In greenhouse tomato production, Botrytis rarely appears “out of nowhere.” Usually there is a predictable sequence: the air cools, humidity rises, VPD falls, and a water film or micro-droplets form on leaves and flowers. The problem is that these windows can be brief, can occur only in certain zones, and are easy to miss if sensors are not placed or interpreted correctly.
VPD (vapour pressure deficit) is an estimate calculated from air temperature and relative humidity, useful for understanding how “eager” the air is to take water from the leaf. But the leaf is not always at the same temperature as the air, and a small difference can determine whether you get condensation or not. That is why, before plant-protection decisions, you should confirm what sensors say with direct observations and simple checks.
Treating based on an unvalidated alarm has costs: money, time, phytotoxicity risk, and resistance pressure. At the same time, ignoring a real risk window can mean losses in trusses and quality. Below is a practical protocol: how Botrytis risk windows form in greenhouse tomatoes, what to watch in the data, how to validate readings, and how to verify the effect of actions (ventilation, heating, hygiene, treatment).
1) Why a “humid night” is dangerous: the VPD–condensation–Botrytis mechanism
In a greenhouse, after sunset radiative energy drops, air cools, and relative humidity can climb quickly. When air approaches saturation, VPD approaches zero and leaf evaporation stalls. If the surface of leaves or flowers is slightly cooler than the air, the dew point is crossed and condensation appears. Botrytis takes advantage of vulnerable tissues (flowers, peduncles, pruning wounds) and periods with free water.
What to watch: charts often show night “plateaus” with very high RH and very low VPD, followed by a morning when leaves are still wet. What to verify independently: enter early with a flashlight and inspect inner-canopy leaves and flowers for a water sheen, fine droplets, or grey mould on plant debris. The practical decision is not automatically “spray,” but “shorten wet-leaf duration” via ventilation/heating and confirm pressure through scouting; verify the result by seeing shorter nighttime periods near-zero VPD and less observed condensation.
2) VPD is not the leaf: how leaf–air temperature differences can mislead you
VPD calculated from air describes the air’s evaporative potential, not the actual state of the leaf surface. In tomatoes, leaves can be cooler than air (radiating to cold film, cold drafts at vents, shaded zones) or warmer (direct sun, near heating pipes). A difference of just fractions of a degree can decide whether the leaf hits dew point, even when the air sensor looks “acceptable.”
What to watch: disease often establishes near walls, row ends, or under drip points where structural condensation falls, even though the greenhouse average doesn’t look critical. What to verify: measure leaf temperature with an IR thermometer, but do it correctly: set leaf-appropriate emissivity if the device allows; aim only at the leaf blade (not string, film, metal, or glossy plastic); stand close enough that the measurement spot is smaller than the leaf (distance-to-spot ratio matters); and avoid interpreting readings from wet, reflective leaves as precise temperatures. The decision: if the leaf is consistently cooler than air in that zone, prioritise air mixing and reduction of “cold surfaces” there; verify by repeating the same leaf–air checks at the same points and times and looking for a smaller gap.
3) Sensor placement to catch the microclimates that create condensation
In tomatoes, the relevant air layer is around leaves and flowers—not up at the ridge and not pressed to the floor. A single measurement point can miss cold, humid “pockets” between rows or at greenhouse ends. A robust approach is zoning: one sensor near a representative row, one in a known hot/dry zone, and one in a cool/wet zone (end wall, corner, near doors, near a north wall).
What to watch: differences in RH and temperature between zones, especially at night and early morning. What to verify independently: make a simple “visible condensation map” (where you find wet leaves at 6–8 a.m.) and compare it with your sensor map; if the wet zones don’t match the sensor story, placement is wrong—not the crop “acting strangely.” The decision: relocate the sensor that reports implausible values (for example, in an air jet, near a heat source, or exposed to dripping), then repeat a week of comparisons; verify when zone differences become stable and explainable by position rather than chaotic. In GrowGuard, zone-based interpretation helps you avoid relying on an average that hides problematic corners.
4) A validation protocol for temperature/RH sensors before you act
Before making treatment decisions based on VPD, check whether the inputs (temperature and RH) are believable. What looks wrong in data: abrupt jumps, “stuck” values (RH at 99% for hours with no variation), temperature that doesn’t track day/night, or one sensor always reading 2–3°C higher than others without a positional reason (pipes, motors, direct sun). In greenhouses, radiation can also overheat housings if sensors are not adequately shielded.
What to verify in practice: (1) a side-by-side spot comparison with a second handheld temperature/RH instrument held for 5–10 minutes at the same height in the canopy; (2) a physical inspection of the sensor: dust, water in the protective filter, insects, placement too close to cold plastic/metal; (3) a coherence check between zones: at night there will be differences, but they should not invert randomly from one night to the next. The decision: clean/replace the filter, reposition, and log the intervention date; verify by watching smaller offsets and smoother curves that clearly match real events (venting, heating, sunrise).
