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Pathogen Biology

Erwinia amylovora is a member of the family Enterobacteriacae. Cells of E. amylovora are gram-negative, rod-shaped, measure 0.5-1.0 x 3.0 mm, and flagellated on all sides (peritrichous) (Figure 9). Physiologically, E. amylovora is classified as a facultative anaerobe. It grows on most standard microbiological media and on several differential media. Optimum temperature for growth is 27°C (81°F), with cell division occurring at temperatures ranging from 5 to 31°C (41 to 88°F). Identification of E. amylovora isolates is based on biochemical and serological tests, inoculation of immature pear fruits and apple seedlings, and DNA hybridization assays.

Figure 9

Figure 10

Epiphytic and endophytic biology

On blossoms, E. amylovora is an excellent colonizer of the surfaces of stigmas and, to a lesser extent, the surface of the nectary. This reproduction on blossom surfaces is called epiphytic growth and occurs without the bacterium causing disease. Epiphytic growth of E. amylovora on stigmas combined with movement of the pathogen from blossom to blossom by pollinating insects (Figure 10) are two important processes that regulate the incidence of blossom infection. Erwinia amylovora also can survive on other healthy plant surfaces, such as leaves and branches, for limited periods (weeks), but colony establishment and epiphytic growth on these surfaces does not occur. Cells of E. amylovora excrete large amounts of an extracelluar polysaccharide (a major component of bacterial ooze), which creates a matrix that protects the pathogen on plant surfaces. In propagation nurseries, cells of E. amylovora surviving on woody surfaces can initiate disease when scions and rootstocks are wounded during grafting. Erwinia amylovora also can reside as an endophyte within apparently healthy plant tissue, such as branches, limbs, and budwood. Migration of the pathogen through xylem is one mechanism by which blossom infections of apple can lead to rootstock infections near the graft union.

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