Method for determining the evolution of leaf wetness

Abstract

A method for determining the evolution of leaf wetness of plants in a cultivated area, comprising the following steps: a) at each of a plurality of acquisition times: a1) emission (S01) of a light beam (8) by the light source (2) towards a position in the cultivated area within its illumination field (4), a2) reception (S02) by the light intensity sensor (10) of a light radiation (16) reflected from the position in the cultivated area, and determination of a light intensity signal (S03) from the reflected light radiation (16), b) for each light intensity signal of a plurality of light intensity signals from different acquisition times for the same position in the cultivated area, determination of a light intensity value (S05) for the position in the cultivated area from the light intensity signal,c) Determination of a leaf wetness evolution (SO6) for the position of the cultivated area during the observation period from light intensity values ​​for the position of the cultivated area. Figure for the abstract: Figure 2,

Description

Title of the invention: Method for determining the evolution of leaf wetness technical field

[0001] The invention relates to the field of plant cultivation, and more specifically concerns a method for determining the evolution of leaf wetness, that is to say, the appearance and disappearance of water on the surface of leaves of cultivated plants. Technological background

[0002] The presence or absence of water on the surface of cultivated plant leaves can lead to significant changes in a plant. In particular, prolonged water on the leaf surface can, when other conditions such as temperature are favorable, lead to the development of diseases, notably the growth of parasitic fungi. The presence of water on the leaves for a sufficient period is indeed the main factor enabling the spores of fungal diseases (downy mildew, rust, powdery mildew, cercospora leaf spot, etc.) to germinate on the leaves of host plants (grapevine, rosebush, wheat, beetroot, etc.) and infect them.

[0003] The presence of water on leaves can be caused by rain, dew, or certain irrigation practices. When certain weather conditions are favorable to the development of fungal diseases, phytosanitary treatments are sometimes undertaken preventively, even though these treatments may not have been necessary because the duration of water on the leaves was too short to allow for fungal infection. The inability to precisely measure the duration of plant wetness introduces uncertainty regarding the probability of infection, and to protect themselves against fungal contamination, growers apply preventive treatments too frequently, whereas precise knowledge of the wetness duration would allow curative treatments to be triggered only when necessary.

[0004] The costs and environmental impact of plant protection treatments have led to a need to better determine the conditions favorable to the development of parasitic fungi, foremost among them leaf wetness. Some existing methods for estimating the presence of water rely on imprecise overall data. For example, meteorological data measure relative humidity, temperature, atmospheric pressure, etc., and, based on physicochemical models, estimate the dew point, which is the temperature below which dew naturally forms due to saturation. Leaf wetness is then determined solely for the case of dew. However, this is often a rather imprecise estimate. precise, or which then requires numerous local measures.

[0005] To improve accuracy, it has been proposed to use artificial leaves that emit a signal roughly proportional to the amount of water wetting them, for example based on the impedance of their surface. However, this is an unreliable estimate, and above all, very localized, not taking into account spatial variations such as a slope of the land, or the presence of a hedge.

[0006] Furthermore, leaf wetness can depend on factors other than dew, such as whether or not precipitation occurs, or even leaf drying, which is difficult to control because it depends on very local characteristics such as wind exposure or sunlight. Thus, simply knowing that it has rained does not tell us how long a leaf will remain dry. However, it is primarily the duration for which water is present on a leaf that determines the risk of fungal disease.

[0007] Finally, the hydrophobicity of the leaf surface (or of a fruit affected by a fungal disease, such as apples with scab) impacts the drying time and therefore the wetting time. An artificial leaf-type sensor, as described above, will only measure the presence of water on its surface, which may not have the same hydrophobicity as that of plants, introducing a bias compared to the actual situation of the leaves.

[0008] There is therefore a need for a means of determining the evolution of leaf wetness in an automated way, on positions spatially distributed in a cultivated space, with sufficient accuracy to best estimate the needs for intervention such as the application of a phytosanitary treatment. Presentation of the invention

[0009] The invention therefore aims to propose a method for determining the evolution of leaf wetness of a plurality of plants in a cultivated area which allows for a precise determination of leaf wetness at a distance from a detection system, and repeatedly over an observation period.

