Time filter

Source Type

Van Beek J.,Catholic University of Leuven | Tits L.,Catholic University of Leuven | Somers B.,Catholic University of Leuven | Deckers T.,PCFruit Research Station | And 2 more authors.
Proceedings of SPIE - The International Society for Optical Engineering | Year: 2014

High spatial resolution satellite imagery provides an alternative for time consuming and labor intensive in situ measurements of biophysical variables, such as chlorophyll and water content. However, despite the high spatial resolution of current satellite sensors, mixtures of canopies and backgrounds will be present, hampering the estimation of biophysical variables. Traditional correction methodologies use spectral differences between canopies and backgrounds, but fail with spectrally similar canopies and backgrounds. In this study, the lack of a generic solution to reduce background effects is tackled. Through synthetic imagery, the mixture problem was demonstrated with regards to the estimation of biophysical variables. A correction method was proposed, rescaling vegetation indices based on the canopy cover fraction. Furthermore, the proposed method was compared to traditional background correction methodologies (i.e. soil-adjusted vegetation indices and signal unmixing) for different background scenarios. The results of a soil background scenario showed the inability of soil-adjusted vegetation indices to reduce background admixture effects, while signal unmixing and the proposed method removed background influences for chlorophyll (ΔR2 = ∼0.3; ΔRMSE = ∼1.6 μg/cm2) and water (ΔR2 = ∼0.3; ΔRMSE = ∼0.5 mg/cm2) related vegetation indices. For the weed background scenario, signal unmixing was unable to remove the background influences for chlorophyll content (ΔR2 = -0.1; ΔRMSE = -0.6 μg/cm 2), while the proposed correction method reduced background effects (ΔR2= 0.1; ΔRMSE = 0.4 μg/cm2). Overall, the proposed vegetation index correction method reduced the background influence irrespective of background type, making useful comparison between management blocks possible. © 2014 SPIE.

Janssens P.,Soil Service of Belgium | Elsen F.,Soil Service of Belgium | Elsen A.,Soil Service of Belgium | Deckers T.,PCFruit Research Station | Vandendriessche H.,Catholic University of Leuven
Acta Horticulturae | Year: 2011

Pear growing (Pyrus Communis 'Conference') is an important part of horticulture in Belgium and The Netherlands. Drip irrigation techniques are implemented to achieve maximum production with a high fruit size distribution. The financial return of fruits having a diameter of ≥60 mm is twice the return of smaller sized fruits (≤55 mm). Maximum fruit size distribution and high productivity on 'Conference' pear trees can be achieved by applying regulated deficit irrigation (RDI). However, unpredictable summer rains and drought periods make it difficult to establish the optimal irrigation thresholds. To overcome these difficulties, an adapted Soil Water Balance model is presented to monitor the evolution of soil water content in the root zone on a daily basis. The input parameters for the model are soil water retention characteristics, rain, irrigation events and evapotranspiration. A specific algorithm has been developed for drip irrigation and tree transpiration in an orchard with weed free strips under the trees and grass strips between tree rows. Gravimetric moisture determination is used to calibrate the model during the growing season. The performance of the model was studied in three commercial orchards in the period 2008-2009. A significant correlation (R>0.80) was observed between predicted and observed soil water content. It illustrates the possibilities of the approach to use the soil water balance model as a base to schedule the irrigation for pear trees under orchard conditions with sufficient accuracy.

Deckers T.,PCFruit Research Station
Communications in agricultural and applied biological sciences | Year: 2010

Fire blight, caused by the bacterium Erwinia amylovora (Burill Winslow et al.), is a very important bacterial disease on apple and pear orchards with devastating effects in some production area and in some years. Fire blight control consists in a whole strategy of measures that should start with control measures in and around the fruit tree nurseries. Only the use of Vacciplant (Laminarin), an inducer of the self-defence mechanism, is registered in Belgium since 2009. In other European countries Fosethyl-Al has been registered for fire blight control. Recently, research trials have been done at Pcfruit research station for several years on the activity of ALiette (fosethyl-Al) against fire blight. Fosethyl-Al, also a plant defence enhancing molecule, applied preventively 3 times at a dose of 3.75 kg/ha standard orchard (3 x 3000 g a.i./ha standard orchard), showed a reduction in the host susceptibility and decreased the disease development on artificial inoculated flower clusters and shoots. Also a clear reduction in the ooze droplet formation on artificially inoculated immature fruitlets has been observed with this molecule. This reduction in the bacterial ooze formation is considered as a very important factor in the spread of the disease in the orchard.

van Beek J.,Catholic University of Leuven | Tits L.,Catholic University of Leuven | Somers B.,Catholic University of Leuven | Deckers T.,PCFruit Research Station | And 2 more authors.
International Journal of Applied Earth Observation and Geoinformation | Year: 2015

Satellite remote sensing provides an alternative to time-consuming and labor intensive in situ measurements of biophysical variables in agricultural crops required for precision agriculture applications. In orchards, however, the spatial resolution causes mixtures of canopies and background (i.e. soil, grass and shadow), hampering the estimation of these biophysical variables. Furthermore, variable background mixtures obstruct meaningful comparisons between different orchard blocks, rows or within each row. Current correction methodologies use spectral differences between canopies and background, but struggle with a vegetated orchard floor. This background influence and the lack of a generic solution areaddressed in this study. Firstly, the problem was demonstrated in a controlled environment for vegetation indices sensitive to chlorophyll content, water content and leaf area index. Afterwards, traditional background correction methods (i.e. soil-adjusted vegetation indices and signal unmixing) were compared to the proposed vegetation index correction. This correction was based on the mixing degree of each pixel (i.e. tree cover fraction) to rescale the vegetation indices accordingly and was applied to synthetic and WorldView-2 satellite imagery. Through the correction, the effect of background admixture for vegetation indices was reduced, and the estimation of biophysical variables was improved (δR2= 0.2-0.31). © 2014 Elsevier B.V.

Janssens P.,Soil Service of Belgium | Deckers T.,PCFruit Research Station | Elsen F.,Soil Service of Belgium | Elsen A.,Soil Service of Belgium | And 3 more authors.
Agricultural Water Management | Year: 2011

The present study examines the need for irrigation in pear trees (Pyrus Communis, cv. 'Conference') under low evaporative demand conditions, like in Belgium, in order to maintain a consistent fruit yield and high fruit size. To determine the sensitivity of the pear yield under low evaporative demand conditions three different orchards were monitored. The study shows that a Ψsoil of -60kPa during shoot growth has no effect on fruit yield but lower Ψsoil values induced a decline in both fruit size and total yield. Just as for arid environments a Ψstem of -1.5MPa is related to negative yield responses. In dry conditions lower Ψsoil and Ψstem values were observed in root pruned trees compared to not root pruned trees in the same irrigation treatment, however without yield decline. In one orchard a biannual bearing tendency was observed after root pruning. Furthermore intensive Ψsoil measurements show a high variation in Ψsoil between orchards, and within an orchard. This underlines the need for irrigation management on a parcel level and the need for new irrigation scheduling techniques which take the spatial variation in the orchard into account. © 2011 Elsevier B.V.

Discover hidden collaborations