Treatments should be implemented when weather conditions permit. Wind and rain are the most important determinants for treatments, but relative humidity and temperature should also be taken into account:
2.5.2.e Taking weather conditions into account
- Wind: the effectiveness of treatments depends on wind speed and direction. This is the most important factor during treatments. Avoid spraying when it is windy. The smaller the droplet size, the greater wind's impact will be. The risk of drift is increased when the nozzles are set to a fine droplet size. In France, the decree of 12 September 2006 prohibits spraying when the wind is higher than three on the Beaufort scale, which corresponds to 19 km/h.
Suggested product application dates for vine treatments (Source: Crédit Chambre d’Agriculture Nouvelle-Aquitaine)
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Rain: it is not recommended to treat in the rain. The risk of leaching is increased. Most contact products require a dry period for application. Rain can cause runoff of the treatment droplets.
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Temperature and relative humidity: when the temperature is higher than 25°C and the humidity is lower than 50%, the size of the spray droplets can be reduced by evaporation. Higher temperatures also increase the risk of spray drift. The optimal temperature is between 12 and 20°C. The humidity of 60-95% is optimal, above this, the risk of runoff becomes too great, while below this, there are risks of evaporation or burns. Dew generally favours the application of products provided that it does not create runoff.