Cegulated deficit irrigation (RDI) offers a way to minimize the effects of a water-short growing season on citrus while improving fruit quality.

The idea is to intentionally stress trees at specific stages of crop development, reducing water use of the orchard by reducing leaf transpiration without cutting fruit yields or quality.

The practice requires that growers have the ability and tools to estimate crop evapotranspiration (ET) for citrus, water application rate, soil-water holding capacity, irrigation application rates and water stress accurately, reports Craig Kallsen, citrus and pistachio University of California Cooperative Extension farm advisor for Kern County.

In one three-year study (1998-2000), Frost Nucellar orange trees were irrigated at only half of full citrus ET from May 15 to July 15. The amount of fruit with puff and crease declined from 30 percent in fully-irrigated trees to 10 percent in the trees where RDI was used. At the same time, 35 percent of fruit from the RDI trees was fancy compared to 22 percent from the fully-irrigated trees. Yield, fruit number and fruit size were not affected by RDI and it saved an average of about 8 inches of water each year during the trial.

Another three-year study revealed how RDI improved quality of later-maturing navel oranges. Lane Late navels were irrigated at 50 percent of full citrus ET evenly throughout the season. At harvest, the RDI oranges peaked on the desirable sizes 56 and 72 compared to the overly large sizes 24 to 40 in oranges irrigated at full citrus ET. Also, the RDI trees had less fruit granulation and there was no loss of yield. As a result, the RDI trees grossed an average of $6,220 per acre compared to $3,610 for the non-treated trees. The RDI trees were irrigated with an average of 17 inches of water a year — 20 inches less than normal.

“Because uncontrolled stress may increase fruit drop or reduce fruit size, attempts to apply this research at the field scale should be approached carefully,” Kallsen says.