- SCAB AND FIRE BLIGHT OF APPLE 379
antibiotics beginning with streptomycin in the 1950s for fire blight man-
agement reduced the necessity for growers to use cultural control prac-
tices such as pruning of active fire blight infections to the point that
these practices are not commonly used today, even as antibiotic resis-
tance has resulted in the loss of streptomycin in several regions of the
United States and elsewhere.
As stated earlier, the shift of apple production to high-density plant-
ings has provided growers with the capacity for significantly higher
yields per acre and yield potential within the second year after estab-
lishment. In addition, the reduction in fungicide use necessary for dis-
ease control has also resulted in significant cost savings to growers.
However, regarding fire blight, high-density plantings introduce several
issues that can have severe negative consequences. First, the empha-
sis in high-density plantings is on maximizing growth, especially in
the first few years after establishment. Unfortunately, growth plays a
key role in susceptibility to fire blight infection (as stated earlier) favor-
ing infection and systemic spread of the pathogen. The second issue
with high-density plantings and fire blight is that the small size and
upright growth habit of the trees reduces the distance the pathogen has
to travel to reach the main trunk of the tree. For example, in a small high-
density tree, infection of a single shoot may lead the pathogen directly
into the main trunk of the tree. Once the pathogen reaches the main
trunk of the tree, further systemic spread can proceed very rapidly to
the rootstock. Even if the rootstock is resistant to fire blight, the pres-
ence of the pathogen in the main trunk of the tree will usually result in
tree removal by the grower. Alternatively, growers can prune the trunk
below severely infected portions of the tree and allow the tree to grow
a new trunk and limbs. However, this procedure, while possibly sav-
ing the tree, will result in a delay of several years in maintaining effec-
tive production from the affected tree. In contrast, larger trees or trees
with larger areas to grow will produce more branch junctions and inter-
sections of wood of different ages; these regions of intersecting branch
tissue are much more difficult for the pathogen to move through and
provide opportunities for pruning and removal of infected tissue before
further spread occurs. Finally, the close proximity of apple trees grown
in high-density plantings enables rapid spread ofE. amylovoracells
between trees by insects or other environmental factors. Loss of signifi-
cant numbers of newly planted trees (5–10% or more) to fire blight from
high-density plantings is an unfortunate and significant economic con-
sequence.
Any examination of the most widely planted apple cultivars in high
apple-producing states such as Washington, New York, and Michigan