- SCAB AND FIRE BLIGHT OF APPLE 375
usually required 400 gal of dilute spray per acre to achieve adequate
coverage. Today, smaller trees (with more open canopy and intensive
pruning) require less than half as much pesticide per acre. This has
resulted in a reduction in pesticide expenses, while at the same time
increasing productivity as compared to the large trees of older orchards.
Rosenberger (2003) estimated that the amount of pesticide required
to produce a pound of apples has been reduced by at least 50% and
perhaps by as much as 75%. However, fire blight susceptible dwarf
rootstocks (like M.9 and M.26) and intensive fertilization practices
used in high density plantings, coupled with consumer-driven demand
for cultivars that are highly susceptible to fire blight, mitigated some
problems for scab management, while creating new ones for fire blight
management.
III. IPM AND FIRE BLIGHT
A. History of Fire Blight
Fire blight is a bacterial disease that occurs on many species in the
Rosaceae including apple, pear, cotoneaster, mountain-ash (Sorbussp.),
serviceberry, quince, and loquat. The fire blight pathogenErwinia
amylovorais thought to have originally evolved in North America on
native rosaceous hosts. In the 1600–1700s, European settlers brought
apple trees with them when colonizing North America, and fire blight
was first observed on apple in New York in 1780 (Denning 1794). The
severity of fire blight disease symptoms on apple and especially pear
is much greater than that observed on native rosaceous hosts (van der
Zwet et al. 2012). The introduction and propagation of apple in North
America significantly increased the availability of much more suscep-
tible and widely planted hosts favoring increases in theE. amylovora
population and spread across the continent with the spread of apple
and pear trees.
Fire blight symptoms on apple readily illustrate the strategies of
pathogenesis ofE. amylovoraincluding a growth phase leading to rapid
increases in population size, systemic movement internally within
hosts, and dissemination externally between hosts.E. amylovoracells
overwinter in cankers on stems; as temperatures warm in the spring
and tree growth reactivates, these cells grow and emerge from cankers
as ooze droplets, which are bacterial cells encased in an exopolysac-
charide matrix (Fig. 8.2a; Norelli et al. 2003). The ooze attracts insects
that land on and vector bacteria from their legs and bodies to the