THE DISPERSAL OF BACTERIA FROM LEAF SURFACES BY WATER SPLASH

Advances in the techniques of genetic engineering have made possible the use of genetically manipulated micro-organisms (GMOs) for the control of pests and diseases. Before GMOs can be widely used in agriculture, however, their fate after release must be understood. Dispersal of released GMOs will have an important influence on their action or survival under field conditions. The object of this study was to quantify the efficiency of rain as a means of removing and disseminating such micro-organisms from foliar crop surfaces. Spontaneous mutants of three species of bacteria (Pseudomonas syringae, Klebsiella planticola and Bacillus subtilis), resistant to the antibiotic rifampicin, were sprayed on two plant species, french bean, Phaseolus vulgaris, and oilseed rape, Brassica napus. The leaves from the plants were then exposed to artificial rain and splash droplets generated by the impact on the leaves were collected on selective nutrient agar in Petri dishes and in sterile glass 28 ml screw-capped bottles. The bacterial content of the splashed drops was assessed, as was the content of the water which ran off the leaf surfaces. Comparisons of the numbers of bacteria removed from the leaves with the numbers applied prior to splashing show that rainfall can be a very efficient means of removing bacteria from foliar surfaces, but that most of the removed bacteria run off to the soil. Only a small proportion was splashed relatively short distances from the source.