Masters Theses

Patrice Smith

Date of Award

12-1997

Degree Type

Thesis

Degree Name

Master of Science

Major

Entomology and Plant Pathology

Major Professor

Bonnie Ownley

Committee Members

Ernest C. Bernard

Abstract

Rhizoctonia solani causes damping-off and root rot of tomato plants. Post-emergence damping-off begins as a canker on the stem located above the soil line. Growers traditionally have managed damping-off and root rot of tomato with chemical pesticides. Due to the increasing awareness of the harmful effects that chemical pesticides impose on the environment and humans, alternative control options are needed. The focus of this study was to assess bacterial isolates for biological control of R. solani on tomato.

Three collections of Bacillus and Pseudomonas isolates were evaluated for biocontrol activity. The collections were from a tobacco field soil, rhizosphere of tomato and within tomato root tissue (endophytes). Of the 204 bacterial isolates screened, 13 decreased disease severity of R. solani, 10 increased shoot height, and 11 decreased plant mortality. Of 94 bacterial isolates collected from tobacco field soils, five reduced disease severity of R. solani BA19, BA77, BA101, BA23, and BA106. Tobacco field soil isolates decreased disease severity from 13% to 64 %, and decreased plant mortality by 25% to 92%. Eight of the 60 endophytic isolates decreased disease severity, ranging from 31 % to 60%. The endophytes also decreased plant mortality (31 to 76 %) and increased shoot height of tomato seedlings from a range of 25 to 76%. Isolates collected from the tomato rhizosphere did not affect disease caused by R. solani.

One tobacco field soil isolate inhibited growth of R. solani in culture on Min 3C agar. Many of the bacterial endophytes suppressed growth of R. solani on Min 3C. Isolates exhibiting the greatest zones of inhibition were BA101, E69, E723, E721, E722, E729, and E730. While antibiosis was not the sole mechanism of action involved, it does appear to be a contributing factor, particularly with the endophytic isolates.

Characterization and identification of the 13 bacterial isolates effective against R. Solani was relevant for determining the possible mechanism of suppression of R. Solani. Bacterial isolate BA106 was Gram-negative, and the remaining isolates were Gram-positive. Tobacco field soil isolates BA77, BA19, BA23, BA101, and all of the endophytic isolates were catalase positive. Bacterial isolates E726, BA77, BA19, E61, and E65 multiplied anaerobically. Starch hydrolysis was negative for isolate BA106, and positive for isolates BA77, BA19, BA101, BA23 and the endophytic isolates. Bacterial isolates BA101, E61, and BA106 utilized citrate and the remaining isolates did not. Both the endophytic isolates and the tobacco field soil isolates were tolerant of 7% NaCl.

Acid production on mannitol medium was positive for bacterial isolates BA101, BA23, and BA106. Acid production on arabinose medium was positive for BA23 and BA106. Acid production on xylose was positive for bacterial isolates BA77, BA19, and BA23. Tobacco field soil isolates and the endophytic isolates digested protein and decomposed chitin, characteristics that might be important for inhibition of R. Solani.

According to the MicroLog identification system, Gram-positive bacterial isolates were identified as bacilli, and the Gram-negative bacterium was identified as a pseudomonad. The bacterial isolates were identified as B. thuringiensis/cereus BA77, B. megaterium BA101, P. corrugata BA106, В. pasteurii BA19, B. azotoformans BA23, B. subtilis E726, B. pasteurii E66, Β. megaterium E61, B. coagulans E69, B. megaterium E65, and B. brevis E21. Two isolates of Bacillus, E727 and E723, were not identified to species.

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