Stem injection of prohexadione carboxylic acid to protect blossoms of apple trees from fire blight infection (Erwinia amylovora)

Stem injection of prohexadione carboxylic acid (pca) was tested to prevent blossom infection by the fire blight pathogen Erwinia amylovora on apple trees, as an alternative to applying streptomycin (Plantomycin (R)). Under glasshouse conditions, the reduction in blossom infection by E. amylovora on two-year-old container-grown plants by injecting 40 mg pca was similar as applying Plantomycin (R). Under field conditions, the injection of just 10 mg pca in four-year-old field-grown trees resulted in reduced primary fire blight infection identical to that of Plantomycin (R). In both experiments, however, shoot stunting in apple trees treated with pca was observed. In addition, yield losses occurred in the field experiment, probably caused by reduced longitudinal growth. Concentrations of residues of pca in the fruits of apple trees treated with 30 and 40 mg pca were 0.0032 ppm and 0.0017 ppm, respectively, and well below the EU's reference value (0.05 ppm). Consequently, the use of stem-injected pca under field conditions is not constrained by residues in the end product. Problems caused by stunting of shoots and associated yield losses after injection with pca and the technical realisation of stem injection in orchards were discussed.