Detection of Biotic Stress (venturia Inaequalis) in Apple Trees Using Hyperspectral Analysis

The potential yield of (capital)-intensive multi-annual crops (e.g., fruit) is seldom harvested in reality. A targeted monitoring and modelling of the growth processes in such agricultural production systems could enable an early detection and treatment of production limiting factors, thereby optimising yield. In Belgium, as in all temperate regions, scab stress caused by the ascomycete Venturia Inaequalis causes the most important stress in apple orchards. The objectives of this study were (i) to determine if Venturia inaequalis leaf infections could be differentiated from healthy leaves in both resistant and susceptible cultivars using hyperspectral spectroradiometer data, (ii) to gauge at which developmental stage Venturia inaequalis infections could be detected and (iii) to identify wavelengths or spectral regions that best differentiate between infected and healthy leaf material. The first objective was related directly to the scientific research question of whether or not infected leaves resulted in a spectral response different from healthy leaves. The second objective addressed the question of whether or not hyperspectral data could serve as part of an early warning system to biotic stress in apple orchards, while the last objective defines the practical implication of such a spectral warning system. Partial Least Squares Discriminant Analysis (PLSDA) was used as classification technique. This technique compresses the dimension of the hyperspectral reflectance dataset, followed by a discriminant classification. Results suggested that good predictability could be achieved when classifying infected plants based on hyperspectral data using PLSDA. Furthermore, a band reduction technique based on logistic regression was used to select the hyperspectral bands that best define differences among treatments. This study showed that the spectral domains centered around 1500nm and the visible region (well-developed infection stage) were best suited to differentiate between infected and healthy plants. INTRODUCTION The structure and physiological status of a plant is represented by reflectance patterns (i). Incident energy of the sun is partly reflected, transmitted and absorbed by the plant. The amount of reflected light depends on an amount of leaf-related factors, such as external morphology, internal structure, concentration, and internal distribution of biochemical components. Each possibility to monitor these factors in an ongoing or periodic manner by means of teledetection offers the potential to model plant production processes, and therefore also to steer processes by means of adapted management measures. Studies have shown that non-intrusive techniques are essential for capturing data in the continuous manner necessary for monitoring vegetative production systems. Much remote sensing research has been done at plant leaf-level to ascertain the amount of stress in plants. Most studies focused on physiological changes and how these changes alter the interaction of light with the foliar medium (ii, iii). The most common and widespread change occurs in the proportion of light-absorbing pigments, most notably chlorophylls a and b which absorb light in the 430–660nm region (iv). Investigators have observed differences in reflectance due to stressinduced changes in pigment concentration in the green peak (525-605nm) and along the red edge (~750nm) (v, vi, vii, viii). © EARSeL and Warsaw University, Warsaw 2005. Proceedings of 4th EARSeL Workshop on Imaging Spectroscopy. New quality in environmental studies. Zagajewski B., Sobczak M., Wrzesień M., (eds) Apple scab stress manifests at different stages. As can be seen in Figure 1, the primary infection is initiated by the release of ascospores. These spores penetrate the leaf cuticle, causing the formation of a flat mycelium between the cuticle and the epidermal cell walls. Sporulation occurs by means of conidiophores, which penetrate the cuticular layer of the leaf surface and release conidia (ix). Until sporulation, V. inaequalis grows without killing host cells. First, olive green spots will appear on the leaf surface, followed by typical necrotic scab lesions. At the end of the vegetation period, the fungus switches to saprophytic growth in the dead leaf tissue and forms fruit bodies (pseudothecia), which overwinter and give rise to ascospores the following spring. (x) Figure 1 : Apple scab disease cycle Little is known about the host-pathogen interaction of apple scab. Resistant apple cultivars generally activate their defense mechanism after infection, which initiates a sequence of metabolic reactions. In some cases it will cause a hypersensitive reaction of the plant cell that has been penetrated by the pathogen, resulting in cell death. When this happens, the pathogen will in many cases die, due to toxic substances accumulating in the dead plant cell. Lateur (xi) listed the most common metabolic modifications a) the reinforcement of the mechanical barrier consisting of a pecto-cellulose wall by incorporation of polysaccharides (callose), phenolic polymers (lignine), polyesters (suberine) and proteins, b) the stimulation of defense enzymes, and c) the production of defense and pathogenesis related proteins, such as chitinases, glucanases, and thaumatine-like proteins. All these factors will influence the reflectance spectra of the vegetation. Figure 2 shows the influence of selected vegetation compounds on a reflectance spectrum from plant material (xii, xiii, xiv, xv, xvi).