If you spray fungicide every ten days and still have Botrytis, the fungicide is probably not the problem.
Vapour pressure deficit, or VPD, expresses the gap between how much moisture the air could hold at its current temperature and how much it actually holds. That single number describes plant transpiration and fungal disease risk far better than relative humidity on its own.
Why relative humidity is not enough
80% relative humidity at 15 °C and 80% at 28 °C are two completely different situations for the plant. In the first the air holds little water and transpiration has all but stopped; in the second the air is very moist yet transpiration continues. VPD separates the two.
The target range
- Below 0.4 kPa: transpiration nearly halted, moisture accumulating on the leaf, high risk of Botrytis and grey mould.
- 0.8–1.2 kPa: the target band for most greenhouse vegetables.
- Above 1.6 kPa: excessive transpiration, stomatal closure and reduced photosynthesis.
The real problem is night
In most Iranian greenhouses the problem is not daytime; it is night. As temperature drops after sunset, relative humidity climbs above 90% and VPD approaches zero. Under those conditions water condenses on the leaf surface and creates ideal circumstances for spore germination. No fungicide compensates for that.
What can be done
The fix is usually a combination: short night ventilation cycles even at some heat cost, avoiding late-day irrigation, plant spacing that allows airflow between rows, and — in better-equipped houses — localised heating to hold leaf surface temperature above the dew point.
Before adding another fungicide to the programme, put a simple data logger at canopy height and record night temperature and humidity for a week. More often than not, the answer is right there.