Pest Management 8. Scab and Fire Blight of Apple: Issues in Integrated
Fig. 8.1. (a) Apple scab infection of “McIntosh” fruit at harvest. (b) Fire blight infection
of “Gala” apple trees.
costs associated with harvesting and grading increase concomitantly
because of the time involved in discriminating between intact and non-
diseased fruit and/or removing diseased fruit during grading.
Fire blight infections result in the death of flower clusters and termi-
nal shoots; further systemic spread of the pathogen within trees can
result in rootstock blight and tree death (Norelli et al. 2003). Under
conducive environmental conditions, the spread of fire blight among
trees can be extremely rapid, overtaking entire orchards (Fig. 8.1b).
The most significant costs associated with fire blight involve tree loss,
orchard removal, and replanting costs. For example, the most severe
fire blight epidemic in Michigan in 2000 resulted in the death of more
than 400,000 trees and the loss of approximately 12% of the total apple
acreage in the state (Longstroth 2001).
The economics of apple production are driven in large part by apple
cultivar and rootstock selection and maximizing the yield potential of
optimally sized apples. Changes in production strategies such as the
incorporation of dwarfing rootstocks and high-density plantings have
reshaped the modern apple production system (Weber 2001; Fallahi
et al. 2002; Robinson et al. 2011a). These shifts in production practices
have also had large impacts on apple disease control. Likewise, dis-
ease control strategies have changed over time with advances in disease
resistance breeding and the introduction of modern synthetic fungi-
cides, favoring the use of IPM practices in disease management. How-
ever, changes in practice resulting in the improved management of one
disease may have consequences on the control of other diseases. Culti-
var choice has a significant impact on disease control, as many of the
most popular cultivars are highly susceptible to economically impor-
tant diseases such as fire blight. Similarly, changing to higher-density