Jane Hollywood - University College, London
The cost of losses and control measures attributed to late blight of potatoes caused by Phytophthora infestans, are estimated to exceed $5 billion annually. This is the single most costly biotic constraint to global food production. Breeding for resistance is difficult owing to the tetraploid genotype of potato and current strains of the pathogen have developed resistance to chemical control. Consequently the search for biological control has assumed greater importance.
In this investigation an in vivo bioassay was used to select soils antagonistic to late blight of potatoes, caused by Phytophthora infestans. Four out of eight samples demonstrated reproducible antagonism as determined by a reduction in the volume of tissue rotted. A total of 292 bacterial and yeast samples and 20 fungal samples were recovered from suppressive soils using a variety of non-selective and selective media. When these organisms were tested individually against P. infestans in the assay, 15 isolates suppressed tuber rotting by >85% in at least three out of four assays. The antagonists were characterised as Pseudomonas spp. (3 strains), Enterobacter spp. (4 strains), Bacillus sp. (1 strain), Pantoea spp. (2 strains), Citrobacter sp. (1 strain), Buttiauxella sp. (1 strain), Trichosporon spp. (2 strains) and Geotrichum spp. (1 strain) by routine bacteriological tests, fatty acid profiling and partial sequencing of the gene encoding 16S or 18S (where appropriate) ribosomal RNA.
This investigation examined the possible mechanisms by which these potential biocontrol agents inhibited P. infestans. Nine isolates showed some evidence of antibiotic production with a Pantoea sp. producing a compound that caused the hyphae of P. infestans to kink and permanently cease growth. Three isolates colonised hyphae of the pathogen and eleven produced siderophores in liquid culture. Hydrogen cyanide, proteolytic, cellulolytic and lipase activity was also evident in some species.
Inoculation of disease conducive soils with a single biocontrol strain rarely leads to the level of suppression observed in naturally disease suppressive soils and the positive effects with single inoculants are often inconsistent. This natural suppression is proposed to result from consortia of microorganisms and mechanisms. The application of combinations of BCAs may more closely mimic the natural environment, resulting in a higher level of protection, with reduced variability and have the potential to suppress multiple plant diseases. It is also possible that the level of disease control achieved by mixtures of effective BCAs may be less influenced by factors such as crop cultivar than applications of a single BCA. Therefore, this investigation looked at the levels of control that could be obtained with a combination of microorganisms.
In order to determine which microorganisms to test in combination, three population levels of the isolates were tested for disease inhibition. Three of the isolates (isolates 3, 7 and 14) exhibited consistently high levels of inhibition at the lowest population levels and these isolates were tested in combination. This achieved disease suppression that was more consistent than isolate 3 alone and was over 30% greater than either isolate 7 or 14.
It is well known that certain rhizobacteria stimulate plant growth. Such rhizobacteria are described ass 'plant growth-promoting rhizobacteria' (PGPR). The properties of PGPR offer great promise for agronomic application. Large-scale application of PGPR to crops could reduce the use of chemical fertilisers and increase crop yield. In 1985, Rhizobia became the first commercial PGPR, marketed as 'nitragin'; now similar products are well established as agricultural products in the USA, Europe, Australia and India. This investigation examined whether the any of the late blight suppressing microorganisms, isolated in this study, also exhibited growth promoting properties. Tomato seedlings were used in place of potato plantlets, because of ease of handling and because late blight also infects tomato. The seedlings were inoculated with the microorganisms, and grown in axenic conditions. Significant growth promotion, as determined by increased stem and main root elongation, was achieved by ten of the isolates.