Arnold Schumann, a professor of soil fertility and water quality with the University of Florida Institute of Food and Agricultural Sciences (UF/IFAS), led the effort to study the effectiveness of citrus under protective screen (CUPS) in protecting trees from HLB. The work resulted in growers having confidence in the practice. Today, Florida CUPS plantings are closing in on 1,000 commercial acres. Schumann discusses his research in the following Q&A.
Q: What got you interested in CUPS and how long have you been studying it?
A: The research has been conducted over nearly nine years. We built our CUPS at the UF/IFAS Citrus Research and Education Center in early 2014 and planted the first trees in summer 2014. The proven ability of screen houses to exclude citrus canker and Asian citrus psyllid from citrus nurseries in Florida was an incentive to try growing mature trees for fruit production under similar screen in larger structures.
Q: For growers considering CUPS, what are some rough estimates for initial establishment costs?
A: I can only reliably quote the $1 per square foot ($43,560 per acre) that we paid in 2014. Prices constantly change, but I know that some growers have constructed large CUPS at less than $1 per square foot by outsourcing the screen and other materials from overseas.
The cost per square foot is less for larger structures (economy of scale), and for structures with a relatively square shape. When designing a CUPS, the spacing of the internal support poles should be compatible with the intended tree-row spacing. For example, if the poles are spaced 30 feet apart, then tree rows spaced 15 feet allow every other row to coincide with the rows of poles.
Q: What rootstocks and varieties have performed the best in CUPS?
A: Semi-dwarfing rootstocks are a good choice for scions with the capacity to grow large trees in order to avoid excessive hedging and topping, which could adversely affect yields. Due to the relatively high planting density and the CUPS roof, available space for tree canopies is limited. US-897 is a good match for red grapefruit in CUPS. We are observing that sour orange at 10 feet by 5 feet high density spacing is running out of space after eight years when compared with the US-897 rootstock.
Ray Ruby, Ruby Red and Flame grapefruit, Dancy, Murcott, W. Murcott, Kinnow, Early Pride, Temple, Minneola, Sugar Belle, Persian lime and Eureka lemon varieties have all performed well in the CUPS. Grapefruit are the most consistent, reliable yielding varieties in CUPS.
Q: What are your thoughts on producing CUPS in pots or in the ground?
A: Pots are a good choice for testing varieties at very high densities with hydroponics, such as for our research spacing of 8 feet by 4 feet. They require trellises to support the trees, and the cost of pots and media is high. Weed control with an herbicide boom is not practical, and herbicide has to be hand-applied. Fruit production from in-ground plantings has exceeded expectations and is the best option.
Q: What has your research shown on planting density?
A: Research experience has shown that planting densities in CUPS can be greater than conventional guidelines would recommend, because the sunlight passing through the screen is diffracted and refracted at many angles. This causes tree canopies to be better illuminated than without screen and mitigates shading effects from high densities.
Q: How has production trended since trees were planted in CUPS?
A: Grapefruit yields have averaged about 800 boxes/acre for six out of eight years at 10 feet by 5 feet spacing in the ground. The cumulative yield for eight years of the grapefruit at 8 feet by 4 feet spacing in pots is 7,329 boxes/acre, and yields have peaked twice at 1,400 boxes/acre/year.
Q: What are the yields now that some of these trees are mature?
A: Yields stabilized as trees fully matured. For grapefruit trees, there is little year-to-year variation. Other varieties like Murcott and Kinnow have major alternate-bearing patterns year to year. The internal quality of fruit from mature trees is top-notch, and external quality depends on how well the peel blemishes caused by rust mites, greasy spot and melanose can be controlled. Packout is nearly 100% most years for grapefruit.
Q: How successful has your CUPS been in protecting against the psyllid and HLB?
A: Psyllids have entered and spread in the CUPS after the screen was breached from storm damage or repairs. The HLB bacteria titer in sampled psyllids was undetectable. Candidatus Liberibacter titer in psyllids is rapidly diluted over a few generations of feeding on uninfected trees and with no new inoculum being added from outside groves. After 8.5 years, the HLB incidence is just over 0.5%.
Q: Are there other pest concerns with CUPS?
A: Citrus canker doesn’t survive in CUPS. Greasy spot and melanose can be controlled with conventional copper, strobilurin fungicides or Bacillus-based biofungicides. Spider mites, rust mites, mealybugs and thrips are controlled with appropriately timed pesticide sprays supported by regular scouting.
Q: How much fertilizer is used in CUPS?
A: The amount of fertilizer used per box of fruit produced is only a fraction of that used in HLB groves because the trees in CUPS grow twice as fast, with healthy roots to absorb the nutrients and healthy trees to utilize them optimally.
Q: What are you hearing from growers about CUPS adoption and how their structures are working?
A: There is growing interest in CUPS in Florida (~650 acres and growing to over 1,000 acres soon) and in other states like Texas, Louisiana and California. In 2022, Florida CUPS growers observed that the screen house structures completely protected the trees from hurricane damage and fruit drop. Fruit drop is already very low in CUPS. It seems that the added value of storm protection is important given that a conventional grove can take up to five years to recover from hurricanes.
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