378 J.L. BECKERMAN AND G.W. SUNDIN
Fig. 8.3. Loss of “SweeTango” apple trees in a young high-density planting due to fire
blight infection. Missing trees killed by fire blight are denoted by arrows.
Thus, the combination of pathogen movement within trees combined
with the potential for rapid dissemination among trees fuels the most
severe fire blight epidemics, with significant tree loss as the final out-
come (Fig. 8.3).
B. Apple Production Practices Favoring the Occurrence
of Fire Blight
Several aspects of modern commercial apple production have favored
the occurrence of fire blight. The first is through the intensive use of
fertilization and irrigation maintaining high growth rates of trees and
increased productivity. Second actively growing apple shoots are signif-
icantly more susceptible to fire blight infection than more slowly grow-
ing shoots. TheE. amylovorapathogen is also more likely to move sys-
temically through infected shoots further into trees with higher growth
rates. The growth of these cultivars in high-density monoculture plant-
ings (Fig. 8.3) has only made it much easier forE. amylovorapopu-
lations to infect and reproduce. Thirdly, the availability of effective