Control strategies of bacterial diseases in European pear growing

The most important bacterial diseases in European pear growing are fire blight (Erwinia amylovora) and blossom blight (Pseudomonas syringae pv. syringae). The economical impact of these bacterial diseases is different : fire blight can destroy a pear orchard completely in a very short time of 4 to 6 weeks when a general primary blossom infection is present or when hail damage during summer occurs; blossom infection of Pseudomonas syringae can destroy totally the crop of one year in a very short time of some days. Pseudomonas infections are often linked with a weakness factor in the pear orchard like the presence of frost damage on the flowers or on the trunk or after excessive overhead orchard irrigation for frost protection. The use of antibiotics or bactericides on a moment with a high infection risk is the standard procedure for the control of the fire blight disease in pear growing in the USA but is not allowed in many countries in the European Union. Biological control agents for fire blight control are developed in many countries; in the USA strains of the bacteria Pantoea agglomerans (Erwinia herbicola) or Pseudomonas fluorescens strain A506 are used as biological antagonists for the Erwinia amylovora bacteria; in Europe different strains of Bacillus subtilis are developed in the research programs as possible fire blight antagonist. Important aspect for this biological control agents is that their activity is not based on the production of antibiotic substances by the antagonist but on another mechanism. A completely new way of biological control of fire blight is in development in a Belgian research project where a phage tail like bacteriocin named Serratine-P with a protein structure is developed for fire blight control. The Serratine-P has a high molecular weight and is very similar to the bacteriophage tail structure but without the genetic DNA-material. These structures interact with the cell wall of the fire blight bacteria which results in a perforation of the cytoplasmic membrane inducing perturbations in cellular exchanges and a final lysis of the bacterial cell. Another important strategy for fire blight control is to change the host susceptibility. Different molecules are tested with the aim to change the host susceptibility. Benzothiadiaozole is an analogue of the acetyl salicilic acid and improves host resistance against fire blight. Prohexadione-calcium is a new growth regulator which inhibits the gibberellin biosynthesis in fruit trees and affects the vegetative behaviour of the trees and this reduces the possibilities of fire blight shoot infections. The treatments of Prohexadione-calcium also induces changes in phenol metabolism of the fruit trees and this also can induce changes in host susceptibility. Carpropamid is a new compound inducing changes in the lignification processes in the fruit trees and is tested against the different types of fire blight infections. Copper is a bacteriostatic compound with some activity as well against Erwinia amylovora as against Pseudomonas syringae infections. This bacteriostatic activity is not strong enough to prevent large scale bacterial infections in pear orchards. In the IPM pear production, the use of copper will be limited in the future.