The effectiveness of phytosanitary treatments depends strongly on the weather conditions during application. Air humidity, temperature, wind speed and precipitation are important data that influence the efficiency of a treatment. Other meteorological parameters such as dew point, wind direction and gust speed should also be taken into account in order to optimize treatment.

The ideal weather conditions change depending on the mode of action of the product. In addition, the indicators to be taken into account depending on the field of application and the selectivity of the active substance influence the decision to consider a meteorological variable or not. This article presents the general interpretation of the various weather parameters that can be performed to optimize the treatments.

Humidity

Humidity is a key parameter for optimizing spraying. Treatments under restrictive conditions reduce the capacity of plants to absorb nutrients and increase harmful effects on the environment. Too low humidity intensifies evaporation, as some of the product's droplets are too light and evaporate as soon as they leave the atomizer. Too high humidity (close to 100%) is also harmful, as the active ingredient of the product appears on the surface in the form of dew and seeps into the environment.

Morning and evening is the best time for spraying, as the humidity level is then usually in the favorable range. The morning hours are particularly suitable when the leaf canopy is still cool from the night and the cuticle of the plants is re-hydrated and more receptive to the product.

(Unit of the x-axis = %)

Wind Speed

Wind speed is the most limiting weather parameter for spraying. This is an important variable subject to legislation. The aim is to limit as far as possible the phenomenon of drift, which has significant environmental consequences. The effectiveness of sputtering is dependent on wind speed because the objective is that the maximum amount of active ingredient reaches its target. Improved treatment efficiency reduces the number of passes and thus the associated input and mechanization costs.

The lowest possible wind speed is most favourable. Above a certain value there is a high risk of wind drift, and then caution is required. Certain devices, such as the shape of the nozzle and optional equipment of the sprayer, make it possible to limit drift despite slightly higher wind speeds. The use of specific aids also reduces the risk of drift.

(Unit of the x-axis = %)

Gusts

The indicator for gusts of wind is an additional parameter to be considered. "Gusts of wind do not allow clear predictions of wind direction and can mean temperature swings" (Deveau, 2009). If the gust speed is high, it is therefore best to postpone spraying to another time.

Temperature

The optimum temperature for treatment depends on the type of pesticide used. Moreover, each product has its ideal area of application. In order to optimize the treatment, it is important to follow the instructions for use of the product to be applied carefully.

The receptivity of organisms also correlates with temperature. Extreme values slow down the metabolism. High thermal amplitudes can also have an inhibitory effect, especially for the post-emergence root products and leaf herbicides used (Verdier et al. , 2012). Amplitudes between day and night of more than 15°C slow down the metabolism of the plants. The active ingredient accumulates in the organism and cannot be extracted, which may increase the risk of phytoxicity (Burgundy Chamber of Agriculture, 2012).

Thus, the temperature has a decisive influence on the quality of the spraying (Chamber of Agriculture Burgundy, 2012). Hot (and dry) air causes a higher evaporation at the sprayer outlet (>25°C) In general, spraying is recommended for a temperature range between 5 and 25°C. However, these recommendations depend on the plant protection product used.

Rainfall

The spraying effectiveness is particularly reduced if it rains during the treatment. A large proportion of the products used end up in the environment and cause considerable damage. The active ingredient is transported into the ground and surface water through run-off and infiltration water. In addition, the outer layer of the plant is covered with a film of water that prevents the product from penetrating.

It is important to keep an eye on the weather forecasts for rain measurements. The plant protection products with contact effect must remain on the plant surface for a longer period of time. Rain can remove this layer of product, causing pollution and reducing the effectiveness of the treatment. Depending on the product, the amount of rain that causes washout varies, but the limit of 20 mm of rain should generally not be exceeded (Davy, Institut Français de la Vigne et du Vin, 2016).

Dew point

Dew is an another indicator to assess the effectiveness of treatment. Dew increases the permeability of the plant (Gauthier, Moigny, 2014). If the cuticle is in good condition and rehydrated, the product can penetrate more easily.

This indicator also makes it possible to evaluate the adhesion of the product on the plant. If the dew is too strong, the product will run off during spraying and the treatment will not be effective. The farmer can observe the conditions by tapping on a leaf. If the dew runs off the leaf, one should wait with spraying (Verdier et al. , 2012).

Weather conditions before and after treatment

Monitoring of weather conditions on the days preceding and following application of the plant protection product may be important depending on the type of pesticide used. In fact, herbicides in particular are 'all the more effective when used in growing weather' (Arvalis, 2014). This means several days in a row with growth-friendly weather conditions: high humidity (over 60%) and cool temperatures (between 4°C and 18°C).

In addition, spraying after severe frost is risky, as the plants may show injuries on the epidermis. There is therefore a high risk of phytotoxicity. Treatment with a systemic foliar herbicide after a drought is also not very effective because the plant cuticula is thickened. It is therefore important to observe the meteorological parameters before and after treatment in order to assess the effectiveness of the treatment.

Conclusion

It is not easy to find a time window for the treatment in which all parameters are favourable for spraying. A compromise has to be found which can be reconciled with the own timetable. The weather conditions are best in the morning when humidity is high and the plants are more receptive. Treatment in the evening is also possible, but one should take into account the weather conditions of the previous day, as the plant may be more sensitive to the product in the evening.

Sources :

ARVALIS - INSTITUT DU VÉGÉTAL, 2014. Réussir ses traitements : l’importance de la météo. [en ligne]. Disponible à l’adresse : https://www.arvalis-infos.fr/pulverisation-sur-cereales-soigner-les-conditions-d-application-ds-phytos-@/view-14762-arvarticle.html.

CHAMBRE D’AGRICULTURE BOURGOGNE, 2012. Optimiser la pulvérisation [en ligne]. Disponible à l’adresse : https://bourgognefranchecomte.chambres-agriculture.fr/fileadmin/user_upload/Bourgogne-Franche-Comte/061_Inst-Bourgogne-Franche-Comte/Actualites/2019/2012_Optimiser_la_pulverisation_Ch_Agri_Bourgogne.pdf.

DAVY, Alexandre et INSTITUT FRANÇAIS DE LA VIGNE ET DU VIN, 2016. Rappel sur le comportement des différents fongicides et impacts pratiques sur leur utilisation [en ligne]. Disponible à l’adresse : https://www.vinopole.com/uploads/tx_vinoexperimentation/Comportement_fongicides_Impacts_pratiques_IFV_Vinopole_2016.pdf

DEVEAU, Jason, 2009. Effets des conditions météorologiques. [en ligne]. Disponible à l’adresse : http://www.omafra.gov.on.ca/french/crops/facts/09-038w.htm#4

GAUTHIER, Marine et MOIGNY, Frédéric, 2014. Optimisation de la pulvérisation [en ligne]. Disponible à l’adresse : http://draaf.auvergne-rhone-alpes.agriculture.gouv.fr/IMG/pdf/2014_optimisation_des_traitements_phytosanitaires_CDA_63_cle47e132.pdf.

VERDIER, Jean-Luc, ARVALIS - INSTITUT DU VÉGÉTAL, CENTRE TECHNIQUE INTERPROFESSIONNEL DES OLÉAGINEUX MÉTROPOLITAINS (FRANCE) et UNION NATIONALE INTERPROFESSIONNELLE DES PLANTES RICHES EN PROTÉINES, 2012. Produire plus et mieux: 56 solutions concrètes pour réduire l’impact des produits phytosanitaires : guide pratique. ISBN 978-2-8179-0110-7.

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