These cankers continue to enlarge during the growing season and may girdle the affected part, resulting in the death of the entire branch or tree. The surface of a canker is somewhat sunken, relative to the surrounding healthy tissue, and the bark is usually darker in color. In some cultivars, it is difficult to determine the margin of the canker without cutting into the wood to expose the discolored and infected tissue. The diseased inner bark of older branches becomes reddish brown and marbled, in contrast with the whitish color of normal wood. Fruits are susceptible to infection until just before maturity.
Diseased fruit is first water-soaked, turns brown, shrivels, and turns black.
The bacteria in the infected blossoms or shoots pass through healthy limbs and trunks and reach the rootstock and produce the cankers. Root suckers and sprouts from susceptible rootstocks also may become infected, much as do the shoots. These infections can also lead to the invasion of the entire root system and the rapid death of the tree. Fire blight is often followed by Black Rot and Wood Rot. Click on image for larger version Figure 5.