DLV-METEO, PO Box 7001, NL-6700 CA Wageningen, The Netherlands

Background and objectives
The (micro) meteorological circumstances before, during and after application of agrochemicals are decisive for their efficacy. A lot of applications are being made under non-optimal circumstances. The non-optimal efficacy of such sprays can lead to a number of (unnecessary) re-sprays. Therefore a decision support system (DSS) was developed, with the acronym GEWIS. In this DSS all the parameters to achieve better efficacy, including formulation type, the dynamic build-up of the wax-layer, the behaviour of the cutin layer and behaviour (sticking, uptake) of active ingredients, are combined with the meteorological circumstances measured in the crop.

Materials and methods
In a project of 2 years, all available knowledge on the influence of weather circumstances on the efficacy of crop protection agents are described and analysed, structured and modelled to build a DSS. The micro-meteorological circumstances were chosen as premises, because these circumstances are the source of the meteorological circumstances at official measuring height (2.00 m). Furthermore, the crop protection agents have to do their work in the crop. The nature of the formulation plays a major role with respect to absorption of the crop protection agents. The formulations can be divided into apolar (water-repellent) and polar (fat-repellent). Almost all formulations are polar, only the emulsifiable concentrates (EC) and the oil-flows (Ofl) are apolar.

Results and conclusions
GEWIS has been used in 1997 in a test version by approximately 60 farmers and extension officers. They were very satisfied, and about 50% of the farmers have changed their application time of agrochemicals due to the recommendations of GEWIS. By applying the agrochemicals at the right moment they reduced the input quantity of active ingredients [1, 2].

1. Bouma E, 1998. In PAV-Special Report no. 3: Proceedings of the workshop on the European Network for development of an integrated control strategy of potato late blight. Eds. H Schepers & E Bouma (in press). 2. Bouma E, Weide RY van der, Floot HWG, 1996. Bulletin OEPP/EPPO Bulletin 26, 651-658.