Deficit Irrigation Could Improve Fruit Quality in CUPS

Josh McGill CUPS, Irrigation, Research

By Fernando Alferez, Daniel Boakye, Susmita Gaire and Tim Gast

Growing citrus under protective screen (CUPS) structures for fresh fruit production is effective in controlling HLB disease by completely excluding the Asian citrus psyllid (ACP, Diaphorina citri). After several years of research on CUPS to grow HLB-free trees in Florida, and with commercial growers increasingly adopting this technology, there is a need for optimizing cultural practices that will sustain higher quality fruit and better yields.

Most growers using CUPS are now in their first producing years. These growers may encounter challenges regarding fruit quality and yield that need to be resolved to remain competitive. The first challenge is that the fruit quantity is not sustained over the years, probably due to alternate bearing in some varieties such as Tango and W. Murcott. These varieties are prone to produce large amounts of fruit one year and then less the following year. This needs to be corrected to ensure consistency, allowing for better financial planning.

The second challenge is the lack of adequate peel color in these varieties, which hampers marketability. Ensuring proper coloration will result in greater economic returns for growers. To help growers deal with these challenges, the immediate benefits that CUPS present can be leveraged since trees are not challenged with HLB. These benefits include normal tree growth and vegetative development, as well as fertilization and irrigation practices that are not dictated by disease alleviation requirements.

In this sense, deficit irrigation (DI) can be a strategy to modulate bloom, fruit set and quality. Flowering can be induced or modulated by managing irrigation. According to the work of the late Gene Albrigo, subtropical areas like Florida need 60 days of drought for flower induction. Floral development may begin as soon as seven to 10 days after the end of the water deficit period (i.e., once normal irrigation is resumed).

Three years ago, a CUPS facility was installed at the University of Florida Institute of Food and Agricultural Sciences (UF/IFAS) Southwest Florida Research and Education Center (SWFREC). This new facility aimed to assay cultural practices that may improve fruit yield and quality for the fresh market. The CUPS consists of two screen houses that accommodate 400 trees (200 in each screenhouse) of four different varieties (Sugar Belle, Early Pride, Bingo and Tango) all grafted on US-897 rootstock.

Half of the trees were planted directly in the ground and the other half in air pots. A zoned automated irrigation system allows for DI experiments. Since how much water a section of the planting receives can be controlled, different irrigation treatments can be established for comparison. Soil moisture, relative humidity and temperature are continuously monitored with digital probes.

So far, the Tango trees in air pots have produced a consistent crop for two years. The other varieties have not reached their full crop capacity yet. Interestingly, all four varieties of trees are more delayed in the ground than in pots. This article reports on the first results of bloom induction, fruit set and quality obtained in the last two years on Tango mandarins in response to DI treatments.

Different irrigation treatments were initiated in early January for two consecutive seasons: 20202021 and 2021–2022. Treatments were designated as abundant irrigation (A) and deficit irrigation (DI).

The A trees were irrigated according to the previously established schedule, i.e., water was applied every other day. On this schedule, pots dried to approximately 66% of field capacity and then were returned to field capacity by irrigation. These trees never displayed signs of water stress during the bloom induction period.

The DI trees, on the other hand, were allowed to dry several times to between 33% and 25% of field capacity before watering to field capacity. On these occasions, the trees rolled leaves and showed other signs of water stress.

Figure 1. Blooming was advanced in Tango trees by one week after deficit irrigation (DI) treatment. The picture was taken in early February. Note how all DI trees are at full bloom, whereas none of the trees that received normal irrigation has open flowers yet.

The number of flowers per tree and the blooming pattern were estimated for two consecutive seasons, 2020–2021 and 2021–2022. In the first year, trees bloomed earlier in the DI treatment. By Feb. 7, most of the DI trees were at full bloom or popcorn stage, whereas trees with normal irrigation were mostly at pinhead stage. Fruit set was reached one week earlier in DI compared to normal irrigation (Figure 1). There were 10% more flowers in the DI treatment than in the well-watered controls.

In the second year, the blooming pattern was very similar, and DI trees bloomed again about one week earlier. Fruit set also followed the same pattern, but no differences in the number of flowers were found. Like W. Murcott, Tango trees tend to overbear and may present alternate bearing. Therefore, it is necessary to maintain good (not excessive) production and to maintain fruit size to prevent alternate bearing.

The most striking results came with fruit maturation. DI treatment performed by late December/early January at the onset of fruit maturation and color change enhanced the external and internal quality of the fruit. Thirty fruit per treatment were flagged to follow color development over time.

Monthly color measurements were taken at four locations in the equator of the fruit flavedo using a portable Minolta color meter. Parameters included: a (negative to positive correspond from green to red, respectively), b (negative to positive, from blue to yellow, respectively), and L (0 to 100, black to white). Color was expressed as the a/b ratio, a classic relationship for color measurement in citrus fruit, where more positive values correspond to more orange/red color and negative values correspond to green color. By the end of maturation, peel color was significantly advanced (Figure 2).

Figure 2. Fruit color of Tango at harvest on March 9, 2021, after deficit irrigation (DI) was performed between December 2020 and January 2021. The upper picture is fruit from well-watered trees. The lower picture is fruit from DI trees. The graph shows color (a/b index) development in flavedo at the end of maturation.

Internal quality was also greatly improved, with fruit from well-irrigated trees having a Brix of 10.7; DI-treated trees had 11.8 Brix. The sugar to acid ratio was also higher in fruit from DI trees: 16.9 as compared to 13.3 in fruit from well-irrigated trees. These results strongly suggest that deficit irrigation performed at the end of maturation can advance internal quality and peel coloration in Tango, a variety in which full coloration is sometimes challenging. This research is being continued with the objective of harmonizing crop load across the years by minimizing alternate bearing while improving fruit quality.

Fernando Alferez is an assistant professor, Tim Gast is a biological scientist, and Daniel Boakye and Susmita Gaire were graduate students — all at the UF/IFAS SWFREC in Immokalee.

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