Biological control of late blight of potatoes : in vivo and in vitro evaluation of microbial antagonists against tuber blight
The cost of losses and control measures attributed to late blight of potatoes caused by Phytophthora infestans, are estimated to exceed $5 billion annually. 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 tuber 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 spp. (1 strain), Pantoea spp. (2 strains), Citrobacter spp. (1 strain), Buttiauxella spp. (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. Subsequently the possible mechanisms by which the potential biocontrol agents inhibited the disease were examined. Nine isolates showed some evidence of antibiotic production with a Pantoea spp. 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 beta-1,3-glucanase activity was also evident in some species. Significant promotion of axenically grown tomato seedlings, as determined by increased stem and main root elongation, was achieved by ten of the isolates. Three population levels of the isolates were retested for disease inhibition at the end of the investigation. Isolates 3, 7 and 14 exhibited the highest levels of consistent inhibition at the lowest population levels and were therefore tested in combination. This achieved disease suppression that, at an antagonist concentration of 25 cfu/nL, was more consistent than isolate 3 alone and was over 30% greater than either isolate 7 or 14.