Vol 120 (2007): Proceedings of the Florida State Horticultural Society

Georeferenced ground photography of citrus orchards to estimate yield and plant stress for variable rate technology

Arnold W. Schumann
University of Florida
Published December 1, 2007
  • digital cameras,
  • GPS,
  • water stress,
  • fruit yield prediction


Citrus canopy measurements with ultrasonic and optical sensors are being used in Florida to control the placement and rate of fertilizers and pesticides with variable rate application (VRA) spreaders. A significant reduction of fertilizer or pesticide consumption is possible simply by applying agrochemicals only to orchard space occupied by trees with dense canopies. Additional refinement of agrochemical VRA may also be possible if fruit load (especially on alternate bearing trees), flowering intensity, and leaf nutrient stress could be measured on the tree canopies. Detection of early (mild) water stress before leaf wilting becomes visible and reduces yield, could be used to schedule irrigation, manipulate flower and leaf flushes, or improve fruit quality. In this study we developed ground-based digital photography systems to study the characteristics of citrus tree canopies over large areas. A color digital camera mounted on a moving vehicle was used to capture georeferenced overlapping images of tree canopies in entire orchards. Images were stored on a laptop computer and were processed using red-green-blue (RGB) pixel ratios and thresholds to identify and quantify numbers of mature fruit. A monochrome digital camera with visible and near-infrared bandpass filters was used to develop a multispectral imaging system capable of rapidly detecting early water stress in tree canopies. Significant correlations were achieved between the water stress index developed with the camera system and stem water potential measurements used for quantifying water stress in citrus trees. The water stress index could also detect, quantify, and map the severity of blight disease in orchard trees for an entire block.