after treatment (DAT). Larvae that consumed viable conidia had greater and more rapid mortality when compared to larvae in the topical application assay. The highest mortalities in topical application and consumption assays were 21 and 77%, respectively, observed 30 DAT.

A separate bioassay examined the effect of H. zea feeding on endophytic tomato foliage established by coating seeds with B. bassiana 11-98 and growing tomato seedlings under gnotobiotic conditions. Starved (24 h) larvae were fed foliage from 10-wk-old tomato plants in both choice and no-choice assays. In choice assays, larvae were offered leaf disks from treated and untreated plants and allowed to feed for 6 and 24 h. Larvae in no-choice assays were fed within bags for 48 and 120 h. While 3rd instar H. zea larvae showed no clear preference for treated or untreated foliage, they exhibited a slight trend for less consumption of foliage from B. bassiana 11-98-treated plants. Larvae that fed on B bassiana 11-98-treated foliage did have mortality (17%) after feeding for 120 h.

Finally, foliage from 8-wk-old, endophytically-colonized tomato plants grown from B. bassiana conidia-coated seeds was assayed for presence of the fungus and beauvericin using mycological media cultures and high pressure liquid chromatography, respectively. In the fungal isolation assay, 3 leaves were collected from the top, middle and bottom sections of 40 plants treated with B. bassiana and 10% of those plants demonstrated endophytic growth on media cultures after 4 wk. The mycotoxic metabolite beauvericin was recovered from lateral foliage of 22 out of 26 eight-wk-old endophytically colonized tomato plants.

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