Submitted to: Tunisia Journal of Plant Protection
Publication Type: Peer reviewed journal
Publication Acceptance Date: 11/30/2007
Publication Date: 12/31/2007
Citation: Nasraoui, B., Hajlaoui, M.R., Gargouri, S., Kremer, R.J. 2007. Biological control of wheat take-all disease: II – rapid selection of bacteria suppressive to Gaeumannomyces graminis var. tritici in laboratory with greenhouse and field confirmation trials. Tunisia Journal of Plant Protection. 2:35-46. Interpretive Summary: The most damaging root disease of cereal grain crops worldwide is “take-all” root rot caused by a soilborne fungus [Gaeumannomyces graminis var. tritici (Ggt)]. Ggt infects plants at any stage of growth when conditions are favorable for development of the pathogen. Current treatment of soils or seed with chemical fungicides often fails to consistently control disease, is hazardous to human health, and may contaminate the environment. For these reasons there is an increasing interest in developing antagonistic microorganisms for use as biological control agents in suppressing growth and disease initiation by the fungal pathogen. We previously collected over 500 bacteria from roots of wheat growing in soils from Missouri and Tunisia and characterized their ability to suppress growth of Ggt. Because large numbers of bacteria must be assessed to determine the best candidates for use in future biological control trials, we developed a rapid screening method for growth suppression of Ggt. Using a series of test tubes containing either vermiculite or the soil from which a bacterial culture was originally isolated, we could rate and characterize inhibition of seedling root infection by Ggt within 7 to 10 days, compared with at least 21 days required for standard greenhouse assays. Follow-up trials in soils under greenhouse and field conditions verified the effectiveness of selected bacteria with the greatest suppressive activity toward Ggt. This information has important implications for scientists in plant pathology and biological control programs in university and industrial settings because it provides a basis for streamlining the selection process for biocontrol agents by reducing the time required for identifying potentially useful cultures. Incorporation of the rapid selection method described here may further reduce the time required to implement a biological control strategy for Ggt in the field.
Technical Abstract: Screening large collections of bacteria for potential biological control activity on economically important diseases is often difficult. A quick test tube assay was developed to rapidly screen selected bacterial isolates for their ability to suppress take-all disease of wheat, caused by Gaeumannomyces graminis var. tritici (Ggt). The assay involved the use of test tubes (18 mm in diameter X 95 mm long) in which either vermiculite or soil was infested with Ggt from agar cultures. Seeds of wheat (Triticum aestivum) cv. ‘Madsen’ inoculated with a bacterial isolates were mixed with vermiculite and overlaid onto the growing medium in the tubes. After 7-10 days, the amount of the take-all symptoms on the roots was determined. Among 23 isolates studied, 3 were selected based on their effectiveness in suppressing the take-all fungal disease symptoms on wheat roots. A greenhouse assay using soils from Tunisia and Missouri with various histories of wheat production and a field trial in Mexico silt loam soil confirmed significant inhibition of take-all symptoms by two bacterial strains: Pseudomonas aureofaciens 6 from Missouri and Bacillus subtilis 177 from Tunisia. Based on results of our screening procedures, these two bacterial strains were selected for their effectiveness as potential biological control agents against take-all to use in specific soils.