Botrytis Prevention Starts with Your HVAC: Humidity Control for Ontario Greenhouse Growers

Botrytis cinerea, the fungus behind grey mould, is the most economically destructive disease in Ontario commercial greenhouse operations. It attacks tomatoes, peppers, cucumbers, ornamentals, cannabis, and virtually every other high-value greenhouse crop. And while growers often reach for fungicides when they see it, the root cause in almost every case is environmental: the wrong humidity level, the wrong air movement pattern, or an HVAC system that is not doing its job.

The good news is that botrytis is highly preventable. Understanding the conditions that trigger it and designing your HVAC system to prevent those conditions is the most effective control strategy available.

What Botrytis Needs to Take Hold

Botrytis spores are essentially ubiquitous in greenhouse environments. They are always present. What triggers germination and growth is a specific combination of conditions: high relative humidity, particularly above 85 percent, combined with cool plant surface temperatures, poor air movement, and the presence of plant debris or damaged tissue.

The most dangerous moment is nighttime. As temperatures drop, the relative humidity in the greenhouse rises. If plant surfaces cool below the dew point, condensation forms on leaves and stems. Botrytis spores sitting on that moist plant tissue germinate within hours and begin spreading. By morning, the conditions may have improved, but the infection is already underway.

How HVAC Failures Create Botrytis Conditions

Most botrytis outbreaks in Ontario greenhouses can be traced directly to HVAC system deficiencies. The most common causes are:

• Undersized or poorly distributed dehumidification: When the dehumidification system cannot keep pace with crop transpiration and ambient moisture, especially during cool Niagara nights, relative humidity climbs into the danger zone.
• Inadequate air movement: Stagnant air pockets form in corners, along end walls, near the gutter zone, and at floor level. These are the areas where humidity climbs first and disease pressure concentrates.
• Heating system cold spots: An uneven heating distribution creates zones within the greenhouse where temperatures are consistently two or three degrees lower than the set point. Plant surfaces in these cold zones cool faster at night and are at higher condensation risk.
• Oversized equipment cycling: A dehumidifier or cooling system that is too large for the load cycles on and off frequently rather than running continuously. This creates humidity swings that are actually more damaging than a steady, slightly elevated RH.
• No monitoring: Many growers do not know what their humidity levels are doing overnight. Without sensors and logging, botrytis conditions can persist for weeks before symptoms are visible.

The HVAC Approach That Prevents Botrytis

Effective botrytis prevention through HVAC is not about one component. It is about a system designed and operated to maintain consistent, controlled conditions throughout the growing space.

Proper dehumidification sizing

Dehumidification capacity must be sized against the actual transpiration load of the crop at peak production, not just the volume of the greenhouse. A pepper or tomato crop in full production transpires significantly more moisture per square metre than most contractors estimate. Alpine sizes dehumidification systems based on crop type, planting density, and projected peak load, not just square footage.

Horizontal airflow fan placement

Horizontal airflow fans circulate air continuously throughout the greenhouse without creating strong drafts that stress plants. Proper placement eliminates the stagnant pockets where humidity concentrates and disease starts. Alpine maps fan placement during the system design phase to ensure coverage of the full growing area including the end wall and gutter zones.

Consistent heating distribution

Eliminating cold spots requires both proper heating system design and distribution layout. In-floor heating, overhead unit heaters with good throw patterns, and hot water distribution systems all approach this differently. The goal is a greenhouse where the temperature at plant level is consistent across the full growing area, including the perimeter.

BAS monitoring and control

A building automation system with humidity and temperature sensors at multiple points in the greenhouse changes the management picture entirely. Growers can see what conditions are doing overnight, receive alerts when humidity climbs above target, and review historical data to understand when and where problems develop. Alpine installs open-protocol BAS systems that give growers full visibility into their growing environment from any device.

The VPD Connection

Relative humidity is a useful metric, but vapor pressure deficit (VPD) is a more precise way to manage the plant-level environment. VPD describes the difference between the moisture content of the air and the maximum moisture it could hold at that temperature. Managing VPD, rather than RH alone, accounts for the interaction between temperature and humidity that determines both disease risk and plant transpiration behaviour.

For most Ontario greenhouse crops, a VPD of 0.8 to 1.2 kPa is the target range. Below 0.6 kPa, disease risk escalates sharply. Above 1.6 kPa, plant stress increases. Your HVAC system and its controls need to manage both temperature and humidity together to hit VPD targets consistently.

Frequently Asked Questions

What humidity level prevents botrytis in a commercial greenhouse?

Keeping relative humidity below 85 percent is the basic threshold, but more precise management targets a vapor pressure deficit of 0.8 to 1.2 kPa based on the crop and growth stage. The goal is not just keeping average RH in range but preventing the overnight spikes and condensation events that give botrytis its foothold.

Can air circulation alone prevent botrytis without dehumidification?

Air circulation significantly reduces botrytis risk by eliminating the stagnant pockets where humidity concentrates, but it cannot substitute for active dehumidification in an Ontario greenhouse. During cool, humid Niagara nights, circulation alone will not prevent humidity from climbing to dangerous levels. Both systems need to work together.

At what temperature does botrytis grow fastest?

Botrytis grows most aggressively between 17 and 23 degrees Celsius combined with high relative humidity. This range overlaps directly with the overnight growing environment in many Ontario greenhouses, which is why nighttime humidity management is the critical control point.

How do I know if my HVAC is causing botrytis in my greenhouse?

Signs that your HVAC system is contributing to botrytis include recurring outbreaks in the same areas of the greenhouse, particularly near end walls, along gutters, or in corners. These patterns indicate cold spots or stagnant air zones. A humidity audit using data loggers placed throughout the greenhouse overnight will identify when and where conditions exceed safe thresholds.

What is the difference between relative humidity and VPD for greenhouse management?

Relative humidity measures moisture content as a percentage of what the air could hold at a given temperature. VPD measures the actual drying pressure the air exerts on plant surfaces and is a more accurate predictor of both disease risk and plant water stress. Managing VPD requires controlling temperature and humidity together, which is why an integrated BAS approach is more effective than monitoring either metric in isolation.