The weather: an ally in optimising your crop protection treatments

The weather: an ally in optimising your crop protection treatments

In order to reduce the development of diseases and pests on crops and thus maximise yields, farmers often resort to the application of phytosanitary treatments. Their action must be carried out in an efficient and environmentally friendly manner, but also with respect to the operators. By 2025, farmers will have to achieve a -50% reduction in phytosanitary use, following the implementation of the Ecophyto II+ plan. Their scope for action is therefore increasingly limited. In addition, the withdrawal of certain active ingredients from the market sometimes leaves farmers without solutions to the damage suffered by their crops. At the same time, the purchase of products represents a substantial budget for farmers. Faced with ever stricter regulations and a need for economic profitability, optimising the effectiveness of treatments by reducing inputs is the best solution.

First of all, it is essential to specify that an effective treatment is a treatment carried out at the right time, under the right application conditions. It is therefore necessary to regularly observe your crops and monitor the evolution of diseases and pests, using ultra-local weather data.

Discover 5 levers to optimise your phytosanitary treatments

1. Spraying with optimal weather conditions

About weather conditions, there are favourable ranges of temperature, relative humidity and wind speed which increase the persistence of the product and favour its efficiency on the root cause.

🌡️ Temperature:

It plays a crucial role in the application of systemic products, which require optimal conditions to be effective. Once absorbed by the plant, they are carried by the sap. For this to happen, the plants must be in the development phase: temperatures between 5°C and 15°C are therefore ideal. In the case of contact products, the aim is to have a maximum amount of active ingredient reaching the plant. Thus, evaporation losses must be limited, so temperatures below 25ºC coupled with RH>70% are more suitable.

🌬️ Wind:

The regulation imposes to treat with a wind lower than 15 mph. But even below this speed, there is a risk of the drops drying out on the leaf, and the cuticle is less receptive. It is therefore recommended to spray with a maximum wind speed of 5 to 10 mph. In most cases, the wind is weakest at night and in the early morning.

📈 Air humidity:

In case of spray application, it must be higher than 80% to limit the risk of evaporation and create ideal conditions for the absorption of the product by the plant. It is more important in the early morning and decreases as soon as the sun rises.

💧 Dew:

It favours the success of the phytosanitary treatment if you use a systemic product. Indeed, it increases the permeability of the cuticle and favours the redistribution of the active ingredients in the plant. Dew is particularly beneficial when it dries out after the product is applied.

🌧️ Rains:

Rain after application dilutes and washes away the applied treatment. It is always recommended not to spray when rain is forecast. Rain also has an impact on the leaching of products, thus favouring drift.

In addition to these different elements, the weather conditions of the days preceding the treatment are very important. For example, after a period of severe frost, the plants have wounds in the epidermis, which favours the penetration of phytosanitary products, but also the risks of phytotoxicity.

Sencrop weather stations offer to collect this data in a localised way, at the crop level, for maximum precision.

2. Assessing the right growth stage for your crop protection treatments

Assuming you are using a product with the appropriate biocidal capacity, the timing of its application has a major influence on its effectiveness as it is highly dependent on the stage of development of the pathogen and the weather conditions.

Indeed, as the pathogen develops, their sensitivity to the action of plant protection products varies, as they may develop genetic resistance, physical forms of resistance such as webs or cocoons, or synthesise substances that protect them, prevent them or reduce the possibility of the product reaching their sensitive organs.

The susceptibility of crops to diseases varies according to the phenological stage and this must be taken into account in order to reduce the risk of severe attacks.

You can view the growth stages of crops (grain maize, forage maize, sunflower, flax, sorghum and grassland) on the Sencrop application, thanks to an indicator provided by Semences de France.

3. Supporting decision-making with DSTs linked to local weather data

Agricultural Decision Support Tools enable in-depth interpretation of weather data to make informed agronomic decisions. By coupling ultra-local weather data to these recognised agronomic models, the accuracy of advice is increased tenfold.

For example, some DSTs can be used to optimise the application of inputs, in particular thanks to indications concerning the best treatment windows. Thus it is possible to reason out one's phytosanitary behaviour and treat only if necessary, at the right dose and at the right time.

