Botrytis on strawberry: prevention and integrated disease management

Botrytis on strawberry is not a disease you treat when it appears: it is a permanent pressure you manage actively throughout the entire season. For professional strawberry growers, Botrytis cinerea is one of the most demanding disease constraints to handle, precisely because its development conditions — mild temperatures combined with high humidity — coincide with the most critical phenological stages: flowering, fruit set, and harvest.

Within a few hours of favourable conditions, a initially moderate pressure can escalate into an uncontrollable epidemic. Losses are not limited to unmarketable fruit: they affect the quality of adjacent batches, cold chain integrity, and sometimes the profitability of the entire season. In organic production especially, a single high-pressure year is enough to turn an acceptable margin into a net financial loss.

This article documents the biological mechanisms of the pathogen, the preventive levers available — cultural and biocontrol — and the principles for reasoning through conventional fungicide use. It covers both professional production contexts: open field and under tunnel.

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Biology and epidemic cycle of Botrytis cinerea on strawberry

Botrytis cinerea is the asexual form of the fungus; the teleomorphic form — Botryotinia fuckeliana — is virtually never observed under production conditions. It is a necrotrophic and polyphagous fungus, capable of colonising living, weakened, or dead tissue. This adaptability is precisely what makes it so difficult to control: it does not wait for a stressed plant to take hold.

Cycle in the strawberry field. The fungus overwinters as sclerotia in the soil and crop debris, and as latent mycelium on organic matter. In spring, once thermal and humidity conditions become favourable again, conidia production resumes. These conidia are dispersed primarily through the air (wind, air currents inside tunnels), but also through handling, harvesting tools, and workers' clothing. Once deposited on a susceptible organ, germination begins as soon as relative humidity exceeds 75% at temperatures between 15 and 25°C. Disease progression is fastest between 15 and 20°C.

Infection windows by phenological stage. Flowering is the stage of maximum vulnerability. Open flowers are direct entry points for the fungus: infection occurs at anthesis, often with no visible symptom immediately. This lag between infection and symptom expression is what catches growers off guard: fruits appear healthy at fruit set, and rot only appears during fruit development or at ripening, when the sugar content of the receptacle becomes an optimal substrate for fungal growth. At full harvest, spread between neighbouring fruits can occur within 24 to 48 hours under humid conditions.

Inoculum sources to monitor. Un-removed plant debris (leaves, fallen petals, overripe fruits left on the plant) are the main spore reservoirs in production. Mechanical wounds are a frequently underestimated entry point: wasp stings, ant bites, hail damage — any trauma to the fruit is a potential infection vector.

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Symptoms by organ and differential diagnosis

Young fruit. Early signs are browning at the base of the fruit, progressive softening, then desiccation if humidity conditions remain insufficient for sporulation. The fruit may mummify without ever showing the characteristic grey felt if conditions stay dry.

Ripe fruit. Under high humidity (RH > 75%, temperatures between 15 and 20°C), affected areas turn dull, the receptacle softens rapidly, then a dense white-grey sporulation develops within 24 to 48 hours. The batch is lost, and fruits in direct contact with sporulating fruit are in turn contaminated.

Flowers and sepals. Open flowers show browning of petals and sepals, sometimes accompanied by a powdery grey film. This symptom on floral organs is an early indicator of high disease pressure — it often precedes fruit damage and should trigger a reassessment of the current protection programme.

Leaves and petioles. Circular spots with a chlorotic halo may appear on leaves, more rarely on petioles. These symptoms have limited direct economic impact, but their presence signals a high ambient disease pressure and a favourable microclimate within the canopy.

Differential diagnosis. Several fungal diseases of strawberry can be confused with Botrytis, particularly in the early stages of an epidemic:

Anthracnose (Colletotrichum acutatum) also causes brown lesions on fruit, but spots are typically sunken (depressed) with sharp margins, without grey sporulation. Anthracnose is more commonly expressed in hot, humid conditions, and also affects petioles and stolons with fusiform lesions.

Crown rot caused by Phytophthora (Phytophthora cactorum) can produce a sudden collapse that, viewed externally, resembles a disease breakdown. However, symptoms are root and crown-level, not on fruit. Examination of the plant crown reveals vascular browning, which is absent in Botrytis.

Hairy rot (Rhizopus stolonifer) produces a blackish or dark brown sporulation, clearly distinguishable from the grey felt of Botrytis, especially under conditions of high post-harvest heat.

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Economic impact: what a Botrytis episode really costs

Botrytis is recognised as the disease with the greatest economic impact in professional strawberry production. Direct fruit losses can represent 20 to 40% of marketable yield in a high-pressure season. In organic production, a single epidemic year is enough to turn an acceptable margin into a net loss — the limited conventional fungicide toolkit means the situation can rapidly become unmanageable if prevention was insufficient.