[0010] To this end, the invention proposes a method for determining the evolution of leaf wetness in a plurality of plants in a cultivated area, using a detection system comprising: - a light source with an illumination field along an optical axis of illumination, - a light intensity sensor with an acquisition field along an optical axis of capture, the light intensity sensor being configured to receive light radiation and to determine a light intensity signal, - a data processing unit configured to receive the light intensity signal and determine the leaf wetness state, including the following steps: a) to each of a plurality of acquisition moments, said acquisition moments covering an observation period of at least 4 hours with intervals between acquisition moments less than 1 hour: al) emission of a light beam by the light source towards a position in the cultivated space within its field of illumination, a2) reception by the light intensity sensor of light radiation reflected from the position of the cultivated area, and determination of a light intensity signal from the reflected light radiation, b) for each light intensity signal from a plurality of light intensity signals acquired at different times for the same position in the cultivated area, determination of a light intensity value for the position in the cultivated area from the light intensity signal, c) determination of a leaf wetness evolution for the position of the cultivated space during the observation period from light intensity values ​​for the position of the cultivated space.

[0011] Being able to measure the presence of water and the duration for which this water is present on plant leaves makes it possible to estimate the risks of spore germination, and therefore of contamination, and thus to apply appropriate control measures against fungal diseases.

[0012] The invention is advantageously complemented by the following various features taken alone or according to their various possible combinations: - plants have hydrophobic leaves, and hydrophobic leaves are considered wet when the light intensity value is greater than a reference value, otherwise hydrophobic leaves are considered dry, or plants have hydrophilic leaves, and hydrophilic leaves are considered wet when the light intensity value is less than a reference value; - a spatial distribution of leaf wetness is determined, for a plurality of positions distributed in the cultivated space; - the light source and the light intensity sensor are part of an optical system configured to move the illumination field and the acquisition field, and step a) is repeated for a plurality of different positions of the cultivated space in its illumination field, the illumination field and the acquisition field being moved for each different position of the cultivated space; - the light source and the light intensity sensor are part of an optical system comprising at least one reflective element on an optical path of the light beam emitted by the light source and on an optical path of the light radiation reflected from the position of the cultivated space and received by the light intensity sensor; - the reflecting organ is mobile and configured to move the optical axis of illumination and the optical axis of capture between two acquisition instants; - the light intensity sensor is an image sensor, and the light intensity signal is a two-dimensional image of the acquisition field; - the light source is configured to emit a polarized light beam, and the detection system includes a separator configured to separate the returned light radiation according to the polarization, and in which a first light intensity signal is obtained for a first polarization, and a second light intensity signal is obtained for a second polarization; - the light source is configured to emit a light beam comprising several distinct wavelengths, and the detection system includes a separator configured to separate the returned light radiation according to wavelengths, and in which a first light intensity signal is obtained for a first wavelength range, and a second light intensity signal is obtained for a second wavelength range; - the detection system includes a distance sensor configured to determine a distance between said system and the position of the cultivated space.

[0013] The invention also relates to a leaf moisture detection system comprising: - a light source with a field of illumination along an optical axis of illumination, - a light intensity sensor with an acquisition field along an optical capture axis, the light intensity sensor being configured to receive light radiation and to determine a light intensity signal, - a data processing unit configured to receive the light intensity signal and determine the leaf wetness state, the system being configured to implement the process according to the invention. Presentation of the figures

[0014] Other features, objectives and advantages of the invention will become apparent from the following description, which is purely illustrative and not limiting, and which should be read in conjunction with the accompanying drawings on which:

[0015] - [Fig. 1] is a diagram showing an example of the implementation steps of a method according to a possible embodiment of the invention;

[0016] - Figure 2 shows an example of a detection system implementing the invention according to one possible embodiment, with a composite light source,

[0017] - Figure 3 shows another example of a detection system implementing the invention according to one possible embodiment, with a combination of optical emission and capture paths,

[0018] - Figure 4 shows another example of a detection system implementing the invention according to a possible embodiment, with a light intensity sensor separating the received light according to a characteristic thereof.

