Efficacy of alternatives to antibiotic chemicals for the control of fire blight of pears

The efficacy of various chemicals as alternatives to antibiotics for the control of fireblight (Erwinia amylovora) on pear trees was tested. The chemicals were applied in two ways. In 1999 and 2000, preselected pear twigs (80–90% bloom stage) were sprayed once either preventively 1 day before inoculation or curatively one or three days after artificial inoculation with pathogen concentrations of 10⁵ and 10⁷cfu ml^?1. In 2000 and 2001, whole trees were sprayed 2 and 4 days before artificial inoculation of the flowers. From the incidence of diseased flowers it appeared that Bion (50% benzothiadiazole) at 0.2 g litre^?1 H₂O and Aliette (80% fosetyl‐Al) at 2.5 g litre^?1 H₂O showed considerable preventive action by eliciting systemic acquired resistance mostly when they were applied in the whole trees. However the best control was achieved with the antibiotic Agrept (20% streptomycin) at 0.5 g litre^?1 H₂O. This showed both preventive and curative action. Kocide (77% copper hydroxide) at 0.9 g litre^?1 H₂O, Dentamet (citric acid in chelate) at 1.5 ml litre^?1 H₂O, Bactosan (an extract from the plant Pongamia pinnatd) at 3.0 g litre^?1 H₂O and Bion at 0.1 g litre^?1 H₂O, showed preventive action, but only when the inoculum concentration was low.