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Tree Health :: Tree Physiology Studies


Mechanical harvesters can cause injuries including leaf and twig loss, scuffing of the bark on trunk and branches, limb breakage, removal of flowers and young green fruit and exposure of shallow roots at the soil surface. Do these injuries affect the tree’s health?


  • Determine long-term effects of mechanical harvesting on tree health and yield. Study long-term effects (over 3 years) to assess trees that are drought stressed in winter and mechanically harvested in consecutive years.
  • Develop drought stress methods that extend the mechanical harvesting window by delay of flowering. It is believed that delaying bloom with drought stress in winter in ‘Valencia’ can improve late season harvesting in ‘Valencia’. If bloom can be delayed 3 weeks, younger fruitlets should be smaller and less susceptible to late season mechanical harvesting losses during May and June.

Key Findings (Updated on 09/10)

We have confirmed that interactions with mechanical harvesting and drought stress result in short term tree stress. This is not unlike previous observations of hand harvested drought stress trees, however, where even hand harvesting represented a significant added drought stress after fruit removal.

1. This work is making progress towards improving safe late season harvesting in previously drought stressed ‘Valencia’ trees. Mechanical harvesting of well managed healthy trees using a trunk shaker and a canopy shaking machine in the third year, did no long term damage to tree health or had any negative impact on yield relative to hand harvested trees. There were no measurable affects on fruit or juice quality.

2. Mechanical harvesting during peak bloom (~March) in ‘Valencia’ can remove some flowers but does not diminish total fruit set. During later season mechanical harvesting of ‘Valencia’, as long as the diameter of young green ‘Valencia’ fruit is less than about one inch, mechanical harvesting does not reduce yields the following year. Once the young fruitlets exceed this size, however, aggressive trunk or canopy shaking will likely depress the following year’s yield by as much as 50%.

3. Winter time drought stress effectively delayed bloom without reducing current yield, percentage of juice or juice quality. Result may allow growers to identify specific blocks for late season harvesting and use winter drought to delay flowering to decrease next season’s young fruitlet loss.

Activities Planned for 2010-2011

We will determine if the winter drought stress increases fruit set in late season ‘Valencia’ orange trees and/or if the relative proportion of vegetative shoots, leafy inflorescences and leafless inflorescences was affected by winter drought.


Melgar, J.C, J. Dunlop, L.G. Albrigo and J.P. Syvertsen. 2010. Winter drought stress can delay flowering and avoid immature fruit loss during late season mechanical harvesting of 'Valencia' oranges. HortScience 45: 271-276.
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Roka, Fritz, Jackie Burns, J. Syvertsen, and Robert Ebel. 2009. Advantages from Registration of an Abscission Product. Citrus Industry (Mar) 90(3): 22-23.
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Syvertsen, J.P. and J.M. Dunlop. 2009. Imidacloprid Has Little Effect On Growth or Drought Tolerance of Citrus Rootstock Seedlings Without Pests. Proc FSHS 122: 81-84.
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Spann, Timothy M., Antonios E. Tsagkarakis and J. P. Syvertsen. 2009. Managing Citrus Tree Growth With Hedging and Plant Growth Regulators: Strategies For Reducing Psyllid Feeding and Huanglongbing Infection. Proc FSHS 122: 161-165.
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Hyman, Barbara, Fritz Roka, J. Syvertsen, Bob Ebel, Tim Spann and Reza Ehsani. 2010. Popular Mechanics: Efforts are being made move citrus mechanical harvesting forward. Florida Grower (Feb) Vol 103(2):20-21.
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Melgar, J.C., A.W. Schumann and J.P. Syvertsen. 2010. Can fertigation frequency with drip alter Citrus growth and improve water and nitrogen use efficiency? Proceedings VI ISHS International Symp. on Irrigation of Horticultural Crops in Vina del Mar, Chile. 2-6 Nov 2009.

Syvertsen, J., Juan Carlos Melgar and Tim Spann 2010. Impact of Mechanical Harvesting on Tree Health and Debris. Citrus Industry (Feb) Vol. 91(2):32.
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For more information:

Jim Syvertsen
Kelly Morgan
Bob Ebel

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