Ultraviolet-induced changes in populations of epiphytic bacteria on beetroot leaves and their effect on germination of Botrytis cinerea spores

There was no consistent reduction in numbers of bacteria isolated from unwetted beetroot leaves following exposure to ultraviolet light. Bacteria increased in water droplets on leaves but were readily killed by short exposures to ultraviolet light. After prolonged exposures numbers of surviving bacteria were similar to those on unwetted leaves. Evidence suggested that the rapid recolonization of leaves after ultraviolet irradiation occurred from surviving bacteria either within the leaf or protected on the surface from direct exposure. The higher numbers of bacteria which developed in water droplets placed on leaves previously exposed to ultraviolet light may have been associated with increased leakage of electrolytes, amino acids and carbohydrates from leaves following irradiation. Germination of Botrytis cinerea spores was more strongly inhibited in vitro in droplets from leaves previously exposed to ultraviolet light. The spectrum of bacterial species on leaves was changed following exposure to ultraviolet light but isolates of bacteria from irradiated leaves were not more antagonistic against B. cinerea spores in vitro.