Abstract
The choice of the phyllotherm value for predicting leaf emergence under field conditions is pivotal to the success of fungicide-based disease risk management in temperate cereals. In this study, we investigated phyllotherm variability for predicting the emergence of the three uppermost leaves (i.e., three last leaves to emerge) in winter wheat and winter barley fields. Data from four sites representative of wheat and barley growing regions in Luxembourg were used within the PROCULTURE model to predict the emergence of F-2, F-1 and F (F being the flag leaf) during the 2014–2019 cropping seasons. The phyllotherms tested ranged between 100 °Cd and 160 °Cd, in 15 °Cd steps, including the current default value of 130 °Cd. The comparisons between the observed and predicted emerged leaf area were qualitatively evaluated using the mean absolute error (MAE), the root mean square error (RMSE) and Willmott’s index (WI). A phyllotherm of 100 °Cd accurately and reliably predicted the emergence of all three upper leaves under the various environmental conditions and crop cultivars of winter wheat and winter barley over the study period. MAE and RMSE were generally <5% and the WI values were most often ≥0.90 for F-1 and F. For phyllotherm values ≥115 °Cd, the prediction errors generally increased for F-1 and F, with MAE and RMSE exceeding 20% in most cases. F-2 agreement between observed and predicted values was generally similar when using 100 °Cd or 115 °Cd. These results tie in valuable, complementary information regarding the variability of phyllotherms within leaf layers in winter wheat and winter barley in Luxembourg. Accurate and reliable leaf emergence prediction from F-2 to F allows for timely fungicide application, which ensures lasting protection against infections by foliar fungal disease pathogens. Hence, understanding phyllotherms can help ensure timely, environmentally sound, and efficacious fungicide application while increasing the likelihood of improved yields of winter wheat and winter barley.
Highlights
The upper three leaves of wheat (Triticum aestivum L.), and the F-1 and F-2 leaves and the ear of barley (Hordeum vulgare L.) contributeAgronomy 2020, 10, 1825; doi:10.3390/agronomy10111825 www.mdpi.com/journal/agronomyAgronomy 2020, 10, 1825 most to yield, respectively [1,2,3]
Despite its relatively small size, the Grand-Duchy of Luxembourg is characterized by noticeable climatic contrasts between and within its agricultural regions, which affect crop growth and the development and severity of foliar fungal diseases in winter wheat and winter barley throughout the cropping season [37,38]
Extending the duration of the green leaf area of the upper leaves in both winter wheat and winter barley through the application of certain foliar fungicides benefits the grain filling period, that is, the fungicide effect leads to extended leaf area greenness, allowing grain filling over a longer period [7,39,40]
Summary
The upper three leaves (the flag leaf ‘F’, and the two leaves below F, F-1 and F-2) of wheat (Triticum aestivum L.), and the F-1 and F-2 leaves and the ear of barley (Hordeum vulgare L.) contributeAgronomy 2020, 10, 1825; doi:10.3390/agronomy10111825 www.mdpi.com/journal/agronomyAgronomy 2020, 10, 1825 most to yield, respectively (in barley, the contribution from the flag leaf is minimal due to its small size) [1,2,3]. The reliable prediction of the emergence of the upper leaves (i.e., last leaves to emerge) in wheat and barley is pivotal to the fungicides applied to control fungal diseases and ensure a satisfactory yield. To safeguard crops against economic losses from diseases, Luxembourgish farmers typically apply two to three preventive foliar fungicide treatments, in conjunction with suitable management practices (e.g., use of disease-resistant cultivars, crop rotation, etc.). Depending on the fungicide’s persistence and prevailing weather conditions, a foliar fungicide applied between growth stage (GS) 37–39 (flag leaf just visible to flag leaf blade all visible) up to GS59 (ear emergence) [9] increases the chances of all three upper leaves being protected against infection from fungal foliar pathogens [6]. Applications at GS59 may result in yield losses due to disease as the upper leaves
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