Epsilon Open Archive

Abstract

Note! Figure 5 on page 41 is not yet published by Epsilon in its entirety, on the author´s request. The rhizosphere microflora and its impact on plant growth is in many respects still unexplored, and biocontrol is part of a growing research area called 'plant-microbe interactions'. The aim of this study was to find bacterial isolates that were able to suppress wheat (Triticum aestivum) diseases caused by Fusarium spp. and Microdochium nivale. A special challenge was the snow mould disease, which brought about the low temperature approach used in this work. Psychrotrophic bacteria were isolated at +1.5°C and 598 isolates were screened in a field-correlated greenhouse bioassay, out of which 163 were tested for disease suppression under field conditions (I). In field experiments during five consecutive years biocontrol of wheat seedling blight, caused by F. culmorum and M. nivale was in many cases as effective as the fungicide (guazatine acetate) control. Three isolates suppressed snow mould in winter wheat and brought about an average yield increase of 1450 kg/ha (I). Of special interest was how selection during the isolation of bacteria could affect the proportion of isolates with disease suppressiveness (II). The isolates were grouped and analysed in search for isolation factors that had a propitious effect on the isolation frequency of such isolates. In total, 30 groups were defined. The colony morphology of the isolates, which is a crude reflection of bacterial species or group, was also analysed. An unexpected discovery was that there was a higher frequency of potential biocontrol agents isolated from plants of the Brassicaceae family. A morphological group containing 56 highly disease-suppressive Isolates having characteristic Optically Denser Spots in their colonies were found and denoted IODS. Members of this group have similar in vitro inhibition spectra of pathogens and produce an antifungal polyketide (DDR) (II, III). A second aim was to identify the mode of action of isolates with disease-suppressive ability. One of the IODS, strain MF 381 produced DDR, pyrrolnitrin, and a novel antibiotic named pseudotrienic acid A (III). In other strains, metabolites such as massetolide and viscosin were found (II). One of the strains (MF 30) had a high disease-suppressive effect against F. oxysporum f.sp. radicis lycopersici causing wilt in tomato. Results by other authors indicate that MF 30 induces plant resistance by means of bacterial lipopolysaccarides through the ISR pathway (IV; Konnova et al. 1999). Yet another mode of action is probably operating in the biocontrol of Fusarium spp. obtained with the non-antibiotic producing strain MF 626 (an atypical Pantoea agglomerans). A set of induced mutants with mutations in in plantae-induced genes has been created and are currently being characterised.