A screening programme was used to search for biocontrol agents against Gaeumannomyces graminis causing take-all disease of wheat. Of the 1800 rhizosphere microorganisms tested, 10% controlled the disease in a secondary screen. The 30 most effective isolates were further investigated for mode of action. Although 72% of the sites sampled for antagonistic microbes were planted to continuous cereals, they yielded only 23% of the most effective isolates. Of all the isolates selected. 63% belonged to the genera Bacillus. Pseudomonas and Penicillium, Beauveria and Rhodococcus were also antagonistic. Fluorescent pseudomonads. all producing siderophores in low-iron medium, accounted for 23"., of the isolates. Over 50% of strains produced ^-glucanases and chitinases. Less than 50% of the strains selected by the in vivo screen inhibited G. graminis in agar plate tests. In the gnotobiotic system used, the Pseudomonas strains were faster in colonizing the wheat roots than the majority of the Bacillus and fungal strains.

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