Yongmei Luo

Document Type

Thesis - University Access Only

Award Date

2000

Degree Name

Master of Science (MS)

Department / School

Biology and Microbiology

Abstract

Tan spot and Fusarium head blight (FHB), caused by Pyrenophora tritici-repentis and Fusarium graminearum (teleomorph Gibberella zeae), respectively, are fungal diseases of wheat which can cause serious crop-yield reductions. Biocontrol agents that antagonize the pathogens could be useful in management strategies for controlling tan spot and FHB. Four Bacillus strains designated lBA, lBC, lBE and 1D3, isolated by previous workers from South Dakota wheat foliage as potential biocontrol agents of tan spot and FHB were the focus of this study. In plate assays challenging the pathogens, cell-free, concentrated ethyl-acetate extracts of the broth supernatants of each bacterial strain produced inhibition zones ranging from 14 to 22 mm depending on the bacterial strain. In greenhouse assays the results showed highly significant reduction of tan-spot symptoms on leaves of spring wheat seedlings that received foliar-applied, cell-free broth supernatant extracts, foliar-applied whole cells, or seed treatment with whole cells of the Bacillus strains. Extracts of lBE and lBC were used in a disease-nursery in the summer of 1998. The results showed significant reduction of tan spot and FHB symptoms on leaves or heads of spring wheat that received cell-free broth supernatant extracts of Bacillus strains. Application of Bacillus strain lBE extract resulted in 18.8% reduction in tan spot severity and 25.9% reduction in FHB disease severity, and application of Bacillus strain lBC broth supernatant extract resulted in 23.8% reduction in tan spot severity and 20.9% reduction in FHB disease severity. The compounds found in cellfree, ethyl-acetate extracts of bacterial culture supernatants were separated by thin layer chromatography (TLC) in a solvent system of 1-butanol:acetic acid: H20 (3: 1: 1) and detected by UV light and iodine vapor. Antifungal compounds were detected by recovering individual spots from TLC plates, eluting them and re-applying them to sterile paper disks used in a plate assay. Twelve spots were found in extracts of each Bacillus strain on TLC plates. Among them, some spots showed antagonism to P. tritici-repentis isolate Ptr 002 in plate assays. Six such spots were found for Bacillus strain IBA at Rr values of 0.30, 0.44, 0.51, 0.63, 0.67 and 0.74; three such spots for Bacillus strain lBC at Rrvalues of0.27, 0.71 and 0.77; six such spots for Bacillus strain lBE at Rrvalues of 0.33, 0.44, 0.51, 0.61, 0.70, 0.76, and 0.81; and four such spots for Bacillus strain 1D3 at Rr values of 0.24, 0.36, 0.67 and 0. 71. None of the individual spots recovered from TLC plates showed antagonism to F. graminearum isolate Fg 4 in plate assays, even though the crude concentrated extracts were antagonistic to F. graminearum. The results showed that whole cells or cell-free broth supernatant extracts of these Bacillus strains helped protect wheat seedlings against tan spot in a greenhouse situation, and mature wheat against tan spot and FHB in a disease-nursery situation, and that the Bacillus strains could be differentiated from one another based on TLC analysis of cell-free, broth-culture supernatant extracts. One or more of these Bacillius strains may prove useful in management schemes for controlling tan spot and/or Fusarium head blight.

Library of Congress Subject Headings

Wheat -- Diseases and pests -- Biological control
Fungal diseases of plants
Fusarium diseases of plants
Bacillus (Bacteria)

Format

application/pdf

Number of Pages

128

Publisher

South Dakota State University

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