Reduction of fire blight infection by manipulation of pear tree water status

Pear tree water status was manipulated in two ways: a) reducing the water supply rate, b) watering trees with salt solution. The soluble carbohydrate content, osmotic potential (ψs) and water potential (ψw) of individual leaves were determined to test the hypothesis that these parameters are related to the fire blight symptom progression in leaves and shoots after experimental inoculation with Erwinia amylovora. The experiment was carried out on potted, one-year-old pear trees of cv. 'Abbé Fétel'/'Quince C' subjected, in a completely randomized design (6 replicates), with the following treatments: 1) control, well irrigated, 2) irrigation rate reduced by 50% of the control; 3) full irrigation rate with water solution of NaCl and CaCl2 to obtain an electric conductivity of 2.1, 4.2 and 8.3 mS cm-1. Treatments were imposed when shoot length was around 10 cm and continued for 45 days. Infection was applied on three apical leaves per shoot (three shoots per tree) and leaf and shoot symptom progression were evaluated 4, 5, 8, and 12 days after inoculation. Irrigation water with salt at a concentration of 8.3 mS cm -1 as well as a 50% reduction of water supply were effective in increasing pear tolerance to fire blight, when compared with control trees. Water with an electric conductivity lower than 8.3 mS cm-1 did not reduce symptom progression in leaves and shoots. A 50% reduction of water supply and a water solution of 8.3 mS cm-1 significantly increased sorbitol concentration in leaf tissue. No relationship between leaf ψs and symptom progression in leaves and shoots was observed. A logarithmic positive relationship between pre-dawn leaf ψw and the extent of shoot necrosis was observed.