By John M. Chater
Arnold Schumann, University of Florida Institute of Food and Agricultural Sciences (UF/IFAS) professor at the Citrus Research and Education Center (CREC), pioneered the citrus under protective screen (CUPS) production method in Florida. He started his CUPS work over 12 years ago to help provide horticultural tools to growers to continue producing high-quality fruit by means of pest exclusion. There are now over 1,000 acres of CUPS with many more being planted this season. Pest exclusion is an ancient method of preventing pest access to crops, often by means of a physical barrier. In CUPS, that barrier is a mesh screen.
Although there have not been enough years of data collected from plantings to recommend a scion/rootstock combination in the CUPS system, there is some information about what has been working and what has not worked out well.
RAY RUBY ON US-897
A common grapefruit fresh market selection in Florida CUPS is Ray Ruby, a high-quality, red-fleshed fruit. Trees can produce nearly 1,000 85-pound field boxes per acre on US-897 rootstock in three to four seasons with proper cultural care under normal growing conditions. Ray Ruby on US-897 appears to be a useful selection in recent seasons.
US-897 aids in controlling tree height, as growers have been working to avoid the use of ladders, which has been a common theme in other citrus-producing regions like California and Spain. Citrus tends to grow faster in CUPS compared to outside groves, and the trees can often produce fruit that is inaccessible if pruning and canopy management are not considered. One potential problem with US-897 is that it is reported to be susceptible to blight, so that should be considered when selecting a rootstock. So far, there has been no sign of incompatibility with the Ray Ruby/US-897 combination.
RES 19-56
Another selection that has been under observation in CUPS is RES 19-56. This mandarin hybrid from UF/IFAS professor Fred Gmitter Jr. is a cross between Daisy (mother) and Mukaku Kishu (pollen plant). The tree appears to crop hard and early in tree age, making it a seemingly precocious selection.
There are not enough years of data to determine whether this will be a better selection than Ray Ruby on US-897. There is, however, evidence that this selection can produce bright-colored, easy peel, seedless fruit about the size of Tango mandarin. Therefore, this cultivar has the potential to meet the Cuties and Halos market.
One benefit of RES 19-56 is its earliness. This cultivar has been ready to harvest by mid- to late October. An interesting trait of this mandarin is that it can produce fruit in bunches, similar to grapes, which makes a unique presentation. The fruit also have a decent postharvest shelf life and are reported to respond to ethylene. Trees with different rootstocks are under observation.
GATOR BITES
There are other UF/IFAS selections under observation in CUPS, although the numbers of trees are fewer than desired. Gator Bites (also known as 18A-9-39), a selection created by UF/IFAS Professor Jude Grosser, has some potential in CUPS, although only a few topworked trees have been observed.
The pedigree of Gator Bites is highly complex. It is a seedless triploid, and embryo rescue was used. In CUPS, it is virtually seedless. The fruit are ready in December and have been able to hold until early January in Central Florida. Fruit are Clementine-sized, easy peel and seedless. Crops have been consistently heavy.
MORE VARIETIES UNDER OBSERVATION
Other selections (recently released by UF/IFAS) are under observation in CUPS and appear to have some potential. This includes two easy-peel seedless mandarins with larger-sized fruit (C7-12-18 and KE 9-9) and the seedless orange-like hybrid 104.
For these selections and others that are being observed in CUPS, it is important to consider canopy management. Hedging mandarins can lead to loss of fruit in productive branches and cause lanky or otherwise suboptimal production of fruit.
Regardless of mandarin selection, the goal is to identify a selection that will bring financial returns near to what grapefruit provides. This would require several hundred boxes per acre, which may be a challenge. To meet this challenge, 100 different mandarins are being planted in a new CUPS structure at the CREC. Researchers intend to learn which mandarins produce quality fruit with commercial yields.
914 is a grapefruit-like hybrid with exceptional, refreshing grapefruit flavor without the typical bitterness of a grapefruit. The trees have been cropping heavy, and the fruit are beautiful. 914 fruit are very much grapefruit-like, although there can be a subset of larger fruit or fruit with more albedo. However, these characteristics seem to subside as the trees age, and the high yields of quality fruit make it even less of a problem. Regardless, 914 fruit and juice have been adored by international audiences, including grapefruit lovers and haters.
The nearly-seedless 18A-10-47 selection of Honey Murcott (newly released from UF/IFAS) is also showing promise in CUPS. Honey Murcott on Kuharske have done well in the CREC CUPS.
WHAT MAY NOT WORK
In terms of what may not be a good selection in a CUPS system, there are quite a few for various reasons. There are major ecological changes when trees are enclosed with a screen. One of those changes is the lack of pollinators. Some cultivars require a pollinator for good production. These types of cultivars would probably do quite poorly due to the lack of bees in CUPS.
Early Pride is a U.S. Department of Agriculture selection that was planted in CUPS. It is an early cultivar that can meet market specifications, but the fruit are susceptible to split and drop. Unless Early Pride fruit split can be mitigated, it may be a cultivar to shelf for now, although some growers may do well with it.
W. Murcott has not done well in the CUPS system in Florida. Honey Murcott and Kinnow do produce high-quality fruit, as does Minneola, although those selections can contain seeds. Sugar Belle® LB8-9 produces excellent-quality, packable fruit with high Brix in the CUPS facility, but rootstock selection and production consistency are under evaluation for optimal production.
Regardless of which mandarin cultivar one selects for CUPS, alternate bearing or inconsistent yields should be a consideration as well as grafting incompatibility for uncharacterized combinations. Alternate bearing and grafting incompatibility with desired rootstocks in mandarin citriculture are considerations for growers both inside the CUPS structure and outside in the grove.
John M. Chater is an assistant professor at the UF/IFAS CREC in Lake Alfred.
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