USE OF SOIL-APPLIED CALCIUM CHLORIDE TO REDUCE FIRE BLIGHT AND BROWN SPOT SUSCEPTIBILITY OF PEAR

The pear industry in Italy’s north-eastern Po river region suffers from important diseases such as bacterial fire blight (Erwinia amylovora) and fungus brown spot (Stemphylium vesicarium) infections which are very difficult to control by chemical sprays. The aim of this study was to evaluate the effectiveness of soil-applied calcium chloride (CaCl₂) in reducing pear susceptibility to both diseases. In two distinct experiments, carried out on one-year-old, potted pear trees of cv. ‘Abbé Fetel’/‘Quince C’, the following treatments were compared in a completely random¬ized design with 6 (fire blight) or 12 (brown spot) replicates: 1) control, well irrigated, 2) irrigation rate reduced by 50% of the control; and 3) full irrigation rate with water solution of CaCl₂ to obtain an electric conductivity of 8.3 mS cm^-1. Treatments were imposed when shoot length was around 10 cm and continued for 4-5 weeks. E. amylovora inoculum (7x10⁸ cfu ml^-1) was applied on 3 apical leaves per shoot (3 shoots per tree), and the progressions of shoot blight symptoms were assessed after 8, and 21 days. S. vesicarium was applied by spraying an aqueous suspension of conidia (2x10⁴ ml^-1), and disease incidence was determined as a percentage of infected leaves. Soil-applied CaCl₂ was effective in reducing pear susceptibility to both fire blight and brown spot as compared to control trees. A 50%-reduction of water supply decreased fire blight but not brown spot incidence. Soil-applied CaCl₂ and the reduction of water supply decreased shoot length, number of leaves and stem water potential as compared to control, although did not affect leaf osmotic potential. Pear susceptibility to fire blight and brown spot was positively related to stem water potential and negatively to leaf Ca concentration.