[0019] The illustrated embodiments can be combined with each other. Detailed description

[0020] Leaf wetness refers to the indication and / or quantification of the presence of water on a leaf. For example, leaf wetness can correspond to the "dry" or "wet" states of a leaf, the wet state being further refined by degrees quantifying the presence of water or its distribution (density or droplet size, for example).

[0021] With reference to the attached figures, the method for determining the evolution of leaf wetness in a plurality of plants in a cultivated area uses a detection system 1 comprising: - a light source 2 with an illumination field 4 along an optical illumination axis 6, configured to emit a light beam 8 - a light intensity sensor 10 with an acquisition field 12 along an optical capture axis 14, the light intensity sensor 10 being configured to receive light radiation 16 and to determine a light intensity signal, and - a data processing unit 20 configured to receive the light intensity signal and determine a leaf wetness state.

[0022] The light source 2 and the light intensity sensor 10 are part of an optical system 30 and are placed as close as possible to each other, typically a few centimeters or less. The optical system 30 is placed near the cultivated plants so that the plants are within the illumination field 4 of the light source 2 and within the acquisition field 12 of the light intensity sensor 10. For example, the optical system 30 can be placed in a field of cultivation or at its edge.

[0023] Preferably, this optical system 30 is raised above the leaves 22 of the cultivated plants and is typically mounted on a support such as a mast, pole, or tripod. Raising the optical system 30 increases the range of the illumination field 4 and the acquisition field 12, thus encompassing a larger area of ​​cultivated plants. Raising the system also allows more of the upper surface of the leaves 22 to be presented to the optical system 30, and therefore within the illumination field 4 and the acquisition field 12, since only the upper surface of the leaves 22 is likely to retain water. The elevation is preferably at least 1 m above the ground surface beneath the optical system 30, and preferably at least 1.5 m, and preferably at least 2 m, at the level of the objective through which the light beam 8 is emitted and / or through which the light radiation 16 reflected by the plants enters.

[0024] The light source 2 may be a lamp, for example similar to a photographic flash, configured to emit intense light for a short period of time. The light source 2 may also be a laser.

[0025] The light source 2 can be configured to emit polarized light, for example with linear, circular, or elliptical polarization. The light source 2 can be configured to emit white light, but preferably the light source 2 is configured to emit monochromatic light. A wavelength of the light beam 8 emitted by the light source 2 is chosen to interact differently with the leaves 22 depending on the presence of water on them. Preferably, in order not to disturb wildlife, the light beam 8 is emitted in the infrared, with wavelengths greater than 780 nm. Several light sources 2 can be provided, forming a composite light source 2 in which each light source 2 emits light having its own characteristics, for example in wavelength.When several light sources 2 are used, they preferably emit at distinct and different wavelengths. In the example of [Fig.2], a first light source 2a may emit at a first wavelength, for example in the green, while the second light source 2b may emit at a second wavelength, for example in the red or infrared.

[0026] The light intensity sensor 10 can be a point photodetector, and is, for example, a photodiode. The light intensity signal is then directly related to the light intensity of the light radiation 16 received by the light intensity sensor 10. The light intensity sensor 10 can be an image sensor, and the light intensity signal is a two-dimensional image of the acquisition field 12.

[0027] The light source 2 emits light into an illumination field 4 along an optical illumination axis 6, while the light intensity sensor 10 captures the light radiation 16 into an acquisition field 12 along an optical capture axis 14. The optical illumination axis 6 and the optical capture axis 14 are close, forming an angle of less than 20°, and preferably less than 10°, and even more preferably less than 5°. To achieve this, the light source 2 and the light intensity sensor 10 are close to each other, and preferably housed in the same unit corresponding to the optical system 30.

[0028] Ideally, the optical illumination axis 6 and the optical capture axis 14 are coaxial and coincide at the output of the optical system 30. In particular, the light beam 8 and the reflected light radiation 16 captured by the light intensity sensor 10 may share the same portion of optical path, and in particular pass through the same optical component of the optical system 30, such as the same optical lens.

