Manfred Stoll

Early pathogen detection under different water status and the assessment of spray application in vineyards through the use of thermal imagery

Remote detection using thermal imagery has potential for use in the pre-symptomatic diagnosis of abiotic stress or of early disease detection. The latter is an issue of great importance since late detection of fungus attacks or poor spray coverage are major factors contributing to weak disease control affecting fruit quality or reducing yield in grapes. In greenhouse experiments the effects on spatial and temporal variability of leaf temperature of grapevine (Vitis vinifera L. cv. Riesling) leaves inoculated with a fungal pathogen (Plasmopara viticola (Berk. & Curt. Ex de Bary) were studied in either well-irrigated or non-irrigated potted plants. Due to the high sensitivity of leaf temperature to the amount of water transpired, infra-red thermography can be used to monitor irregularities in temperature at an early stage of pathogen development. Evidence for characteristic thermal responses in grapevines was apparent well before visible symptoms appeared. Contrasting thermal effects due to the pathogen attack were found between measurements on well-irrigated and water-stressed plants. Furthermore, from a technical point of view, thermal imagery has the potential to assess the evenness of spray coverage within a canopy, hence optimizing pesticide application efficiency.

Soil water-holding capacity mediates hydraulic and hormonal signals of near-isohydric and near-anisohydric Vitis cultivars in potted grapevines

Grapevine (Vitis vinifera L.) expresses different responses to water stress, depending not only on genotype, but also on the influence of vineyard growing conditions or seasonality. Our aim was to analyse the effects on drought response of two grapevine cultivars growing on two soils, one water draining (WD) containing sand 80% volume and the other water retaining (WR), with no sand. Under these two different water-holding capacities Syrah, displaying a near-anisohydric response to water stress, and Cabernet Sauvignon (on the contrary, near-isohydric) were submitted to water stress in a pot trial. Xylem embolism contributed to plant adaptation to soil water deprivation: in both cultivars during late phases of water stress, however, in Syrah, already at moderate early stress levels. By contrast, Syrah showed a less effective stomatal control of drought than Cabernet Sauvignon. The abscisic acid (ABA) influenced tightly the stomatal conductance of Cabernet Sauvignon on both pot soils. In the near-anisohydric variety Syrah an ABA-related stomatal closure was induced in WR soil to maintain high levels of water potential, showing that a soil-related hormonal root-to-shoot signal causing stomatal closure superimposes on the putatively variety-induced anisohydric response to water stress.

Exploring the sensitivity of thermal imaging for Plasmopara viticola pathogen detection in grapevines under different water status

The effect on spatial variability of leaf temperature of inoculating grapevine leaves (Vitis vinifera L. cv. Riesling) with a fungal pathogen (Plasmopara viticola) was studied in either well irrigated or non-irrigated plants. The results from thermal imagery were compared with stomatal conductance measured by leaf gas exchange. The high sensitivity of leaf temperature to stomatal conductance means that infrared thermography can be used to monitor irregularities in temperature at an early stage of development after either infection or other stress-related changes affecting the amount of water transpired. Contrasting thermal effects due to the pathogen attack were found between measurements on well irrigated and water stressed plants. With irrigated vines, pathogen development caused an increase in leaf temperature at the point of infection. In contrast, under severe water stress, the inoculated plants showed a lower temperature at the sites of inoculation compared with the rest of the leaf. Analysis of the spatial and temporal sensitivity of the temperature profile, obtained from the deviation of individual pixels from the mean along a straight line, successfully distinguished between healthy and infected positions on the leaf irrespective of the plant water status. Under greenhouse conditions and for predefined areas of the leaf surface, evidence was also acquired for characteristic thermal responses to be apparent not later than 4 days past inoculation; that is, at least 3 days before visible symptoms appeared. Thus, early and remote detection using thermal imagery has the potential for pre-symptomatic diagnosis of biotic stress.

Growth, Yield and Fruit Quality of Grapevines under Organic and Biodynamic Management

