Improved prediction of leaf emergence for efficacious crop protection: assessing field variability in phyllotherms for upper leaves in winter wheat and winter barley

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 greater or equal to 0.90 for F-1 and F. For phyllotherm values greater or equal to 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

Quantifying the potential effects of regional climate change on wheat leaf rust disease in the Grand-Duchy of Luxembourg

Climate change will affect wheat crop production both in the main processes of plant growth and development and in the occurrences and severities of plant diseases. We assessed the potential infection periods of wheat leaf rust WLR in a changing climate at two climatologically different sites in Luxembourg using a threshold-based model for WLR infection and progress that involves night-time hourly data of air temperature, relative humidity and rainfall. Field experiments were conducted during the 2003–2013 period at the selected sites to test the model. Projected climate data, from a multi model ensemble of regional climate models (spatial resolution 25 km) as well as an additional projection with a higher spatial resolution of 1.3 km, were used for investigating the potential WLR infection periods for two future time spans. Results showed that the infections of WLR were satisfactorily simulated at both sites during 2003–2013 period: probabilities of WLR detection were close to 1 and the critical success index ranged from 0.80 to 0.94. Regional climate projections indicated an increase in temperatures over the 2014-2050 and 2091-2100 periods compared to the reference period 1991–2000. Annual precipitation is also expected to increase slightly in the future. Further, trends in increased favourable days of WLR infection would likely occur at both sites due to projected climatic conditions more conducive than in the reference period. The methodology presented here can be readily transferred to other regions and other fungal diseases by adjusting the meteorological threshold values essential for a specific disease development

Weather conditions conducive to infection of winter wheat by Puccinia striiformis sp. tritici race ‘warrior’

Wheat stripe rust (WSR) (caused by Puccinia striiformis sp. tritici) continues to be a major threat in most wheat growing regions of the world, with potential to inflict regular yield losses when susceptible cultivars are grown and weather conditions are favourable. A recent isolated strain of P. striiformis sp. tritici, warrior, first identified in 2011 in Europe, is now virulent on adult plants of susceptible wheat cultivars across most of wheat growing regions, including Luxembourg. Daily weather conditions were monitored and related to development of WSR during the 2012-2014 period in Luxembourg. Favourable weather conditions were determined by (i) analysing Dennis model outputs generated through a Monte Carlo method, and (ii) identifying the best correlation between the frequencies of weather condition classes and the area under the disease progress curve on the uppermost three leaves (L1, L2, and L3; L1 being the flag leaf). Our results showed that combined weather conditions, including relative humidity >92% for ≥4 hours and air temperatures between 4°C and 16°C for ≥36 hours are necessary for WLR development, assuming inoculum is available. Furthermore, comparisons with reported WLR outbreaks in previous years showed that in recent years the disease is occurring at earlier stages in the growing season, suggesting a likely effect of climate change and/or climate variability

Economics of a decision–support system for managing the main fungal diseases of winter wheat in the Grand-Duchy of Luxembourg

We evaluated the cost effectiveness of a decision–support system (DSS) developed for assessing in real time the risk of progression of the main fungal diseases (i.e., Septoria leaf blotch, powdery mildew, leaf rusts and Fusarium head blight) of winter wheat in the Grand-Duchy of Luxembourg (GDL). The study was conducted in replicated field experiments located in four agricultural locations (representative of the main agro-ecological regions of the country) over a 10-year period (2003–2012). Three fungicide spray strategies were compared: a single DSS-based system and two commonly used spray practices in the GDL, a double- (2T) and a triple-spray (3T) treatment; there was also a non-treated control. In years with a high disease pressure, the DSS-based recommendation resulted in protection of the three upper leaves comparable to that achieved with the 2T and 3T treatments, with significant grain yield increases (P > 0.05) compared to the control (a 4 to 42% increase, depending on the site and year). Overall, the financial gain in treated plots compared with the control ranged from 3 to 16% at the study sites. Furthermore, in seasons when dry weather conditions precluded epidemic development, the DSS recommended no fungicide spray, reducing use of fungicide, and thus saving the cost of the product. The gain in yield for the 2T and 3T plots (compared with control) did not necessarily result in a financial gain during the duration of the experiment. This study demonstrates the potential advantages and profitability of using a DSS-based approach for disease management