Plant pathogenic bacteria are not restricted in their occurrence to infected plant tissue, but are widely dispersed throughout the external environment. This chapter will consider the general occurrence of plant pathogenic bacteria in the aerial and soil/water environments, environmental interactions at the micro-level and the association of these bacteria with invertebrates (vectors).

The aerial environment

The aerial occurrence of plant pathogenic bacteria is clearly of particular relevance to those pathogens that infect aerial parts of plants, including leaves, flowers and fruit. The aerial environment includes both physical aspects (e.g. occurrence of bacteria in rain and aerosols) and biotic aspects (occurrence of bacteria on plant surfaces and aerial dispersal by vectors).

Occurrence of bacteria in rain and aerosols

The aerial environment presents a potentially important medium for both survival and transmission of plant pathogenic bacteria, particularly where cells are contained in rain water or fine water droplet dispersions (aerosols).

Rain-water from infected foliage may contain high levels of phytopathogenic bacteria, and may be important in the spread of bacteria both within and between plants. A good example of rain dispersal of pathogen within single plants is provided by the studies of Miller on Erwinia amylovora (reported in Van der Zwet & Keil, 1979), who showed that if a source of inoculum was present in the upper part of a tree, the region of secondary infection below was cone-shaped due to downward dispersal of bacteria by rain-splash.