Molly J. McGrath,
Jessica M. Koczan,
Megan M. Kennelly, and
George W. Sundin
First, second, third, and fourth authors: Department of Plant Pathology, Michigan State University, East Lansing 48824; and third author: Department of Plant Pathology, Kansas State University, Manhattan 66506.
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Accepted for publication 2 February 2009.
Mechanisms of fire blight control by the shoot-growth regulator prohexadione-calcium (ProCa) were investigated by comparing disease development in ProCa-treated potted apple trees (cv. Gala) to paclobutrazol (another shoot-growth regulator)-treated and nontreated trees and in ProCa-treated cv. McIntosh trees in the field. Twenty-eight days after inoculation with Erwinia amylovora Ea110, disease incidence on ProCa- and paclobutrazol-treated shoots was significantly reduced compared with that on nontreated shoots. Disease severity (percent shoot length infected) was also significantly lower on both ProCa- and paclobutrazol-treated shoots than on nontreated shoots. However, bacterial populations within inoculated shoots were high and bacterial growth occurred in all treatments. In addition, the mean cell wall width of the cortical parenchyma midvein tissue of the first and second youngest unfolded leaves of ProCa- and paclobutrazol-treated shoots was significantly wider both 0.5 and 2 cm from the leaf tips compared with the cell walls of the nontreated tissue. Taken together, these results suggest that reduction of fire blight symptoms by ProCa and paclobutrazol is not the result of reduced populations of E. amylovora in shoots. Moreover, because paclobutrazol also reduced disease severity and incidence, changes in flavonoid metabolism induced by ProCa but not paclobutrazol does not appear to be responsible for disease control as suggested in recent literature. Finally, although this study did not directly link disease control to the observed cell wall changes, the possibility that an increase in cell wall width impedes the spread of E. amylovora should be investigated in more depth.
© 2009 The American Phytopathological Society