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., Aissa, A.D., Kremer, R.J. 2007. Biological control of wheat take-all disease: I - characterization of antagonistic bacteria from diverse soils toward Gaeumannomyces graminis var. tritici. Tunisia Journal of Plant Protection. 2:23-34. Interpretive Summary: “Take-all” root rot caused by a soilborne fungal pathogen [Gaeumannomyces graminis var. tritici (Ggt)] is the most damaging root disease of cereal grain crops worldwide. Plants can be infected at all stages of growth under conditions favoring the development of Ggt. Favorable conditions for the disease involve a combination of numerous factors including susceptible host plant, low plant nutrient availability in soil, an imbalance in nitrate to ammonium nitrogen, and high soil pH. Thus a high level of management is required to prevent or control the disease in cereal production fields. Seed or soil treatment with fungicides is often inconsistent in control 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. Because take-all is a disease problematic globally in cereal production fields, we collected numerous bacteria from roots of wheat growing in soils from Missouri and Tunisia. The collected bacteria were characterized for ability to suppress growth of Ggt. We also determined if certain soil properties or management systems were related to growth suppressive activity of the root bacteria. Bacteria from wheat seedlings growing in different soils were tested under laboratory conditions for suppression of Ggt alone in culture and when residing on wheat roots causing infection. Of over 500 cultures tested, 23 were most inhibitory to Ggt growth in pure culture; 13 of these were able to inhibit colonization of roots and onset of disease by Ggt. Interestingly, the groups of inhibitory bacteria differed between the Missouri and Tunisian soils suggesting an influence of soil properties in selecting the most effective bacteria in each region. For example, the acid pH, high organic matter, and moderately moist soils of central Missouri favored one type of bacteria compared with those associated with the alkaline pH, low organic matter, and dry soils of Tunisia. The two groups of bacteria also suppressed Ggt through different modes of action based on different antibiotic production and strategies for making certain nutrients inaccessible to Ggt. The information gained from this research has important implications for other scientists in extension and industry and farmers in two areas. First, knowledge of a variety of bacteria with growth-inhibitory ability toward Ggt offers the opportunity to combine these bacteria for potential application as a biological control strategy. Secondly, overall management practices might be developed to exploit the naturally-occurring soil microorganisms for disease suppression directly in the cereal grain production field.
Technical Abstract: Take-all is a world-wide disease of cereal crops that varies in severity depending on numerous ecological factors including soil nutrient contents and microbial composition. Microbial interactions with the take-all pathogen, Gaeumannomyces graminis var. tritici (Ggt), have not been fully explained. To determine the impact of rhizosphere bacteria on the activity of Ggt, soil samples were collected from Tunisian and Missouri fields under different crop management systems and histories of take-all disease. Bacterial isolates were collected from wheat rhizospheres in each soil and screened in vitro for their antagonistic activity against Ggt. Twenty-three bacterial isolates were selected and tested in vitro against three Ggt strains using three different culture media. Dual cultures of the antagonists showed that fungal inhibition depended on media type and presence or absence of supplemental iron. A second assay based on detached wheat roots on potato dextrose agar revealed antagonistic activity in only half of the bacterial isolates classified as effective in the in vitro assays. The results suggest rigorous testing is required to confirm the most effective rhizosphere bacteria for development as biological control agents of cereal root diseases due to Ggt.