Botrytis in a Winter Capsicum Glasshouse
Kim manages a 0.3 ha glasshouse near Tuakau growing red capsicum for the Auckland market. It is mid-July. Night temperature is set at 14 degrees C and the vents are kept closed at night to hold heat and hold down the LPG bill, which has risen 40% this season. Over the past two weeks staff have found spreading soft brown lesions on pruning wounds and on fruit near the head of the plants, and a consultant has confirmed Botrytis cinerea. Humidity loggers show 92-96% relative humidity inside from about 8 pm to 9 am, and leaf wetness sensors show 8-10 hours of continuous wetness. Day temperatures are fine at 22-24 degrees C when the sun is out, and the crop is at high fruit load with daily picking. Vents open at 8 am. Options are: raise night temperature to 17 degrees C, run the vents a crack at night, install a dehumidifier at $18,000 capital cost, or cut back pruning and hand-remove affected fruit. The crop is worth about $180,000 at current prices and the disease is spreading at roughly 1% of plants per day.
1. Explain why the current night-time climate management is favouring Botrytis, referring to humidity, temperature and leaf wetness.
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Why the current regime favours the disease
- The glasshouse is trapping water. Plants transpire all day and continue at night. With the vents shut and 14 degrees C holding the air temperature, that water has nowhere to go, so relative humidity climbs to 92-96%.
- Relative humidity near saturation means a vapour pressure deficit close to zero. At that point there is no driving force for water to evaporate from a pruning wound, a damaged flower or a fruit surface. Surfaces simply stay wet.
- Cold cover and cold fruit surfaces condense. The structure loses heat to the night sky, so surfaces fall to or below the dew point of the humid air inside. Water then condenses directly onto plants and, more importantly, onto the fruit and wounds.
- Botrytis needs free moisture to infect. Conidia will not germinate on a dry surface. The 8-10 hours of continuous leaf wetness recorded here is well beyond the roughly four hours of wetness usually needed for a successful infection, so every pruning wound created in the afternoon is a near-certain entry point overnight. Once inside the tissue, the fungus grows at 15-20 degrees C - the glasshouse night temperature sits squarely in that band, and closing the vents removes both the drying and the cooling that would slow it.
- Wounds and senescing flowers are the food source. Pruning creates a wound at every cut, and high fruit load means constant picking damage. The disease is therefore being supplied with both the climate and the entry points it needs.
2. Recommend a combination of actions for the next two weeks and justify your choices against cost and the value of the crop.
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Recommendation: run the vents a crack at night, tighten pruning and sanitation immediately, and order the dehumidifier as the durable fix. Raising the night temperature is the weakest of the four options on its own.
Immediate actions (this week, low cost)
- Vent a crack at night. Even a small exchange of air carries the transpired moisture out. It costs some heat, but far less than losing fruit, and it is the fastest way to break the wetness cycle.
- Sanitation. Hand-remove affected fruit and any infected pruned material into a sealed bin, not onto the floor. Prune in the morning while vents are open and wounds can dry, not late in the afternoon, and avoid pruning on overcast days.
- Adjust irrigation so the crop is not overwatered going into the night, which is the main source of the humidity.
The dehumidifier
At 1% of plants affected per day, the epidemic compounds: about 10% of plants in ten days, and losses on a $180,000 crop are many times $18,000 well before that. The dehumidifier removes water rather than merely moving it, so it keeps working even on still, cold nights when venting loses too much heat. It is the option that addresses the cause.
Why not just raise the temperature to 17 degrees C? Raising the temperature by 3 degrees C increases the air's water-holding capacity and will pull relative humidity down, reducing condensation, but it costs LPG on every cold night for the rest of the season and does nothing about the water already in the air. If used at all, it should be paired with venting or the dehumidifier. Note also that 17 degrees C is still within the Botrytis growth range.
3. Describe a monitoring and decision rule Kim could use to decide when to escalate controls, and explain why a rule is better than reacting to visible lesions.
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A workable decision rule
- Log the two key numbers continuously: relative humidity and leaf wetness duration in the crop canopy, not just at the sensor head near the roof.
- Trigger point: if canopy relative humidity stays above 90% for more than four consecutive hours, or leaf wetness exceeds four hours, the night protocol escalates - vents opened further, dehumidifier run harder, and no pruning that day.
- Weekly scouting: inspect 100 plants across fixed points, count new lesions on wounds and fruit, and record them.
- Escalation threshold: if new lesions exceed about 1 in 100 plants per week, tighten sanitation further, review the spray programme with the consultant and check that the dehumidifier is keeping up.
- Record all actions taken and their effect, so next winter's settings are based on the farm's own data rather than a general rule of thumb.
Why a rule beats reacting to visible lesions
Botrytis lesions are the end of the process, not the beginning. By the time a soft brown lesion is visible, infection happened days earlier, the fungus is sporulating and spreading conidia, and that fruit is unsaleable. A climate rule catches the conditions that allow infection, during the 8-10 hours of wetness, when the action still makes a difference. The rule is also objective: staff on different shifts apply the same trigger, decisions can be reviewed after the season, and a rising trend shows up before it becomes an epidemic rather than after.