Submitted to: International Journal of Fruit Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/17/2018
Publication Date: 8/17/2018
Citation: Glenn, D.M., Tabb, A. 2018. Evaluation of five methods to measure Normalized Difference Vegetation Index (NDVI) in apple and citrus. International Journal of Fruit Science. https://doi.org/10.1080/15538362.2018.1502720.
Interpretive Summary: Accurate and low cost sensors are needed to assess crop status in order to identify problems with pest damage, nutrient, and water stress before they limit yield and quality. We evaluated three methodologies to calculate an index of crop status (NDVI) using low cost modified web cameras without the standard near-infrared block filter (NIR) in addition to sophisticated cameras suited to measuring crop reflectance and calculating NDVI. Calibration equations were developed for all cameras using apple and citrus trees, and evaluated in acute water deficit studies. Useful NDVI values were calculated using low-cost modified web cameras; however, acute water stress did not result in reduced NDVI values. Low-cost modified web cameras without a NIR filter can be used to estimate NDVI using mathematical methods. NDVI is a useful tool when evaluating long-term crop changes such as pest damage, chronic water shortage, and nutrient deficiencies, but NDVI is not useful for acute stresses such as an irrigation pump failure or plugged irrigation lines that have an effect within days.
Technical Abstract: Normalized Difference Vegetation Index (NDVI) is a common remote sensing calculation used to assess green biomass in addition to nutrient, pest, and water stress. The approach used in NDVI has potential for a broader use of remote sensing of plant physiological status. The purpose of the study was to evaluate techniques that utilize combined NIR and visible detection by low cost web cameras in comparison with a dedicated NDVI sensor and direct measurement of RGB and NIR bands to calculate NDVI and identify water stress. Established techniques as well as new techniques for NDVI computation from cameras were evaluated and compared to soil plant analysis development (SPAD) chlorophyll meter values to assess the accuracy of NDVI for predicting plant stress in apple and citrus. NDVI is a useful tool when evaluating long-term crop changes such as pest damage, chronic water shortage and nutrient deficiencies that affect chlorophyll, but NDVI is not useful for acute stresses such as an irrigation pump failure or plugged irrigation lines that have an effect within days.