Arnold Schumann recently reported on what he called “the most successful and practical intervention for improving color break and quality in CUPS fresh fruit.” CUPS stands for citrus under protective screen, a growing technique that Schumann, a University of Florida Institute of Food and Agricultural Sciences (UF/IFAS) researcher, helped pioneer in Florida. He reported on his work at a Feb. 8 Citrus Irrigation and Nutrient Management Workshop at the UF/IFAS Citrus Research and Education Center in Lake Alfred.
The researcher used what he called “smart fertilizer timing” to compensate for warmer fall temperatures that can impede color break and quality in early- and mid-season varieties.
Schumann began the successful color break experiment in the 2020–21 season by stopping all nitrogen fertilizer on Sept. 22, 2020. He reduced the remaining daily fertigation to 25% of the maximum rate to allow depletion of leaf nutrients in the fall. That effort resulted in “excellent early color break and Brix quality in all varieties grown in the CUPS,” he reported. He added that visible symptoms of nitrogen deficiency, including pale green leaves, developed as a result of the technique.
According to Schumann, the leaf nutrients nitrogen, phosphorus, potassium, sulfur and iron were negatively correlated with peel color and Brix. This suggests that surplus amounts in the fall could harm fruit quality and color break. Leaf nitrogen, phosphorus and sulfur correlations with peel color were strongest, and leaf phosphorus correlation with Brix was strongest. In his presentation, Schumann suggested that CUPS growers focus on nitrogen and phosphorus fertilizer timing to ensure the best fruit color break and quality.
On the other hand, leaf magnesium was positively correlated with both peel color and Brix. This suggests that a sustained supply of magnesium nutrition could support higher Brix and complete color break in the fall.
Schumann offered the following recommendations for growers:
- Apply most phosphorus fertilizer in the pre-bloom to post-bloom period. Omit phosphorus if leaf and soil levels are high.
- Apply 50% of nitrogen by the post-bloom period.
- Apply 75% of nitrogen by the time of physiological fruit drop (May/June).
- Apply 100% of nitrogen by the middle to end of summer, depending on the maturity date of the variety.
- Regarding leaf nitrogen and phosphorus concentrations, aim for the high end of optimal in spring to early summer, and low optimal or low in late summer and fall.
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