382 J.L. BECKERMAN AND G.W. SUNDIN
in Canada in 2013 and was registered for use in the United States in
late 2014. Kasugamycin may represent the last effective blossom blight
spray material in that, should resistance occur inE. amylovora,there
will likely be no other fire blight control compounds in the industry
pipeline.
D. Alternate Management Options
Prohexadione-calcium (ProCa; Apogee, BASF) is a plant growth reg-
ulator that inhibits late steps of gibberellin biosynthesis resulting in
reduced shoot growth. A byproduct of the reduced shoot growth is
excellent control of the shoot blight phase of fire blight, as the bac-
terium preferentially infects actively growing shoots (Yoder et al. 2000).
We have found that the cell walls of cortical parenchyma cells in plant
leaves are thickened in ProCa-treated shoots (McGrath et al. 2009); these
cells must be infected byE. amylovoraduring the initiation of a shoot
blight infection. The thickened cell walls provide a highly effective bar-
rier in preventing infection, thus providing control of shoot blight. Tim-
ing of application is critical to maximize the efficacy of Apogee as it
takes approximately 10–14 d for the Apogee “effect” of growth reduc-
tion and cell wall thickening to occur (Sundin 2014). Initial application
of Apogee at king bloom petal fall is optimal, as an application at this
time affects growth just as shoots are initiating their period of maxi-
mal growth. Three applications of Apogee, spaced at 2-week intervals,
typically provide shoot blight control throughout the summer months
(G. Sundin, unpublished information). Thus, while ProCa use does not
affect the incidence of blossom blight, this material can play a critical
role in limiting the development of fire blight epidemics.
In the western United States, two applications of Blossom Protect dur-
ing bloom of the thinning treatments lime sulfur plus fish oil resulted
in reduced blossom blight incidence in four of five field trials (John-
son and Temple 2013). These thinning materials, applied at 30% and
70% bloom also suppressed flower infection byE. amylovoraand pre-
sumably controlled blossom blight both by reducingE. amylovorapop-
ulations and by killing flowers, which obviously lowers the number of
flowers available for infection (Johnson and Temple 2013).
E. Genetic Resistance to Fire Blight
The possibilities for breeding for fire blight resistance in apple have
also been tempered because fire blight tolerance is a quantitative or
multi-genic trait, not easily bred and difficult and time-consuming to