The 2011 ASHS Annual Conference

Leafy inflorescences in citrus have been reported to favor higher fruit set and fruit persistence than leafless inflorescences. In subtropical humid climates like Florida, drought stress during winter may be used as a management strategy to delay flowering in late-season sweet orange cultivars destined for late-season mechanical harvesting to avoid the loss of young fruit from next year’s crop. We studied the effects of winter time drought stress treatments on the ratio of young leaves to open flowers and fruit set to assess treatment affects on yield. During 3 consecutive seasons (2007-2009), starting in December 2006, three irrigation treatments were applied to 13- to 15-year-old ‘Valencia’ trees: 1. Drought, no irrigation and soil covered with a rain shield groundcover; 2. Rain only, no irrigation, no cover; and 3. Well-irrigated, normal irrigation with rain and no cover. Covers were removed in spring after 100 days of treatment and all trees were well-watered and fertilized. Previously drought-stressed trees had fewer open flowers than well-irrigated trees in 2007 and 2009 but not in 2008 which seemed to be a consequence of the previous drought stress increasing alternate bearing. Previously drought stressed trees had more leafless inflorescences and fewer leafy shoots without flowers than well-irrigated trees. The young leaf / open flowers ratios on drought-stressed trees were 1.3-3.6 fold higher than on well-irrigated trees in 2007 and 2009. However, fruit number and yield in 2007 and 2009 were not significantly different between drought-stressed and well-irrigated trees, confirming the higher fruit set in drought-stressed trees than in well-irrigated trees.