But purely quantitative loss estimates understate the real damage. A Botrytis episode affects the quality of batches that are not yet rotten: fruit developing near an active focus show reduced Brix and lower firmness, which degrades their logistical handling and commercial value. Batches that pass quality sorting at harvest may be downgraded within 48 hours in cold storage.

Direct losses and hidden costs. The financial impact of an episode unfolds at several levels. The first is obvious: unmarketable rotten fruit representing a direct loss on gross output. But this is compounded by costs that often go uncounted: the labour time spent sorting and removing affected fruit during picking, which slows harvest pace and increases the harvesting cost per hectare; the downgrading of adjacent batches showing subclinical defects (browning peduncles, early spots); and rejected deliveries or deductions applied by retail buyers on batches with Botrytis levels exceeding contractual thresholds.

Open field vs tunnel differential. In open field, pressure is directly driven by rainfall events and dew conditions — interannual variability is high. A wet season from May to June can cause 30 to 40% losses on poorly managed plots, while a dry season may limit damage to a few percent. Under tunnel, pressure is more predictable but potentially more sustained: nocturnal relative humidity is structurally high, and inadequate ventilation or crop management can maintain a chronic low-level pressure that silently degrades batch quality throughout the season, without any dramatic visible episode.

Cold chain and shelf life. Botrytis is the primary factor limiting strawberry shelf life: batches carrying latent contamination deteriorate within 3 to 7 days in cold storage. In a context of exacting specifications — multiple retail, foodservice, premium organic — this window can be prohibitive, particularly for growers shipping to distant markets or supplying shelf replenishment on a J+2/J+3 basis. Harvesting dry fruit early in the morning partially addresses the problem; it does not resolve latent contamination already established in the crop.

A concrete figure from field data: a well-conducted leaf removal operation can reduce by up to 40% the rate of downgraded fruit due to grey mould at the end of the season. This single lever, often undervalued because it is labour-intensive, delivers a highly significant labour return on investment in high-volume operations.

Hail damage deserves a specific mention: the mechanical wounds on fruit caused by hail are immediate entry points for Botrytis cinerea, triggering massive losses within a few days. Growers exposed in open field factor this risk into their physical protection strategy — anti-hail nets — but also into their post-event treatment planning.

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Preventive levers: from cultural hygiene to microclimate management

Botrytis prevention follows a simple logic: reduce the duration and intensity of windows favourable to spore germination, and limit the available inoculum sources in the plot. Here are the operational levers, ranked by impact.

Humidity management within the canopy. This is the primary lever, particularly under cover. Under tunnel, confinement creates nocturnal relative humidity peaks that regularly reach 90 to 95% even without rainfall. Active ventilation — opening both sides of the tunnel in the late afternoon, before the overnight temperature drop — is essential for removing humidity before germination conditions are reached. Plant density is directly linked to this: overly dense rows create a closed canopy where air stagnates and humidity persists.

Sanitary leaf removal. Regular leaf removal — elimination of aged, yellowing, or lesioned leaves — improves air circulation through the canopy and reduces the biomass available as fungal substrate. All removed material must be taken out of the plot, not left between rows or on the ground. Under tunnel, this includes removing overripe fruit and fallen petals, which are inoculum reservoirs in a confined space. Key figure: a well-conducted leaf removal programme reduces by up to 40% the rate of fruit downgraded for grey mould.

Irrigation. Drip irrigation is the reference system in professional production, precisely because it keeps foliage and fruit dry. Micro-sprinkler irrigation under cover is strongly discouraged during flowering and throughout the high-risk period: it wets flowers and foliage and creates immediate germination conditions. In open field, overhead irrigation can be tolerated outside sensitive periods, but watering must systematically be done in the morning to allow foliage to dry during the day. For more detail on precision irrigation practices, see our article Strawberry irrigation: water requirements and management.

Nitrogen fertilisation. Excess nitrogen produces lush, water-filled tissue and increased susceptibility to fungal pathogens. In strawberry production, nitrogen inputs must be carefully calibrated by growth stage: over-application before flowering directly increases Botrytis sensitivity. This is covered in detail in our article Strawberry fertilisation: N-P-K ratios by stage.

Mulching. The black plastic film on raised beds serves a dual function: it limits splash dispersal of soil-borne spores onto fruit, and keeps fruit away from direct contact with moisture. Perforated agrofilm, used in soilless tunnel production, allows more regulated evaporation. In both cases, mulching is only effective when irrigation is well managed: excess moisture under the plastic creates a favourable microclimate for pathogens.

Wound management. Wasp stings and ant bites are direct Botrytis contamination vectors: every breach in the fruit skin is a potential entry point. Installing wasp traps close to the rows is a simple, high-impact preventive measure during the harvest period. For integrated management of the pests causing these wounds, see our article Strawberry pests: suzukii and mites.

