REDUCING SECONDARY FLOWERING IN BARTLETT PEAR TO REDUCE FIRE BLIGHT

Abstract

Fire blight (Erwinia amylovora) is a major bacterial disease of pear. Secondary flowering (bloom) shoots in 'Bartlett' pear provide infection sites for fire blight. The authors have endeavoured to learn about the dynamics of secondary flowering throughout the growing season. In addition, they are identifying production practices that may aggravate as well as mitigate the production of secondary flowering shoots. An orchard was selected in Courtland, CA. USA with 'Bartlett' trees at least 100 years old and growing on 'Winter Nelis' rootstock. Multiple experiments were conducted from 1997 through 2000. The authors characterized and identified five different types of secondary flowering shoots produced under California, USA conditions. Types I and V appear to be most often associated with fire blight infection. Type I's appear10 to 40 days after full bloom. Type V's are associated with pruning cuts and are commonly found 15 to 55 days after full bloom. Plant growth regulators including gibberellins (GAs), ethephon, surfactants and the GA biosynthesis inhibitor, Apogee® (Prohexadione-calcium; BASF), have been used in an attempt to reduce the production of secondary flowering shoots. Apogee® applied at 250 mg.liter-1 near full bloom or shortly thereafter, reduced the production of Type I flowering shoots. GA applied in June or July reduced the production of Type V's in the following season. However, neither growth regulator eliminated the production of secondary flowering shoots. Summer and postharvest (~1 month after harvest) pruning reduced Type V flowering shoots compared with dormant pruning. Older shoots were more likely to produce Type V's than younger shoots. NAA applied in a 1.15% paint solution to the pruned shoot cut surface and first two nodes below the cut inhibited Type V development. We are attempting to integrate these findings and other procedures into a best management program designed to reduce secondary flowering shoots in 'Bartlett' pear, thereby reducing the incidence of fire blight.