5 Conclusions

Pesticides Research No. 116 2008 of fungicide application in winter wheat

The project has confirmed that there is a spatial variation in septoria disease and that this variation is to a large extent related to crop canopy structure and leaf N concentration, for which proxy estimates are available from tractor mounted sensor measurements. In particular the use of ratio vegetation index (RVI) from measurements of canopy spectral reflectance at GS39 seems to be a good indicator of crop susceptibility to septoria disease. A high RVI thus gives a high risk of septoria attack.

Analysis of yield gain from fungicide application showed significant positive correlations between yield gain and leaf N concentration or RVI at GS39, in particular at the site with the highest septoria attack and the smallest error variation in grain yields. There were also significant positive relationships between normalised disease response to fungicide application and the measurements of leaf N concentration and RVI, which indicates that fungicides may be less effective in controlling disease in dense and nutritious crops.

The disease free yield was found to be better correlated to crop sensor measurements (LAI, leaf N concentration and RVI) at GS39 than to soil measurements taken in spring. However, the usefulness of the RVI measurements at Schackenborg was reduced due to a large weed population. The best prediction of disease free yield was obtained using a combination of RVI and EM38 measurements.

The relevant crop characteristics could all be measured using handheld sensors. Analyses of the tractor-mounted measurements using the MobilLas measurements indicate that it is possible to perform these measurements in practice for the RVI measurements, whereas it is considerably more difficult to measure leaf area index (LAI) with sufficient precision using tractor mounted sensors. This is due to the saturation of the laser sensor measurements at LAI above 2.5-3.0.

The fungicide deposition in terms of tracer concentration per leaf area varied strongly between the top three leaves with the highest concentration on the upper leaves. There was a tendency for the highest concentrations at high LAI, which may be related to leaf inclination, as there was a significant correlation to mean leaf angle. However, this effect was strongest for leaf 3 and there were only weak relationships for leaf 1 and 2. As it is the disease control on the upper two leaves that provides the main effect on yield gain from disease control, the effect of varying leaf fungicide concentrations with varying canopy density could be ignored in the models.

Two different algorithms (an empirical model and a causal model) for spatially varying fungicide application were developed. Both models make use of RVI and EM38 measurements. EM38 describes the soil characteristics, in particular the soil clay content; and this measurement should be made only once and used subsequently, whereas the RVI measurement needs to be taken at GS39 at the time of fungicide application. Both types of measurements are operationally available today.

Both algorithms were estimated to give a higher need for fungicide application for crops having a higher RVI. This was an indirect effect for the empirical model, whereas both higher grain yield and a higher septoria occurrence and high RVI contributed to this for the causal model.

The evaluation of the model showed that the estimated variation in fungicide rate within the field varied between experimental sites, but the standard deviation was generally in the order of 0.1 to 0.2 L ha^-1 Opus. There were only small yield gains from applying the spatially variable fungicide rate using data on variation in soil and crop conditions at the normal N rate in the field experiments conducted in this project when comparing with a uniform fungicide rate of 0.4 L ha^-1 Opus. There was mostly a reduction in fungicide deposition on the soil from applying the sensor-based fungicide rates compared with the uniform fungicide rate.

The evaluation showed that the largest yield gains from sensor-based fungicide application were obtained in situations with either very low or high disease infestation. There may thus be some scope for applying sensor-based fungicide rates in winter wheat. However, this will require further testing under a wider range of soil and climatic conditions.

The evaluation was performed for wheat grain price of 1000 DKK per ton DM, and the price has during 2007 increased considerably. This will result in increases in optimal fungicide dose, but not necessarily in increased variation in fungicide dose across a wheat field.