[0029] To this end, the optical system 30 may include at least one reflective element 32, such as a mirror, a semi-reflective mirror, or any other optical component enabling the combination and / or separation of optical paths, onto which the light beam 8 and the light radiation 16 returned to the light intensity sensor 10 are incident, as in the example of [Fig. 3]. Such a reflective element 32 is disposed on an optical path of the light beam 8 emitted by the light source 2 and on an optical path of the light radiation 16 returned from the position of the cultivated space and received by the light intensity sensor 10.

[0030] The optical system 30 is configured to move the illumination optical axis 6 and the capture optical axis 14, and therefore the illumination field 4 and the capture field. The movement can be achieved by moving the optical system 30, for example, by rotating the optical system 30. Preferably, a reflective element 34, such as a mirror, arranged on an optical path of the light beam 8 emitted by the light source 2 and / or on an optical path of the reflected light 16, is movable, and by changing its inclination moves the illumination optical axis 6 and the capture optical axis 14. It is then possible to scan the positions of the cultivated area.

[0031] The method comprises an acquisition step implemented at a plurality of acquisition times, preferably for several positions within the cultivated area. The observation period covers an observation duration compatible with the duration of the measured leaf wetness phenomenon, and with intervals between acquisition times allowing for the precise capture of the evolution of said phenomenon. The acquisition times thus cover an observation period of at least 4 hours, with intervals between acquisition times of less than 1 hour. Preferably, the acquisition times are separated by intervals of less than 40 minutes, and even more preferably by intervals of less than 20 minutes. Preferably, the intervals between acquisition times for the same target position within the cultivated area are greater than one minute.For example, if a two-dimensional image of the entire acquisition field 12 is acquired at each acquisition time, this acquisition is repeated at each interval mentioned. If the acquisition involves scanning the acquisition field position by position within the cultivated area, then all the target positions are traversed during this interval. The interval between acquisition times is a compromise between the advantage of detecting leaf wetness with good temporal resolution and the inefficiency of multiplying acquisition times too frequently, at the cost of excessive hardware and processing requirements.

[0032] The acquisition times may be regularly distributed over the observation period, or may have a temporal density that varies with the probability of the leaves 22 being wet. For example, there may be more acquisition times when conditions are favorable for dew formation, namely, shortly before and after sunrise and sunset and during the night for dew, and during and after rain to measure the time of wetting due to rain. The observation period is typically at least 4 hours, but is preferably greater than 6 hours, and preferably also greater than 10 hours. There are at least 4 acquisition times during the observation period, preferably at least 10 acquisition times, and preferably at least 20 acquisition times.

[0033] This acquisition step comprises two successive parts which may, however, overlap temporally. In a first part (step SOI), the light source 2 emits a light beam 8 towards a position in the cultivated space within its illumination field 4. Several light sources 2 can be used, preferably with coaxial optical illumination axes 6, for example with semi-reflective blades 3. The [Fig.[2] shows an example of the use of two light sources 2a, 2b forming a composite light source 2: a first light source 2a emits a first light beam 8 with first characteristics (wavelength, polarization), a second light source 2b emits a second light beam 8 with second characteristics (wavelength, polarization), and the first light beam 8 and the second light beam 8 are combined by a semi-reflecting mirror 3, thus giving a composite light beam 8 directed towards the position of the cultivated space.

[0034] After possible reflection from a reflective element 32 of the optical system 30, the light beam 8 exits the optical system 30 and illuminates a position in the cultivated space where plant leaves 22 are located. The leaves 22 may or may not have water on their surface. When the plants have hydrophobic leaves 22, the water forms more or less spherical droplets 24 on them. The droplets 24 behave as a reflective device (reflector or retroreflector) that reflects the incident light back in the same direction of incidence, i.e., towards the light source 2, regardless of the angle of incidence of the light rays with these droplets 24. This is the effect known as "heiligenschein". When the plants have hydrophilic leaves 22, this heiligenschein effect generally does not occur.On the contrary, water on the surface of leaves 22 tends to form a film 26 that diffuses light, and therefore causes a reduction in the light intensity reflected back to the source, as in the example of [Fig.3]. However, it is possible that at a low degree of leaf wetness, droplets may appear even on the surface of hydrophilic leaves 22.

