C-DRONE GUIDE · 12 SEPTEMBER 2026

Onions and drones: detecting downy mildew before it destroys the crop, method and price

Next to wheat or sugar beet, onions remain a discreet crop on France's agricultural map — but a high-value one, where a poorly spotted disease outbreak can ruin an entire campaign within weeks. Its number-one threat has a name: downy mildew, caused by the oomycete Peronospora destructor, capable of destroying an entire plot's foliage within a month with no intervention. The subject remains open enough that CTIFL, France's fruit and vegetable technical institute, runs a dedicated research programme on it. Two international studies, carried out on other foliar diseases of onion, document what a drone brings to spotting and mapping an outbreak: here is the detail, the limits worth knowing, and prices observed in France in 2026.

Published on 12 September 2026, reviewed on 12 September 2026 — regulations in force as of September 2026.

Onions in France: a demanding crop facing a mildew that can destroy everything in a month

Centre-Val de Loire, particularly the Eure-et-Loir and Beauce basin, accounts for a large share of French colour-onion production grown for fresh consumption and processing. In Brittany, the Oignon de Roscoff AOC — obtained in 2009 — protects a pink variety grown on a little over 100 hectares around Saint-Pol-de-Léon, yielding more than 3,000 tonnes harvested each year between 20 July and mid-August depending on the planting date; according to local tradition, the variety was brought from Portugal in the 17th century by a monk, who is said to have taught its cultivation to the local community.

On this crop, the number-one threat has a precise name: onion downy mildew, caused by the oomycete Peronospora destructor. Early in the growing season, affected plants stay stunted, with chlorotic, deformed leaves covered in a velvety grey mycelium; later, elongated yellowish spots become covered in secondary spores as they age. The fungus sporulates under near-saturating humidity (above 95% relative humidity) and cool temperatures (optimum around 16°C), with a complete cycle of just 10 to 16 days: with no treatment, four cycles are enough to destroy the entire canopy, and the disease can spread across an entire plot within a month. An early attack can cut yield by 70%, with a loss risk that frequently reaches 30% even in a milder year; volunteer regrowth and debris from a previous harvest remain the main source of inoculum early in the season.

The subject is far from secondary for the industry: CTIFL, France's fruit and vegetable technical institute, runs a dedicated research programme on it (ORION) and holds regular professional workshops on managing onion downy mildew — a sign that no simple, reliable scouting solution has yet settled into practice, particularly since the withdrawal of mancozeb-based fungicides, which already complicated the treatment calendar.

A study on onions: an onboard near-infrared camera against the human eye

On the ground, the reference remains the technician's eye, rating the percentage of affected foliage plot by plot — a reliable method, but slow, prone to fatigue and to variability between observers, and poorly suited to repeated monitoring across large areas. A study by McDonald, Tayviah and Gossen, published in 2022 in Remote Sensing, tested this exact limitation on onion: in variety trials run at Holland Marsh, Ontario (Canada), the authors compared visual ratings of Stemphylium leaf blight (Stemphylium vesicarium, a foliar blight whose symptoms resemble downy mildew) made by experienced raters against automated analysis of aerial photographs taken by an octocopter fitted with a near-infrared filter camera (see the study on Google Scholar).

The result: image analysis from the drone distinguished disease severity levels with a consistency comparable to that between two different human raters scoring the same plots — without the fatigue or variability inherent to repeated visual rating across dozens of plots. An honest caveat: this study documents Stemphylium leaf blight, a disease mostly reported in North America so far and not yet established as a major issue on French fields; it does not therefore directly concern Peronospora destructor downy mildew. Only the method itself — an onboard near-infrared sensor can quantify a foliar onion disease as finely as an experienced eye — is presented here as transferable, pending validation specific to this pathogen, still to be carried out.

Mapping the extent of an outbreak at plot scale

Spotting an outbreak early is one thing; measuring its exact extent across a plot of several hectares is another, essential for choosing between a localised treatment and a blanket pass. A study by Alberto, Rivera, Biagtan and co-authors, published in 2020 in Spatial Information Research, developed an automated classification method (object-based image analysis combined with a support vector machine) applied to drone-captured aerial images over onion plots in the Philippines, to delineate areas affected by anthracnose-twister disease (see the study on Google Scholar). Across 30.26 hectares surveyed in three municipalities of Nueva Ecija province, the method quantified the share of infected area municipality by municipality — from 19% to nearly 58% of a single site's area — with enough precision to steer a targeted intervention rather than a uniform treatment.

Here too, anthracnose-twister (Colletotrichum gloeosporioides) is not the downy mildew found on our plots; only the method — automatically mapping, to the hectare, the real extent of a foliar onion disease from drone imagery — remains transferable. On French fields, the constraint specific to downy mildew is timing: with a 10-to-16-day cycle and the ability to spread across an entire plot within a month, a single early-season pass is not enough — only repeated monitoring, at close intervals during humid spells, can catch an outbreak before it takes over the whole crop. These flights fall in practice under the European open category (subcategory A3, away from third parties, below 120 m), with no process beyond what any agricultural flight already requires; our guide comparing multispectral and RGB cameras in agriculture covers choosing the right sensor for the goal — early near-infrared scouting, or visual mapping of the outbreak's extent in high-resolution RGB.

Who commissions this kind of mission, and 2026 prices

This kind of monitoring is commissioned by arable farmers in the Beauce and Eure-et-Loir basin, by growers working under the Oignon de Roscoff AOC specification in Brittany, by packing cooperatives contracting volumes with retailers, and by growers' groups wanting to share close monitoring across several neighbouring plots during the highest-risk weeks. Ranges observed in France in 2026, excl. VAT:

These amounts cover the flight and delivery of a usable map or report within 24 to 48 hours; they do not include fungicide treatment or the spray-timing decision, which remain the grower's or their technical adviser's call. This onion downy mildew monitoring is distinct from potato late blight (Phytophthora infestans, covered in our dedicated guide) and from vineyard downy and powdery mildew (Plasmopara viticola, in our guide to vineyard mildew and powdery mildew): three diseases sharing a common French name, without being the same pathogen or the same monitoring protocol. For a grower, a cooperative or a growers' group, request a quote stating the area, the production basin and the intended period — during a humid spell for early scouting, before harvest for an end-of-season review. Our guide to emergence counting and sowing density completes the monitoring from the crop's earliest stage onward.

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Also worth reading

• Drone services in the Bouches-du-Rhône (13): Marseille, Fos, the Calanques and the Camargue — zones, missions and 2026 prices
• Drone Services in Bas-Rhin (67): Entzheim, EU Institutions, the Rhine and the Hop Fields
• Drone services in the Côte-d'Or (21): the UNESCO Climats seen as a land register, from Dijon to Beaune