Field spectra and airborne digital imagery for detecting phytophthora foot rot infections in citrus trees

Abstract

Phytophthora foot rot, caused by Phytophthora parasitica (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 (P ≤ 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.