These DSTs are often specialised on a type of crop or disease and act as a trusted third party. They provide real support in diagnosing diseases and pests, but also in recommending phytosanitary interventions. The correlation of ultra-local weather data with these DSTs allows for a rapid reduction in the IFT (Indice de Fréquence de Traitement) and a more concrete approach to the objectives set by the French government for 2025, with a -50% reduction in phytosanitary products.

Sencrop, allows you to connect your local weather data to the most recognised DSTs of the market. This way, your precision data can be fed into these tools, for ever more accurate advice. The list of compatible DSTs is growing regularly.
Discover Sencrop's DST partners.

4. Reduce copper doses thanks to weather forecasts

Copper is traditionally used in agriculture to prevent and treat certain fungal diseases on several types of crops. Whatever the formulation of the copper product, it is the cuprous ion (Cu++), released in an aqueous medium, which has an action that disrupts the respiratory, enzymatic and membrane activities of the fungus. It is not, or only slightly, absorbed by the plant (unless it is applied as a foliar fertiliser). It is present on the surface of the vegetation that will protect it from possible contamination during a rainy episode (rain, dew).

There are 3 types of copper, which should be applied according to the upcoming rainfall and the level of pressure. We have given a table summarising the main characteristics of each formulation:

Copper hydroxides


Copper sulphates

Rapid release and action




Resistance to leaching






++ (especially at acid pH))


Other characteristics

Some adjuvants can slow down the shock action effect, (e.g. pine oils) 

More concentrated than other products (which reduces the volume to be handled)

Bordeaux mixture is the best known.

When to renew a treatment?

It is always difficult to take a stance on the remanence of the different products used. In addition to the quality of the application and the chemical characteristics of the product, other parameters must be taken into account, such as

  • The development of the crop biomass : if it has grown since the last treatment, it is important to protect the newly formed tissue. In the case of vines, for example, 15 cm of growth can be achieved in 7 days during the growing season.
  • The degree of leaching depends on the amount of rainfall but also on the dose used. If the weather is dry, renewal may not be necessary, it should be done when another rainfall is approaching or in wet weather (dew, fog...).
The Raincrop rain gauge as well as a subscription to the Sencrop app will allow you to collect the key data for a copper treatment: rainfall accumulation, humidity level... Then process this information in the best way with maps or the weather forecast.

5. The impact of weather on the fungicidal action of sulphur

Like copper, sulphur is also one of the most widely used fungicides in agriculture due to its multi-site activity. Indeed, these two products can control the fungus by acting on several cell sites. Today, modern fungicides based on these two molecules are designed to act only on one site of the fungus' metabolism and are very effective with low quantities per hectare with a longer remanence compared to the old synthetic molecules.

Which sulphur to choose?

There are different types of sulphur on the market: powdered (triturated / sublimated / mixed) and wettable (concentrated suspension and granulated). Regardless of the formulation chosen, sublimation (change from solid to gas without passing through the liquid state) is the physical phenomenon that gives sulphur its anti-fungal power. Thus, a good sublimation is esse

On the other hand, the particle size will be crucial to have a good persistence and a reduced phytotoxicity. The spraying of homogeneous and dense particles will allow to cover a larger perimeter of plants and therefore to obtain an increased particle/atmosphere contact surface, which directly translates into a much higher efficiency.

Treatment conditions

Light levels are an important factor in improving sublimation at equivalent temperatures. The higher the temperature, the greater the sublimation effect. However, above 28ºC and at low ambient humidity (< 70%), the phytotoxicity effect increases considerably. Furthermore, the presence of water on the leaves decreases the effectiveness of the treatment as it can be washed off or evaporated with this water.

On the Sencrop app, you will find the temperature and humidity results on your dashboard, with live readings. This way you will be able to detect favourable conditions for a sulphur treatment.

The effectiveness of a plant protection treatment is therefore dependent on many criteria. The weather remains the main concern of farmers when applying plant protection products. The presence of a connected weather station (rain gauge and anemometer), installed on the plot, will allow to intervene in the best possible conditions, with a precise follow-up. And don't forget the essentials: personal protective equipment that protects you from the risks of toxicity and avoids any possible health problems.