Infection of plants by pathogenic bacteria can generally be considered in terms of three interrelated phases:

1. Population build-up, competition and migration of bacteria at the plant surface.

2. Bacterial entry into plant tissue.

3. Migration of bacteria within the plant to and from regions of multiplication.

Build-up and activity of epiphytic populations

Population level

The presence of epiphytic pathogens on host plants does not imply that disease will necessarily develop, and many cases have been reported where quite high levels of pathogenic bacteria were present on symptomless foliage. This has been noted, for example, for Pseudomonas syringae pathovars on red maple (Malvick & Moore, 1988) and snap beans (Legard & Schwartz, 1987) and for Erwinia amylovora on apple and pear blossom (see later).

In other situations, the presence of epiphytic bacteria does lead to disease development. This was initially noted by Crosse (1957), who reported the presence of Pseudomonas syringae pv. mors-prunorum as an epiphyte on cherry foliage, leading to canker formation. The relationship between epiphytic occurrence and disease development has subsequently been investigated for a wide range of bacterial pathogens by monitoring naturally occurring populations and carrying out experimental inoculations of plant surfaces. These studies have shown that plant infection and disease development depend on a number of factors, including the particular host—pathogen combination, critical environmental conditions, physiological stress of the host plant and the attainment of minimal threshold levels of the pathogen.