Harvest hygiene. Picking dry fruit, preferably in the morning before temperatures rise, limits mechanical spore dispersal. Training staff to avoid contact between healthy and suspect fruit, and regularly disinfecting harvesting tools between rows, are practices to integrate into farm protocols. For leaf removal and crop management operations linked to disease control, see Strawberry crop management: runners and leaf removal.

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Biocontrol: products, application logic, and limits

The primary rule for Botrytis biocontrol is also the most commonly misapplied: these products must be applied before pressure builds, not in response to it. A grower who waits for the first visible outbreaks before applying a biocontrol product has already missed the optimal efficacy window.

Bacillus subtilis and Bacillus amyloliquefaciens. These antagonistic bacteria — found in products such as Serenade ASO (B. subtilis) and Amylo-X (B. amyloliquefaciens) — work through complementary mechanisms: production of antifungal lipopeptides, competition for the niche on plant organs, and stimulation of plant defences. Efficacy is highest at temperatures between 15 and 30°C. Application from pre-flowering, renewed every 7 to 10 days based on weather pressure, is the reference strategy in integrated and organic production. Serenade is particularly well documented for spray application ahead of flowering.

Potassium bicarbonate. This biocontrol product works by modifying the surface pH of plant organs, creating an environment unfavourable to spore germination. It is used preventively, often combined with antagonistic bacteria. Its action is rapid but short-lived — frequent reapplication is needed, particularly after rainfall or overhead irrigation.

Copper in organic production. Copper-based preparations (Bordeaux mixture) are authorised in organic production against Botrytis on strawberry, but should be used sparingly given the regulatory annual copper metal limit (3 kg Cu/ha/year averaged over 7 years). Optimal positioning is preventive, ahead of flowering stages. Copper applied curatively, once symptoms are already visible, is of limited efficacy.

Note on Trichoderma. Trichoderma harzianum is registered on strawberry for control of soil-borne diseases — Phytophthora in particular — not against aerial Botrytis. For Phytophthora and Verticillium management, see Strawberry Verticillium and Phytophthora.

Programme logic. In professional integrated production, biocontrol forms the backbone of a systematic preventive programme: starting at pre-flowering, renewed every 7 to 10 days, with tighter application intervals during high weather-risk windows. This preventive programme does not preclude the use of conventional fungicides during high-pressure episodes, but significantly reduces the number of applications required. For a broader view of integrated strawberry protection, see Strawberry protection: prophylaxis and biocontrol.

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Conventional fungicides: active substances, resistance, and FRAC rotation

In conventional production, fungicides remain a necessary tool, particularly during high-risk episodes at full flowering. Their efficacy depends directly, however, on the rigour of their application timing and mode-of-action rotation — Botrytis cinerea is classified among the pathogens at very high risk of resistance development.

Key active substances. The two reference molecules documented on strawberry are fenhexamide (FRAC 17, hydroxyanilide family, marketed as Teldor) and fludioxonil (FRAC 12, phenylpyrroles). These are often combined with or alternated against SDHI fungicides (FRAC 7) or benzimidazoles (FRAC 1). For any application in France, the current list of registered products must be checked on the official ANSES E-Phy database. Registrations change regularly, and several major active substances have been withdrawn over the past decade.

Documented resistances. Botrytis has developed resistance to multiple modes of action, particularly FRAC 1 (benzimidazoles), 7 (SDHIs), and 17 (fenhexamide). Multi-resistant strains are commonly identified in European strawberry operations. This is not a theoretical concern: in operations where mode-of-action rotations have been neglected over several seasons, it is not uncommon to observe reduced efficacy from well-timed treatments. The general rule is to apply no more than two consecutive applications from the same FRAC group per season, systematically alternating with a different mode of action.

Building a rotation programme. A sound programme for a production season with staggered flowering might be structured as follows: first treatment at pre-flowering with a biocontrol product (Bacillus); at F2–F3 (full flowering), first conventional fungicide FRAC 12 or FRAC 17 based on local resistance history; during fruit development, a second fungicide from a different FRAC group (FRAC 7 or FRAC 1 if still effective); at the start of harvest if pressure persists, possible return to FRAC 12 in compliance with the PHI. Inserting biocontrol products between fungicide applications allows intervals between chemical treatments to be extended without leaving uncovered windows.

Pre-harvest intervals (PHI). The PHI is a non-negotiable constraint in commercial production. PHIs vary by active substance and formulation — some anti-Botrytis fungicides have PHIs of 1 to 3 days, others 7 days or more. Any treatment decision during the harvest period must factor in this interval: a treatment applied on a Friday on a plot harvested the following Monday may expose the grower to an MRL (Maximum Residue Limit) exceedance, with the attendant commercial and regulatory consequences. Checking the PHI of each product before any application during active harvest is not optional.

