1Institute of Food and Agricultural Technology-CeRTA, University of Girona, Avda. Llufs Santa 6 s/n, 17071 Girona, Spain; 2 INRA Station de Pathologie Vegetale et Phytobacteriologie, Centre de Recherches d'Angers, rue Georges Morel, 49070 Beaucouz6, France

Background and objectives
Blast of pear caused by Pseudomonas syringae pv. syringae is one of the bacterial diseases that limit pear production throughout the world. Symptoms are characterized by blast of buds, blossoms, leaves or fruits wich occur in periods of cool wet weather during bloom and postbloom stages. Control of bacterial blast of pear with chemicals is difficult and is based on copper compounds and antibiotics. Fosetyl-Al, a phosphonate derivative used to control diseases caused by fungy, has been evaluated for control of several bacterial diseases on plants showing moderate efficacy [1]. The purpose of this work has been to determine the effect of several phosphonate derivatives on P. syringae pv. syringae -pear interaction and evaluate their efficacy in disease control, under controlled environment conditions.

Materials and methods
Phosphonates used were potassium phosphonate, aluminium tris-o-ethylphosphonate (fosetyl Al), 2-chloroethyiphosphonate (etephon) and 2-epoxypropylphosphonate (fosfomicyn). Selfrooted pear plants of cultivar Conference (CAV clone) obtained by micropropagation were irrigated with 100 ml of either 0.62, 1.86, 3.72 or 6.20 g HPO32-/1 depending on the experiment. Treatments were applied once a day for five days proceeding inoculation of the bacterial pathogens. Inoculation with P. syringae pv. syringae strains was performed by the detached leaf assay method [3] or by a whole plant leaf microinfiltration method [2]. Inoculated leaves or plants were incubated at optimal conditions for disease development. Leaf discs of cv. Passe Crassane pear plants, treated with fosetyl-Al and non treated, were infiltrated with P. syringae suspensions and incubated in sterille buffer at 250C for 48h. The conductivity of the incubation medium was measured at initial time and every 4-8 h during the incubation period. Conductivity values were fitted to a mathematical model in order to describe kinetics of electrolyte leakage in each treatment and to compare the parameters in the model for all treatments.

Results and conclusions
Plant treatment with phosphonates induced particular symptoms in leaves, structural modifications of plant cell tissues and changes in electrolyte leakage, after inoculation with P. syringae pv. syringae, similar to incompatible reaction. Drenching the root system of pear plants with either potassium phosphonate, fosetyi-Al, etephon or fosfomicyn decreased significantly and consistently disease levels after inoculation with P. syringae pv. syringae. The effect of phosphonates in the control of bacterial blast of pear is related to product, dose and number of applications. Fosfomycin and etephon were the most effective products in reducing disease levels, with median effective dose (ED50) of 0.5 and 1.47 g HPO32/1 , respectively. Potassium phosphonate and fosety]-Al were moderately effective, with ED50 of 2.9 and 5.5 g HPO32-/1, respectively. Otherwise, in vitro antibacterial activity of these products is very low for several strains of phytopathogenic and non-phytopathogenic bacteria tested [2]. Although the ED50 of potassium phosphonate and fosetyl-Al applied by irrigation to pear plants may be too high for a practical use in the field, ft is expected that improvement of plant absorption and application methods may increase the efficiency of these compounds.

1. Chase AR. 1993. Plant Disease 77, 771-776.
2. Moragrega C, Manceau C, Montesinos E. 1998. European Journal of Plant Pathology (in press).
3. Yessad S, Manceau C, Luisetti,J. 1992. Plant Disease 76, 370-373.