Managing Drought Stress of Oranges to Optimize Flowering and Productivity under Sao Paulo–Mina Gerais, Brazil, and Florida Conditions

Winter weather in the Sao Paulo–Minas Gerais (SP–MG), Brazil, and Florida citrus production areas often varies from adequate to inadequate cold for flower induction and excessive drought for induction in the north of the main Brazilian citrus production area.  A program of monitoring winter temperature and rainfall data for farms from near Conchal, SP, to north of Frutal, MG, was instituted starting in 2008 and Florida winter weather for flower bud induction has been monitored since 1998. Controlled irrigation experiments were instituted in the northern third of Sao Paulo and at Lake Alfred, FL.  Cool temperature induction hours (19 °C or less) ranged from less than 200 to more than 1300 hours by location and year in (SP-MG). Florida usually ranged from 600 to 1200 hours. Winter drought stress by withholding irrigation increased flower bud induction in Florida in years when winter rains were sparse. Based on an ultralow volume irrigation scheme ULVI), deficits in cool temperature induction in SP-MG were compensated for drought stress management to achieve adequate flower bud induction without excessive drought stress. In an ULVI experiment soil moisture tensiometers indicated that the surface 20 cm could be kept at adequate moisture levels to prevent severe drought with less than 1 mm/day. This kept trees in adequate condition without initiating bud sprouting for up to 120 days, until normal timing of bloom could occur. The moderate stress could be released at any time in order to initiate an earlier bloom as long as water was available for normal irrigation until the rainy season started. Thirty-five to 40 days were required from initiating full irrigation until full bloom. Yields were very satisfactory under this program, which is now being used on over 10,000 hectares in SP-MG.