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An Insecticide-Free Way to Grow Citrus in the Face of Greening

Citrus greening disease is devastating Florida citrus. This disease is spread by an insect, the Asian Citrus Psyllid. Growers are faced with expensive decisions about spraying pesticides to control the psyllid to reduce the spread of the disease. Are there any ways to control psyllids without resorting to chemicals? Researchers at UF are looking into growing citrus under an insect excluding screen enclosure. Insect excluding screens have much smaller openings than the screen enclosures around pools. Pool screens will not exclude psyllids. Insect excluding screens are so fine that they can impact temperature, humidity, wind, and light levels. There are many questions to answer about how this system, called Citrus Under Protective Screen or CUPS, will affect plants. Plants must be kept smaller and planted at a higher density to be kept in an enclosure. Researchers wanted to know what impacts growing in a screen enclosure will have on the plants, and whether they can be grown in pots to help control size.

Experiment

Researchers tested ‘Ray Ruby’ grapefruit grown in pots and in the ground, in an insect excluding screen enclosure and outside in the open air. Trees were on sour orange and US-897 rootstocks. There were no rootstock differences in analysis, so they pooled that data for the final analysis. Plant density was 792 trees per acre with 5.5 ft in-row and 10 ft between row spacing. Plants in pots were grown in 10-gallon plastic Accelerator pots filled with 50% clean, washed silica sand, 15% Florida peatmoss, 7.5% coconut fiber, 20% cypress sawdust, and 7.5% perlite. A ceramic tile was placed under each pot to prevent roots from growing into the ground. All plants were irrigated with two 2-gallon/hour flow drip emitters. Plants were watered to replenish the reference evapotranspiration for their respective growing environments. Fertigation was supplied equally to all treatments. Researchers looked at canopy surface area, water use efficiency, leaf area, transpiration, leaf nitrogen concentration, and the changes in environmental conditions caused by the screen enclosure.

Results

Trees grown inside screen houses developed a larger canopy surface area, water use efficiency, and leaf area than trees grown outside in the open air. Leaves under cover also transpired more (lost water through the leaves). Higher transpiration could be a good thing because it means the stomates were open more, allowing more photosynthesis to occur, possibly resulting in the larger leaf area and canopy surface. Although monthly leaf nitrogen concentration was greater in container grown trees in the open-air compared with trees grown in-ground and inside the screen house, trees grown in-ground and inside the screen houses did not have severe leaf N deficiencies and were the largest trees at the end of the one-year study. The protective screen enclosures accelerated young tree growth compared with open-air plantings while protecting trees from greening infection.

The enclosures effectively excluded the psyllids from the plants under cover. Environmental monitoring inside the enclosures indicated air temperature was greater, wind gusts were lower, evapotranspiration was lower, and light was lower compared to open-air. In general, the environmental conditions inside the protective screen enclosures were suitable for grapefruit production.

For complete information on the research:

“Protected Fresh Grapefruit Cultivation Systems: Antipsyllid Screen Effects on Plant Growth and Leaf Transpiration, Vapor Pressure Deficit, and Nutrition” and “Protected Fresh Grapefruit Cultivation Systems: Antipsyllid Screen Effects on Environmental Variables inside Enclosures” by R.S. Ferrarezi, A.L. Wright, B.J. Boman, A.W. Schumann, F.G. Gmitter, and J.W. Grosser. HortTechnology October 2017 27(5).

More information on the CUPS system

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