Citrus RDI shows dual gains

It’s called regulated deficit irrigation (RDI), and a University of California, Davis water management specialist says it can boost navel orange quality and fruit size while at the same time conserving water in a drought year.

David Goldhamer, who is based at the Kearney Ag Center at Parlier, said RDI amounts to “knowing when best to impose water stress and doing so only during the trees’ stress tolerant periods. It’s actually forcing the trees to use less water.”

Speaking during a spring citrus meeting at Tulare, he said the goal is — without harming the trees’ sustained productivity — to save consumptive water use and to solve horticultural issues, such as fruit quality relating to peel appearance and fruit size, for the greatest grower returns.

“RDI can mean increased profits while reducing consumptive use of water for the best of both worlds,” he said.

His conclusions are based on the past 10 to 15 years of experiments, some of which are continuing, with three navel varieties: Frost Nucellar, Parent Washington, and Lane Late. All have been conducted for at least four years.

The San Joaquin Valley field trials were done on mature trees under micro sprinkler irrigation in plots with several replications of various moisture-stress regimes compared with normal, full irrigation. Effects of RDI vary by cultivar and other conditions.

Among those tested on Frost Nucellar, known for creasing problems, two early-season stress regimes of either delaying irrigation until early June or irrigating from mid-May to mid-July saved about 22 percent of consumptive water use along with providing the optimum in grower returns.

The quality issue of creasing, or puff and crease, occurs when the outer surface of the rind separates from the inner fruit. It is thought to be caused by different growth rates between the inner fruit and the white layer under the peel.

Goldhamer pointed out that after the reduced irrigation period the trees showed few visual symptoms of water stress when compared with fully-irrigated trees.

“Citrus,” Goldhamer pointed out, “is not very good at manifesting stress levels. You can see some reduction in growth of shoots, but it is not a good idea to rely on visual symptoms for managing moisture stress.”

On fruit load, expressed as number of fruit per tree over a three-year period, he continued, “we found no statistically significant differences for any of the stress regimes. In other words, Frost Nucellar trees are relatively tolerant of stress, regardless of when it is imposed, in terms of fruit load.”

The stress regimes also showed no statistically significant difference in fruit size or weight, relative to the normal irrigation control.

“That was somewhat surprising, because the stress regimes did impact the rate of fruit growth at maybe 20 percent to 25 percent less. When the stress kicked in, the trees had significantly smaller fruit. But when we reintroduced full irrigation in mid-July, the growth of the fruit had an apparent acceleration, probably because of rehydration,” he explained.

Thus, at harvest, the fruit was no different between the RDI and fully irrigated trees. “That’s the theme of what we saw in all these experiments. Given enough time on the tree and return to full irrigation, the fruit size recovered.” As an added benefit with Frost Nucellar, Goldhamer discovered that, based on the mean of the RDI treatments compared with the fully irrigated treatments over three years, creasing in the RDI fruit was reduced by two-thirds.

That meant that a number of the RDI regimes gave higher grower revenue, because of the improvement in fruit quality due to reduction in creasing, plus less consumptive use of water.

“The best of these,” Goldhamer said, “was irrigation at 25 percent of normal on Frost Nucellar from mid-May to mid-July for $600 to $700 more per acre in gross revenue while using about 8 inches less water.”

Turning to his studies on Parent Washington, which does not have the tendency for creasing, he said the objective was to impose four different RDI treatments to learn how much less water could be used without negative impact on productivity. Improvement of fruit quality was not an issue in these trials.

The treatment with delaying irrigation with 25 percent of normal during mid-May through mid-July had no significant impact on gross revenue.

However, one of the treatments was delaying irrigation until the end of May and applying only 50 percent of normal for the entire season. This resulted in appreciably smaller fruit size and less gross revenue.

“With Parent Washington, an early harvest cultivar,” Goldhamer said, “if you have only 17 inches of water you are not going to be able to produce fruit with the same size and number of fruit per tree as if you had full irrigation.”

In describing the Lane Late research, Goldhamer said the variety can be a hazard because it remains on the tree longer and can develop unmarketable, larger sizes.

“Early season stress with Lane Late can mean 15 percent smaller fruit, but it recovers to almost the equivalent size under full irrigation if full irrigation is returned later in the season.”

Season-long stress, however, produced smaller fruit. The full irrigation trees had about 43 percent size 24-36, unmarketable fruit. The stress shifted that proportion toward the more valuable, smaller sizes.

Goldhamer said the control regime produced about $3,600 per acre in gross revenue, while the season-long stress (17 inches) and late-season stress regimes pushed gross revenue to more than $6,000 per acre.

“We are not recommending that growers irrigate with 17 inches of water,” he said. “We are recommending that they use late-season stress (50 percent of normal, starting Sept. 1) by reducing consumptive use by 4 or 5 inches to get about $3,000 an acre more than with full irrigation.”

Goldhamer cautioned Lane Late growers that if their trees are heavily loaded with fruit, that alone can produce fruit too small when RDI stress is imposed.

For guidance in integrating RDI into their operations, growers can contact Goldhamer at [email protected].

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