5) Building “risk windows” from duration, not a single number
Botrytis risk is linked to time spent in favourable conditions, not just a minute of high RH. A workable method without universal setpoints is to use durations and your own greenhouse baseline. What to watch: nights when RH climbs much earlier and stays high until after sunrise are generally more risky than nights with a brief spike. What to verify: over several typical nights, note when RH begins its rapid rise and when humidity “breaks” through heating/venting; then correlate that with morning condensation presence in the same zones.
One practical criterion example (to be adjusted for your greenhouse): trigger an alert when, in a zone, you have very high RH or very low VPD for a cumulative nighttime duration (for example, more than 2–3 hours within 22:00–06:00), not for a 5-minute excursion. An even more robust alternative: set a threshold from your history, such as the 90th percentile of nighttime RH from the last 14 nights, and alert when you exceed that level for a minimum duration. The decision: treat the “window” as an action trigger (vent/heat, intensify scouting), not as a diagnosis; verify by seeing the total duration in the critical zone drop on nights with similar outside temperature. GrowGuard can help with duration-based, zone alerts and visual comparisons, but the rule must be calibrated locally.
6) Condensation doesn’t come only from the air: canopy mass, late irrigation, and cold surfaces
In tomatoes, a dense canopy reduces airflow and creates higher-humidity layers inside the plant. Transpiration from leaf mass accumulates, and if the night starts with wet foliage (after a humid day) or with very wet substrate/soil following late irrigation, vapour loading stays high. In addition, cold surfaces (film, metal elements, glazing) can generate dripping that wets leaves and trusses in specific spots.
What to watch: RH increases immediately after late irrigations or after fully closing ventilation; disease clusters under structural points that drip condensation. What to verify: (1) inspect in the morning for structural drip sources; (2) compare irrigation timing with nighttime RH evolution; (3) check whether the canopy interior stays damp even when the aisle looks dry. The decision: adjust practices that prolong humidity (for example, avoiding late pulses, managing leaves to improve internal airflow) and fix structural condensation points; verify by seeing fewer nights where late irrigation aligns with persistently high RH and by finding fewer wet inner-canopy leaves during morning checks.
7) From climatic risk to a plant-protection decision: avoiding “spraying the chart”
Microclimate data shows conducive conditions; it does not confirm the pathogen’s presence. Botrytis can be introduced or maintained by plant debris, planting material, or water, and a VPD alarm cannot tell you which of these is driving your problem. What to watch: repeated risk windows in a zone followed by consistent symptom patterns (brown lesions on older leaves, grey mould on flowers, lesions on stems/pedicels), especially after operations that wound plants.
Decision-making should be stepwise: (1) if you have climatic risk only, increase inspection frequency and correct microclimate; (2) if you have climatic risk plus early signs or a clear history in that house, consider treatment according to the label and your anti-resistance strategy; (3) if you confirm nothing, do not turn alarms into automatic spray routines. Not observing wet leaves is not proof of no risk—especially without a leaf-wetness sensor or leaf-temperature checks in cold zones—so the minimum safe approach is “data + inspection.” Verify outcomes with a simple audit: after intervention, risk windows shrink and the number of new symptomatic points drops at the same scouting intensity.
8) Checking the effect: how to know whether your measure actually closed the infection window
Any intervention (ventilation, heating, dehumidification, leaf management, treatment) should be checked through a measurable indicator. What to watch in data when you’re improving: RH no longer stays “stuck” high for hours, VPD no longer sits near zero until after sunrise, and zone differences shrink or become predictable. In the crop, morning checks find fewer wet inner-canopy leaves, not just drier aisles.
What to verify concretely: select two nights comparable for outside temperature and cloud cover and compare the duration of “critical humidity” in each zone before/after the measure. If you moved sensors or performed maintenance, first confirm the data is coherent; otherwise you can wrongly credit “success” to a measurement artefact. The decision: if risk reduction shows up only in one zone, treat the greenhouse as non-uniform and correct that specific area (drafts, sealing, cold sources). In GrowGuard you can compare zones and history more easily, but your audit must tie back to the same hours, the same points, and the same in-crop observations.
Conclusion
In greenhouse tomatoes, Botrytis risk windows are born from short combinations: nighttime cooling, high RH, low VPD, and leaf–air differences that lead to condensation. Sensors help you see them, but they don’t replace validation: microclimate-based placement, a handheld cross-check, morning wet-leaf inspection, and targeted scouting after events. Microclimate indicates “favourable conditions,” not proof of the pathogen.
When you build criteria based on duration and your own greenhouse history, alerts become tools rather than noise. Work in steps: confirm the data, reduce condensation time, then confirm in the crop before treatments. If you use GrowGuard, set zone-based alerts and keep a field-check log; if you want, invite the team to track the same windows so you close the loop quickly between “the chart” and what is happening on the leaves.