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Remote Sens. 2009, 1(4), 858-874; doi:10.3390/rs1040858

Hyperspectral Reflectance and Fluorescence Imaging to Detect Scab Induced Stress in Apple Leaves

3,* , 3
1 VITO, Flemish Institute for Technological Research, Boeretang 200, BE-2400, Mol, Flanders, Belgium 2 Crop Bio-engineering, Katholieke Universiteit Leuven, W. de Croylaan 42, 3001 Heverlee, Flanders, Belgium 3 M3-BIORES, Katholiek Universiteit Leuven, W. de Croylaan 34, BE-3001, Flanders, Belgium 4 Laboratory of Molecular and Physical Plant Physiology, Centre for Environmental Sciences, Hasselt University, Agoralaan, Bldg D, 3590 Diepenbeek, Flanders, Belgium 5 Centro de Estudios Avanzados en Zonas Aridas, Universidad de la Serena Benavente 980, Casilla 599, 172-0170 La Serena, Chile
* Author to whom correspondence should be addressed.
Received: 14 September 2009 / Revised: 28 October 2009 / Accepted: 2 November 2009 / Published: 6 November 2009
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Apple scab causes significant losses in the production of this fruit. A timely and more site-specific monitoring and spraying of the disease could reduce the number of applications of fungicides in the fruit industry. The aim of this leaf-scale study therefore lies in the early detection of apple scab infections in a non-invasive and non-destructive way. In order to attain this objective, fluorescence- and hyperspectral imaging techniques were used. An experiment was conducted under controlled environmental conditions, linking hyperspectral reflectance and fluorescence imaging measurements to scab infection symptoms in a susceptible apple cultivar (Malus x domestica Borkh. cv. Braeburn). Plant stress was induced by inoculation of the apple plants with scab spores. The quantum efficiency of Photosystem II (PSII) photochemistry was derived from fluorescence images of leaves under light adapted conditions. Leaves inoculated with scab spores were expected to have lower PSII quantum efficiency than control (mock) leaves. However, besides scab-induced, also immature leaves exhibited low PSII quantum efficiency. Therefore, this study recommends the simultaneous use of fluorescence imaging and hyperspectral techniques. A shortwave infrared narrow-waveband ratio index (R1480/R2135) is presented in this paper as a promising tool to identify scab stress before symptoms become visible to the naked eye. Low PSII quantum efficiency attended by low narrow waveband R1480/R2135 index values points out scab stress in an early stage. Apparent high PSII quantum efficiency together with high overall reflectance in VIS and SWIR spectral domains indicate a severe, well-developed scab infection.
Keywords: hyperspectral and fluorescence data; scab detection hyperspectral and fluorescence data; scab detection
This is an open access article distributed under the Creative Commons Attribution License (CC BY) which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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Delalieux, S.; Auwerkerken, A.; Verstraeten, W.W.; Somers, B.; Valcke, R.; Lhermitte, S.; Keulemans, J.; Coppin, P. Hyperspectral Reflectance and Fluorescence Imaging to Detect Scab Induced Stress in Apple Leaves. Remote Sens. 2009, 1, 858-874.

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