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

Stephanie Delalieux,Pol Coppin,Stefaan Lhermitte,Annemarie Auwerkerken,Ben Somers,Roland Valcke,Johan Keulemans,Willem W Verstraeten

doi:10.3390/rs1040858

Abstract

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.

Highlights

Apple scab, caused by the fungus Venturia inaequalis (Cooke) Wint, is a major problem for apple growers in many parts of the world
The main objective of this study is to evaluate the potential of nondestructive chlorophyll fluorescence and hyperspectral sensors for early detection of scab infections caused by Venturia inaequalis (Cooke) Wint in a susceptible apple cultivar, Braeburn
The Photosystem II (PSII) quantum efficiency (Fm' − Fs)/Fm' or ΦPSII is affected by the level of electron acceptors, usually NADP+, available at the acceptor side of PSI

Summary

Introduction

Apple scab, caused by the fungus Venturia inaequalis (Cooke) Wint, is a major problem for apple growers in many parts of the world. The ascospores in the overwintering pseudothecia are dispersed by wind to susceptible young leaves, sepals and young fruits where they can cause infections. The growth of hyphae between the cuticle and epidermal cell wall over several days result in the development of stroma and in the development of conidiophores and conidia that rupture the cuticle When this occurs, a scab lesion is clearly visible macroscopically [1]. When left untreated, scab infection can cause severe damage to leaves and sepals, and to fruit, petioles, blossoms, young shoots and budscales. Cultivars with genetic resistance to scab do not meet the expectations of growers in terms of yield, fruit quality, and flavor [2]. Recent research efforts have shown the potential of hyperspectral and chlorophyll fluorescence (CF)

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Conclusion