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Pesticides Research No. 116 2008 of fungicide application in winter wheat 1 Introduction
1.1 Fungicide use in winter wheatFungicides are commonly used for controlling leaf diseases in winter wheat. On average the number of treatments today is two, and the average dose of fungicides per treatment varies between 20 and 50% of the full dose (Farmstat and Kleffmann, 2002). The main targets for disease control in winter wheat are powdery mildew (Blumeria graminis) and septoria leaf blotch (Septoria tritici). Septoria tritici has during the last 20 years been the most yield reducing disease and been the major factor influencing the fungicide input. Input of pesticides in Denmark is today generally measured as Treatment Frequency Index (TFI), which quantifies the number of full dosages applied in the field. The target figure for 2003 in Pesticide Action Plan II for fungicide input in winter wheat was recommended to 0.75. In the new action plan extending to 2009 the recommended TFI is 0.65. The target of 0.75 was reached in general in 2002 (Anonymous, 2003). A further reduction requires increased use of resistant cultivars and new techniques to optimise the input. More than a 50% reduction in fungicide input in winter wheat has already taken place during the last 20 years mainly due to use of appropriate and reduced dosages. 1.2 Spatial variation in diseasesMildew often occurs near the borders of fields, where local shelter effects may give favourable conditions for infection and spreading of mildew spores (Koch, 1980; Secher et al., 1995; Bjerre et al., 1998). This is probably caused by more favourable temperature and humidity conditions near shelterbelts, and topographic variation may give rise to similar variation in occurrence of mildew. Mildew has also been found to increase at higher crop densities during early crop development (Bødker et al., 1994). The occurrence of mildew on the upper leaves (including the flag leaf) on the other hand seems to be more related to nitrogen (N) application rates and N concentration in the leaves around growth stage 39 (Olesen et al., 2000a, 2003b). Several investigations have demonstrated a relationship between incidence of septoria and crop density (Jørgensen, 1997; Bjerre et al., 1998; Olesen et al., 2000a), showing that septoria increases at low crop densities, where the fungal disease spores more readily spreads upwards in the crop canopy. However, other investigation have found higher disease incidence at high crop densities (Broscious et al., 1985; Tompkins et al., 1985). The different results obtained may be an effect of a correlation between crop density and leaf nitrogen (N) concentrations, since a large crop N supply often leads to dense crops. However, if a low crop density is determined by a low plant density, then high leaf N concentrations may also occur at low densities. High leaf N concentrations at growth stage 39 have in several investigations been found to promote Septoria tritici (Leitch and Jenkins, 1995; Olesen et al., 2003b; Simon et al., 2003). 1.3 Spatially varying fungicide applicationControl of leaf diseases in cereals is normally performed with the same fungicide dosages throughout the field from an assessment of the disease occurrence in the field as a whole. However, there may be large differences in the need for disease control in different parts of a field (Bjerre et al., 1998). A site specific fungicide application may improve disease control under the following conditions (Bjerre et al., 2006): (a) if the disease varies spatially, (b) if the crop sensitivity to disease varies, and/or (c) if the effect of disease control varies spatially. Analyses of yield gains from fungicide control have shown a tendency for higher yield increases at sites with high yield potentials (Paveley et al., 1996; Oerke and Dehne, 1997; Dansk Landbrugsrådgivning, 2003). The causes of this have not been fully clarified, but it may be at least partly related to low yielding crops primarily occurring on sandy soils, where the full potential of disease control cannot be exploited, since the crop yield is often reduced by early senescence. The crop density and the leaf area also affects the amount of fungicide deposited on the individual leaves, and thus also the effect obtained from a fungicide treatment (Secher, 1998). The qualitative deposition in the crop canopy with conventional spraying technology is primarily varied through changes in driving speed and drop size and nozzle choice. A low driving speed and a large drop size gives the largest penetration in the canopy, whereas a high speed and fine drops lead to a deposition in the upper part of the canopy. The deposition is often characterised by a capture efficiency, which is correlated with the crop leaf area index (Jagers op Akkerhuis et al., 1998; Gyldenkærne et al., 1999). A constant capture efficiency will lead to a steeper decline of fungicide deposition in a dense compared with an open crop canopy. However, Secher (1998) found the same deposition profiles at different crop densities, but generally a lower deposition per unit leaf area in the more dense canopies. If the fungicide treatments within a field are varied based on crop density and the same amount of fungicide is used, then a spatially varied application may lead to less fungicide being deposited on the soil surface. Experiments have shown that there is a small, but significant yield gain from adjusting the fungicide rate (+/- 20%) according to the crop density (Secher, 1998). A spatially varied fungicide rate requires that the relevant influencing factors can be measured using tractor-mounted soil and plant sensors for mapping the spatial variation in soil and plant characteristics. The soil characteristics may be measured using time domain reflectrometry (MobilTDR) (Thomsen and Schelde, 2006) or electrical conductivity (EM38) (Greve et al., 2002). These soil characteristics mostly reflect the soil water retention capacity, which is related to yield potential and therefore possibly also to yield gain from fungicide application. The plant characteristics may be measured by using spectral reflectance combined with laser measurements to characterise crop canopy geometry (MobilLas) (Thomsen and Schelde, 2007). These plant characteristics mostly reflect leaf area index and plant nitrogen content, which are related yield potential, fungicide deposition and disease susceptibility. There thus appears to be a potential for spatially optimising the fungicide application in winter wheat. This may increase the overall efficacy of fungicide applications and at the same time reduce the amount of fungicide lost to the soil surface. However, there is a need to assess a number of factors, which thus have been the focus of this project:
1.4 ObjectivesThe objectives were to clarify which factors influence the need to spatially vary fungicide treatments in winter wheat, and how these factors can be monitored using available tractor mounted sensors. This requires clarification of the relationships between the influencing crop parameters and a range of processes including disease occurrence, fungicide deposition on leaves and disease control effects of fungicides. The investigations were performed with the objective of developing algorithms for spatially varying the fungicide dose when spraying at heading in winter wheat. The following specific objectives were considered:
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