[0035] Thus, part of the light beam 8 is reflected back to the light source 2 by the optical path taken by said light beam 8, and is therefore reflected back to the light intensity sensor 10. The intensity of the light radiation 16 reflected back to the light intensity sensor 10 therefore depends on the leaf wetness.

[0036] The light intensity sensor 10 receives (step S02) the light radiation 16 reflected from the position of the cultivated space, and determines a light intensity signal from it. The light intensity signal is a function of the amount of light in the light beam 8 reflected from the position of the cultivated space, and therefore depends on the presence or absence of water on the leaves 22. In the example of [Fig. 2], the light intensity sensor 10 directly receives the reflected light radiation 16. In the example of [Fig. 3], the reflected light radiation 16 travels back along the same optical path as the lighting light beam 8, and in particular is reflected by a reflective element 34, then is deflected from the optical path of the lighting light beam 8 by a semi-reflective mirror-type separator 32 to reach the light intensity sensor 10. In the example of [Fig.[4] A separator 36 separates the reflected light 16 according to the polarization or wavelength constituting the reflected light 16: a first polarization or color is directed to a first light intensity sensor 10a, and a second polarization or color is directed to a second light intensity sensor 10b. It is thus possible to acquire a distinct light intensity signal for each polarization considered and / or for each wavelength considered. For example, a dichroic filter or a dichroic mirror can be used to separate wavelengths of the reflected light 16. Since the depolarization angle depends on the wavelength, it is advantageous to separate according to polarization, then according to color, or vice versa.

[0037] It is possible to perform this acquisition step several times in quick succession, for example at intervals of less than 10 seconds, for the same target position, in order to obtain several intermediate light intensity signals which are then combined (for example by averaging) to give a light intensity signal for the target position at a given acquisition time. This approach makes it possible to compensate for certain noises, particularly when the intensity of the returned light radiation 16 is low.

[0038] In order to compensate for ambient noise, particularly when acquisition is carried out during the day, some (preferably all) of the acquisition steps may include a preliminary step of acquiring an ambient light intensity signal in the absence of the emission of a light beam 8. This ambient light intensity signal may be subtracted from the light intensity signal subsequently acquired, in order to subtracting radiation not originating from the emission of the light beam 8.

[0039] Once this acquisition step is completed, and thus once a light intensity signal has been obtained for the targeted position in the cultivated area, the optical system 30 can move (step S04) the illumination optical axis 6 and the capture optical axis 14 to another position in the cultivated area, and a new acquisition step is implemented for this other position. It is thus possible to scan a whole set of positions spatially distributed within the cultivated area. The distance at which it is possible to effectively target a position in the cultivated area depends primarily on the attenuation of the illumination light beam 8 and the sensitivity of the light intensity sensor 10.Typically, the target positions extend to a distance of at least 5 m from the optical system 30, and preferably at least 10 m from the optical system 30, and even more preferably at least 20 m. The sensitivity of the light intensity sensor 10 will allow the measurement distance to be increased further. The angular extent of the target positions around the optical system 30 depends on the ability of the illumination and acquisition fields to move, and can even reach 360° if the optical system 30 is rotated, although an angular range of approximately 40° to 180° is more common for scanning with a moving mirror, for example at the edge of a field.

[0040] The acquisition step is repeated for each target position in the cultivated area for several acquisition times covering the observation period. This yields, for each target position in the cultivated area, a sequence of light intensity signals from different acquisition times. For each light intensity signal in the plurality of light intensity signals from different acquisition times, a light intensity value is determined (step S05) from the light intensity signal. This light intensity value is representative of the light intensity of the light radiation 16 reflected from the position in the cultivated area. If the light intensity sensor 10 is a point sensor, such as a photodiode, the light intensity signal is directly a light intensity value.

