Field Spectra and Airborne Digital Imagery for Detecting Phytophthora Foot Rot Infections in Citrus Trees

Abstract

<jats:p>Phytophthora foot rot, caused by <jats:italic>Phytophthora parasitica</jats:italic> (Dast), can result in economic losses for the citrus industry in the Lower Rio Grande Valley of Texas. Therefore, locating foot rot-infected trees in citrus groves is important to citrus growers. Symptoms of the infection include leaf yellowing, canopy defoliation, twig dieback, and short growth flushes. This study evaluated the use of the latest remote sensing technology, that of airborne digital imagery, for the detection of citrus trees exhibiting mild symptoms of foot rot infection. Airborne color-infrared (CIR) digital imagery of two citrus orchards having problems with foot rot infection was acquired. In addition to the aerial digital imagery, ground spectroradiometric measurements were conducted to determine the visible and near-infrared (NIR) spectral reflectance differences between healthy and infected trees. These measurements were also used to help interpret the color tonal renditions between the trees. The CIR digital imagery distinguished infected from noninfected trees. The noninfected trees had a bright red-magenta color rendition, while the infected trees had a dull grayish red tonal response. The NIR spectroradiometric and digital readings were significantly lower for infected trees than for healthy ones (<jats:italic>P</jats:italic> ≤ 0.05), whereas the visible reflectance and digital data revealed no significant differences between the trees. The infected trees' dull tonal response in the CIR image was attributed to their lower NIR light intensity. These results indicated that digital imagery has potential for detecting foot rot-infected trees in citrus groves based upon NIR spectral differences. The advantage of airborne digital imagery is its real-time survey for quick field assessment.</jats:p>

Coloured ornamental traits could be effective and non-invasive indicators of pollution exposure for wildlife

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Evaluating Airborne Normalized Difference Vegetation Index Imagery for Citrus Orchard Surveys

“…I magery acquired by airborne sensors has been used to detect and assess stresses affecting citrus trees. Airborne color-infrared imagery has been primarily used for these surveys (Blaquez and Horn, 1980;Blaquez et al, 1988;Everitt et al, 1994;Fletcher et al, 2001;Richardson and Blaquez, 1989). Vegetation indices have been developed that reduce multispectral observations to a single numeric index for examining vegetation conditions (Jensen, 2000).…”

United States Department of Agriculture–Agricultural Research Service research in application technology for pest management

“…Analysis of CIR imagery has been the preferred method for assessing biological systems. A variation of the three‐camera system has been developed for detection of foot‐rot infections in citrus trees 30. The authors used a true‐digital system consisting of three black‐and‐white Kodak Megaplus cameras with interference filters for derivation of CIR images.…”