- SCAB AND FIRE BLIGHT OF APPLE 377
further escape host tissue as ooze droplets, where bacterial cells are
disseminated by wind, rain, and insects to actively growing new
shoot tips, the next site of infection (Hildebrand 1939). Shoot blight
infection can occur throughout the summer until the terminal buds set.
Infected shoots appear wilted and take the form of a shepherd’s crook
(Thomson 2000). The brown and then blackened leaves give shoots the
appearance of being burned; the name fire blight originated from this
symptom (Fig. 8.2c and 8.2d). Because ooze extrusion occurs readily on
infected shoots, the spread of fire blight between shoots and between
trees can be quite rapid. Extensive shoot blight infections, especially
on young trees, can ultimately lead to tree death.
Following blossom or shoot blight infection,E. amylovoracells can
move systemically through the host via xylem and cortical parenchyma
tissue of stems. This net downward migration of pathogen cells ends at
the rootstock crown, whereE. amylovoraincites cankers that eventually
girdle the rootstock resulting in tree death (Aldwinckle and Beer 1978),
a form of the disease called rootstock blight.
In seasons with conducive weather conditions for disease, blossom
blight infections can significantly reduce current season yields, and
even more importantly result in enhanced spread of the pathogen
within and between trees resulting in extensive shoot blight infection
that can ultimately lead to tree death. Because ooze extrusion occurs
readily on infected shoots, the spread of fire blight between shoots and
between trees can be quite rapid, especially in high-density plantings.
Furthermore, the occurrence of strong storms with high winds and hail
is associated with trauma blight, a phase of the disease where spread
is extensive, and infection rates of trees can rise from<10% to near-
ing 100% after strong storms (J.L. Beckerman and G.W. Sundin, unpub-
lished observations).
The most important issue with fire blight in commercial apple pro-
duction is the potential for this disease to kill trees, especially in
younger (<8-year old) orchard blocks of highly susceptible hosts. The
fact that the most popular apple cultivars in terms of consumer prefer-
ence are often the most highly susceptible to fire blight further exac-
erbates this problem. For example, cultivars such as “Gala,” “Fuji,”
“Pink Lady,” and “Granny Smith,” and the widely planted processing
cultivar “Jonathan” are all highly susceptible to fire blight. Following
blossom or shoot blight infection,E. amylovoracells can move sys-
temically through the host via xylem and cortical parenchyma tissue
of stems. This net downward migration of pathogen cells ends at the
rootstock crown, whereE. amylovoraincites cankers that eventually
girdle the rootstock resulting in tree death (Aldwinckle and Beer 1978).