[0041] When the light intensity sensor 10 is two-dimensional, as in the case of an imager or a plurality of photodiodes, then the light intensity signal can include several representative values ​​of the light intensity, such as several gray level values. In this case, a light intensity value is derived for each position in the cultivated space considered. This can be, for example, a mean or a median, possibly local, or simply an association between a value of the light intensity signal and a position. Thus, in the case of an image, the gray level of one or more pixels can be associated with a corresponding position in the cultivated space. In the case where the optical system 30 moves the lighting field 4 or the acquisition field 12, an indexing of this movement makes it possible to associate each value of light intensity with a position in the cultivated space.

[0042] It may be envisaged that the optical illumination axis 6 and the optical capture axis 14 will not be moved, and that the same position in the cultivated area will always be targeted. This is particularly feasible if the light intensity sensor 10 is an image sensor, and the light intensity signal is a two-dimensional image of the acquisition field 12, which associates to each of a plurality of positions in the cultivated area, a respective light intensity value, typically a gray level.

[0043] To improve the localization of the target position, the detection system can be equipped with a distance sensor 9, for example, a Lidar sensor, and a distance measurement associated with the target position can be acquired at each acquisition time, as in the example in [Fig. 2] or 4. This distance measurement then makes it possible to locate the target position more precisely relative to the detection system, and thus to assign a light intensity value to a position more accurately. Target positions can then be more easily organized spatially, for example, on a map. Knowing this distance also makes it possible to correct distortions due to perspective. The role of the distance sensor 9 can also be played by the light source 2 when it includes several lasers, by using the "time of flight" of the light radiation 16 reflected after the emission of the light beam 8 by the light source 2.

[0044] Separating the reflected light 16 according to wavelength or polarization makes it possible to obtain several light intensity signals for the same target position. It is then possible to better highlight the interaction between the light beam 8 and any water present on the leaves 22.

[0045] It is known that the polarization of a polarized ray incident on a water droplet is modified according to the shape and size of the droplet. This information thus makes it possible to infer, from the size of water droplets 24 on the surface of the leaves 22, or from the thickness of the film, the shape (dew or rain), and the density of these droplets 24 on the surface.

[0046] On the other hand, it is also known that leaves 22 absorb wavelengths that depend on their structure and composition. For example, they absorb red and blue for photosynthesis, but reflect green, hence their color, and also reflect near-infrared. The presence of water on the surface of leaf 22 can modify the relative fraction of each wavelength reflected by the leaf, but can also provide information as to whether it is a leaf 22 or another surface, such as soil or a stem. This thus provides information contextual on the type of surface hit by the incident beam, and therefore conclude that a reduction in the intensity of the reflected signal comes from the fact that it is earth rather than a leaf, and therefore eliminate the measurement so that it does not bias the measurements on leaves 22. The interpretation of these different factors (depolarization, variation in light intensity, nature of the support) can partly involve the use of machine / statistical learning techniques, based on measurements made on different known situations and which serve for possible supervised learning.

[0047] Leaf wetness is then determined at the position of the cultivated area based on the light intensity value associated with that position. To this end, the detection system 1 includes a data processing unit 20 configured to receive the light intensity signal and determine a leaf wetness state at the target position from the light intensity signal. The data processing unit 20 includes at least one processor and memory, and communication interfaces for receiving data and communicating or interacting with a user. The data processing unit 20 can also be used to drive the optical system 30, causing the emission of the light beam 8 and the acquisition of the light intensity signal, or controlling the movement of the illumination field 4 and the acquisition field 12.

[0048] The determination of leaf wetness may, in particular, involve a reference value, which typically corresponds to the light intensity that would be reflected by dry leaves 22 at the target location in the cultivated area when illuminated by the light beam 8. This reference value may be provided beforehand, or preferably obtained or updated in situ, for example by performing a calibration measurement when it is known that the leaves 22 are dry. These calibration measurements may be repeated at regular intervals (one to three times per week when the leaves 22 are dry) to avoid drift and to take into account plant growth, which in fact modifies the analyzed leaf area and the bare soil area.The device, with a plurality of wavelengths, can indeed measure the evolution of leaf area over time, for example by calculating the normalized difference vegetation index (NDVI) based on the choice of wavelengths.

