EFFECTS OF WATER STRESS/DROUGHT ON FIRE BLIGHT

Commercial apple orchards in British Columbia, Canada depend on irrigation for their survival. The object of this study was to determine the effect of water stress on infection of apple by Erwinia amylovora. Two trials were conducted on 'Gala' in 2005 comparing "well irrigated" trees to "water stressed" trees. The "well irrigated" trees received 1500 ml of water every 2–4 days starting at green tip and ending a week after petal fall. Similarly deficit irrigated trees received 500 ml of water on the same dates. The trees were inoculated with a low (1 × 10⁵ CFU/ml) and high (1 × 10⁷ CFU/ml) concentration of E. amylovora at full bloom followed 48 h later by 3 h of wetting. In 2006 the experiment was repeated on 'Aurora' trees but in this case 0, 3 and 12 h of wetting also were evaluated. Results of the 2005 trials showed that water stressed trees inoculated with the low concentration of E. amylovora had fewer blighted blossoms than the irrigated trees. However, at the high concentration there was no significant difference in blighted blossoms between irrigated or water stressed trees. In 2006 on 'Aurora' the results were similar for 3 and 12 h of wetting but when zero wetting was used; only the well irrigated trees became infected. Based on these trials, it would appear that by limiting soil irrigation in apple orchards where fire blight is a threat, it may be possible to reduce the disease incidence.