In organic production, the conventional toolkit is excluded. Strategy relies exclusively on the cultural preventive levers and biocontrol products described above. Bordeaux mixture remains the authorised copper option, within the regulatory cap.

Decision support tools for open field. The epidemiological models MS-BOT and BoMa calculate Botrytis epidemic risk based on local weather data (temperature, relative humidity, leaf wetness duration). They allow treatments to be timed to when actual pressure justifies them, rather than following a fixed calendar. These DSS tools are particularly relevant in open field, where weather variability is greater than under tunnel, and where the number of interventions can be optimised by avoiding preventive treatments during weather windows that ultimately carry no risk. Under tunnel, continuous internal humidity monitoring serves a similar function — connected sensors allowing nocturnal peaks to be objectively tracked and ventilation strategy adjusted before risk materialises.

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Why a "standard" Botrytis programme will never be enough for your operation

Technical guides, regional disease alert bulletins, and advisory sheets have genuine value: they document biological mechanisms, provide reference thresholds, and list registered products. But they face a structural limitation that every experienced grower knows: they do not make the day-to-day decisions you need to take on your own operation.

The difficulty with Botrytis is precisely this variability of situations: the same climatic pressure does not produce the same results from one operation to another — or even from one plot to another.

Variety matters. Genotypes differ in canopy architecture, foliage density, and how quickly they dry after dew. A dense-canopy everbearing variety grown in soilless tunnel systems is exposed very differently from a June-bearing variety on outdoor raised beds. New variety selections specifically target Botrytis tolerance as a selection criterion — some recent introductions show improved resistance linked to a more open leaf architecture or to epicuticular properties of the fruit skin. But trade-offs with flavour, yield, and post-harvest firmness remain real, and varietal choice does not resolve crop management decisions during the season.

Phenological stage changes everything. Moderate pressure during flowering calls for a different response to the same moderate pressure during fruit development. At flowering, the risk is a latent infection on flowers that will not express itself for several weeks — the decision to treat preventively is a decision about an invisible future risk. During fruit development or at harvest, risk is immediate and visible. The decision to spray, wait, or reinforce prevention depends on the exact position in the phenological calendar — a factor that shifts by several weeks depending on variety, growing system, and annual climatic configuration.

Growing system redefines thresholds. Relative humidity in your tunnel can exceed 90% at night while the regional disease alert bulletin, built from outdoor weather station data, signals moderate risk. A grower in a multi-span structure with forced ventilation has management levers unavailable to a grower under a non-ventilated cold tunnel. A grower in soilless gutter production manages humidity problems differently from one growing in soil on plastic-mulched raised beds. Optimal strategies differ, and generic recommendations cannot anticipate these differences.

Plot history carries weight. A previous season with high Botrytis pressure left sclerotia in the soil and crop residues. The initial pressure of the following season will be structurally higher than on a clean plot, even with identical weather. This factor is not accounted for in general recommendations, which reason from a "standard" plot with no known history.

Local weather overrides the regional bulletin. The disease alert bulletin is a broad pressure indicator — useful for strategic orientation, insufficient for tactical decisions at the farm scale. A localised persistent dew episode in your area, or a humid night following poorly timed irrigation, can trigger an infection without the regional bulletin having flagged it.

Fungicide resistance is local. The resistance level in the Botrytis cinerea population present on your operation depends on your plots' own treatment history — not a national average. An operation that has used fenhexamide for four consecutive seasons without alternation may face a FRAC 17-resistant population even if regional data shows no issue. This individual dimension of resistance management is never addressed in general guides.

These questions — when precisely to treat, with what, how often, how to sequence biocontrol and conventional fungicides across your specific season, how to interpret what your plants are showing today, how to adjust tunnel ventilation based on the next 48 hours of weather — have no universal answer in a summary table. It is the variability of on-farm situations, more than the biology of the fungus itself, that drives losses in professional operations.

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Conclusion: managing Botrytis at the scale of your operation

Sustainable Botrytis control in professional strawberry production rests on three interconnected pillars: rigorous cultural prevention reducing germination windows, a biocontrol programme systematically positioned ahead of disease pressure, and a reasoned use of conventional fungicides in mode-of-action rotation, targeting priority phenological stages.

No single pillar is sufficient alone. And calibrating all three, at every decision point, depends on your variety, your growing system, your local weather, your plot history, and the stage your plants are at today.

It is precisely this level of contextualisation — which goes beyond what general guides can offer — that Fraisibot provides: a specialist strawberry AI agronomist, available in real time to help you reason through crop protection decisions in the specific context of your operation.

To go further on the overall disease management of your strawberry crop, see also our article Powdery mildew on strawberry: identification, prevention and treatment, and for an integrated view of all protection strategies, Strawberry protection: prophylaxis and biocontrol.

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