[0049] In particular, the light intensity value can vary depending on the nature of the surface but also on the distance between the target position and the optical system 30, because the power of the light beam 8 reaching the target position, and of the reflected light radiation 16, decreases with distance due to divergence or dispersion. The use of a distance sensor 9 such as a LIDAR allows, thanks to the precise measurement of the distance between the target position and the optical system 30, to compensate for these distance effects, for example by modulating the exposure time of the light intensity sensor 10 for a position, or by applying a correction factor to the measured values ​​or to the reference values ​​to which they are compared.

[0050] When plants have hydrophobic leaves 22, the hydrophobic leaves 22 are considered wet when the light intensity value is greater than the reference value, due to the Heiligenschein effect mentioned previously. Preferably, the light intensity value is considered greater if it exceeds 115%, and preferably 130%, of the reference value. Otherwise, the hydrophobic leaves 22 are considered dry. Indeed, in the absence of water droplets 24 on the leaves 22, the light from the light beam 8 is only slightly reflected because it is absorbed by the leaves 22. It should be noted that the light intensity increases with the density of droplets 24 on the leaves 22, and the light intensity value therefore allows not only the presence of water on the leaves 22 to be determined, but also the quantification of this presence of water.

[0051] Conversely, when plants have hydrophilic leaves, hydrophilic leaves are considered wet at this position when the light intensity value is lower than the reference value. Preferably, the light intensity value is considered lower if it is less than 90%, and preferably 80% of the reference value, and even more preferably less than 70% of the reference value. As mentioned above, the Heiligenschein effect can, however, appear for low degrees of leaf wetness even for hydrophilic leaves. Thus, hydrophilic leaves can be considered wet at this position when the light intensity value is higher than the reference value. Preferably, the light intensity value is considered higher if it exceeds 115%, and preferably 130% of the reference value.If the light intensity value is close to the reference value, and cannot be considered sufficiently lower or higher than the reference value corresponding to a dry leaf, hydrophilic leaves are considered dry. It is also possible to consider a change in the light intensity value. For example, for a hydrophilic leaf, the appearance of dew may initially result in an increase in the light intensity value (appearance of droplets), followed by a decrease (appearance of the water film).

[0052] The light intensity values ​​recorded when the leaves 22 are dry can be used to update the reference value. For example, an average value can be derived from the light intensity values ​​closest to the The previous reference value is used to establish a new reference value. Updating the reference value allows for adjustments to account for changes in the cultivated area, particularly plant growth.

[0053] Leaf wetness can be determined by one or more comparisons resulting in a classification of the leaf wetness state for the target position at the time of acquisition. Several reference values ​​can be used, each corresponding to a degree of wetness, with the classification for obtaining leaf wetness then being based on the closest reference value. An important reference value, however, remains the light intensity value corresponding to the absence of water on the leaves 22.

[0054] It is possible to use a neural network, for example convolutional, previously trained to classify the degree of leaf wetness according to the value of light intensity, a reference value being a value used during the training of the neural network.

[0055] Since light intensity values ​​are available for different acquisition times at the same target location, it is possible to deduce the evolution of leaf wetness (step S06) during the observation period. It is then possible to determine when the leaves 22 become wet and when they become dry, and therefore how long the leaves 22 remain wet, which is the main factor allowing fungal disease spores to spread. It is then possible to determine the fungal risk for each target location, and consequently, it is possible to plan and carry out any necessary intervention, such as the application of a plant protection product, if required.

[0056] By having several target positions distributed in the cultivated space, a spatial distribution of leaf wetness can be determined, thus making it possible to establish a map of this evolution of leaf wetness, and therefore of the fungal risk, which makes it possible to intervene only in the areas which require such intervention.

[0057] The invention is not limited to the embodiments described and shown in the accompanying figures. Modifications remain possible, particularly with regard to the constitution of the various technical features or by substitution of technical equivalents, without departing from the scope of protection of the invention.