The main objective of this study was to determine growth, yield and fruit quality of grapevines under organic and biodynamic management in relation to integrated viticultural practices. Furthermore, the mechanisms for the observed changes in growth, yield and fruit quality were investigated by determining nutrient status, physiological performance of the plants and disease incidence on bunches in three consecutive growing seasons. A field trial (Vitis vinifera L. cv. Riesling) was set up at Hochschule Geisenheim University, Germany. The integrated treatment was managed according to the code of good practice. Organic and biodynamic plots were managed according to Regulation (EC) No 834/2007 and Regulation (EC) No 889/2008 and according to ECOVIN- and Demeter-Standards, respectively. The growth and yield of the grapevines differed strongly among the different management systems, whereas fruit quality was not affected by the management system. The organic and the biodynamic treatments showed significantly lower growth and yield in comparison to the integrated treatment. The physiological performance was significantly lower in the organic and the biodynamic systems, which may account for differences in growth and cluster weight and might therefore induce lower yields of the respective treatments. Soil management and fertilization strategy could be responsible factors for these changes. Yields of the organic and the biodynamic treatments partially decreased due to higher disease incidence of downy mildew. The organic and the biodynamic plant protection strategies that exclude the use of synthetic fungicides are likely to induce higher disease incidence and might partially account for differences in the nutrient status of vines under organic and biodynamic management. Use of the biodynamic preparations had little influence on vine growth and yield. Due to the investigation of important parameters that induce changes especially in growth and yield of grapevines under organic and biodynamic management the study can potentially provide guidance for defining more effective farming systems.

Postponing First Shoot Topping Reduces Grape Cluster Compactness and Delays Bunch Rot Epidemic

Field trials investigating the impact of the timing of first shoot topping in a vertical shoot positioning (VSP) trellis system on (i) the cluster morphology, (ii) the canopy structure in the cluster-zone, and (iii) the progress of the bunch rot disease severity, were conducted in two white Vitis vinifera (Pinot gris, Riesling) cultivars in the years 2012 and 2013 in Luxembourg. The first shoot topping was carried out at seven different times from one week prior to beginning of flowering (BBCH 57) to four weeks after the end of flowering (BBCH 75–79) at approximately weekly intervals. Late first shoot topping reduced cluster compactness, delayed bunch rot epidemics, and thereby prolonged the potential ripening period compared to the standard treatment, in which the first shoot topping was performed one week postbloom. First shoot topping four weeks postbloom delayed the moment when 5% disease severity was reached up to eleven days and increased total soluble solids by 0.77 to 2.24 Brix compared to the standard treatment, with no significant impact on yield. Postponing the first shoot topping to the latest technically possible moment is recommended to improve grape architecture and health as well as to optimize grape maturity and potential wine quality under cool climate conditions. Late first shoot topping can be implemented in bunch rot control strategies without imposing additional costs and might support the reduction of pesticide use.

Indirect Estimation of Leaf Area Index in VSP-Trained Grapevines Using Plant Area Index

Leaf area index (LAI) and canopy structure are important parameters affecting grape quality and yield of grapevines. Two different experimental protocols as well as the average LAI value of the different protocols for indirect estimation of LAI by gap fraction analysis in VSP-trained grapevines (Vitis vinifera L. cv. Riesling) were tested in this study using plant area index (PAI). Measurements were performed using a plant canopy analyzer. Directly measured LAI and estimated PAI were compared. Protocol SFC (sensor facing the canopy) gave accurate estimates of LAI by measuring PAI along a diagonal transect including eight vines on each side. The correlation between directly measured LAI and estimated PAI was very high (R2 = 0.93) and the root mean square error was lowest of the methods tested here (RMSE = 0.21). Eight measurements below the canopy were enough to accurately estimate LAI. By applying the empirical calibration equation, the measurements provide accurate LAI estimates. Nevertheless, local calibration is required. The method presented provides a useful tool for rapid and precise LAI estimation in VSP training systems and for supporting canopy or management decisions based on LAI.

Grapevine rootstocks result in differences in leaf composition (Vitis vinifera L. cv. Pinot Noir) detected through non-invasive fluorescence sensor technology: Rootstock and leaf composition detected by sensors

Background and Aims The main objective of the study was to test if differences in leaf composition induced by the rootstock could be detected through non‐invasive fluorescence sensor technology. Methods and Results Multiplex Research measurements on three dates over three consecutive growing seasons showed that Pinot Noir (Vitis vinifera L.) grafted onto high vigour rootstocks (125AA and SO4 with double the amount of pruning mass) had also leaves with high chlorophyll index and low flavonol index when compared to that of Riparia and Schwarzmann. Multiplex measurements ‘on‐the‐go’ gave similar results. Amino acids in juices, however, was not directly related to vine vigour as values for R110 and Riparia were significantly lower than that for 125AA. Conclusions The applicability of new optical sensors used both manually or ‘on‐the‐go’ to characterise rootstock influence on scion leaf composition was established and could be related to vine vigour conferred by the rootstock. Significance of the Study Our study provides evidence that non‐invasive fluorescence sensors used for the first time over consecutive growing seasons and dates are able to reveal differences in scion leaf composition induced by the rootstock in relation to vine vigour and to provide the basis for a better understanding of its impact on fruit and wine composition.