Claims

1. Demands Method for determining the evolution of leaf wetness of a plurality of plants in a cultivated area, using a detection system (1) comprising: - a light source (2) with an illumination field (4) along an optical illumination axis (6), - a light intensity sensor (10) with an acquisition field (12) along an optical capture axis (14), the light intensity sensor (10) being configured to receive light radiation (16) and to determine a light intensity signal, - a data processing unit (20) configured to receive the light intensity signal and determine a leaf wetness state, comprising the following steps: a) to each of a plurality of acquisition moments, said acquisition moments covering an observation period of at least 4 hours with intervals between acquisition moments less than 1 hour: a1) emission (SOI) of a light beam (8) by the light source (2) towards a position of the cultivated space in its illumination field (4), a2) reception (S02) by the light intensity sensor (10) of a light radiation (16) returned from the position of the cultivated space, and determination of a light intensity signal (S03) from the returned light radiation (16), the light intensity signal being a function of the quantity of light in the light beam (8) returned from the position of the cultivated space, b) for each light intensity signal of a plurality of light intensity signals from different acquisition times for the same position of the cultivated space, determination of a light intensity value (S05) for the position of the cultivated space from the light intensity signal, the light intensity value being representative of the light intensity of the light radiation (16) returned from the position of the cultivated space; (c) Determination of a leaf wetness evolution (SO6) for the position of the cultivated area during the observation period from light intensity values ​​at acquisition times for the position of the cultivated area, by comparing said light intensity values ​​to at least one reference value corresponding to the light intensity that would be reflected by dry leaves (22) at the position targeting of the cultivated area when said leaves (22) are illuminated by the light beam (8), resulting in a classification of the leaf wetness state for the targeted position at the time of acquisition.

2. A method according to claim 1, wherein the plants have hydrophobic leaves (22), and hydrophobic leaves (22) are considered wet when the light intensity value is greater than a reference value, otherwise the hydrophobic leaves (22) are considered dry, or the plants have hydrophilic leaves (22), and hydrophilic leaves (22) are considered wet when the light intensity value is less than a reference value.

3. A method according to any one of the preceding claims, wherein a spatial distribution of leaf wetness is determined, for a plurality of positions distributed in the cultivated space.

4. A method according to any one of the preceding claims, wherein the light source (2) and the light intensity sensor (10) are part of an optical system (30) configured to move the illumination field (4) and the acquisition field (12), and step a) is repeated for a plurality of different positions of the cultivated space in its illumination field (4), the illumination field (4) and the acquisition field (12) being moved for each different position of the cultivated space.

5. A method according to any one of the preceding claims, wherein the light source (2) and the light intensity sensor (10) are part of an optical system (30) comprising at least one reflecting element on an optical path of the light beam (8) emitted by the light source (2) and on an optical path of the light radiation (16) returned from the position of the cultivated space and received by the light intensity sensor (10).

6. A method according to the preceding claim, wherein the reflecting element is movable and configured to move the optical illumination axis (6) and the optical capture axis (14) between two acquisition instants.

7. A method according to any one of the preceding claims, wherein the light intensity sensor (10) is an image sensor, and the light intensity signal is a two-dimensional image of the acquisition field (12).

8. A method according to any one of the preceding claims, wherein the light source (2) is configured to emit a polarized light beam (8), and the detection system includes a separator configured to separate the reflected light radiation (16). depending on the polarization, and in which a first light intensity signal is obtained for a first polarization, and a second light intensity signal is obtained for a second polarization.

9. A method according to any one of the preceding claims, wherein the light source (2) is configured to emit a light beam (8) comprising several distinct wavelengths, and the detection system comprises a separator configured to separate the returned light radiation (16) according to the wavelengths, and wherein a first light intensity signal is obtained for a first wavelength range, and a second light intensity signal is obtained for a second wavelength range.

10. A method according to any one of the preceding claims, wherein the detection system includes a distance sensor (9) configured to determine a distance between said system and the position of the cultivated space.

11. Leaf moisture detection system comprising: - a light source (2) with an illumination field (4) along an optical illumination axis (6), - a light intensity sensor (10) with an acquisition field (12) along an optical capture axis (14), the light intensity sensor (10) being configured to receive light radiation (16) and to determine a light intensity signal, - a data processing unit configured to receive the light intensity signal and determine a leaf wetness state, the system being configured to implement the method according to any one of the preceding claims.

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